Diffraction WRECKS sharpness: Photography physics
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- เผยแพร่เมื่อ 2 ก.ค. 2024
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Diffraction is a bizarre property of physics that ruins the sharpness of your pictures at high f/stop numbers. There's a lot of information on the web about diffraction and photography, but none of it seems to agree, and most of it is wrong. I got deep into researching the physics of light, and in this video, I give you my best guess as to what happens to cause the reduced sharpness at small apertures.
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Tony, What's with the sound. Your voice is sounding thin. Mic? Post processing? Just not as good as the studio mix!
Tony - What happens, with regards to diffraction, when I put a 2x extender on my 300mm f2.8 lens? I realize that I'm shooting at F5.6, losing 2 stops, but does diffraction come into affect quicker, or differently, because of the 2-stop loss?
@@sfink16 I take it the 300/2.8 is a prime of some sort. At 5.6 the diffraction should still be ok. Most of the time it becomes noticeable after F11 and up.
Shashank Singh What "corners" are you referring to? My apertures are almost round; they don't have no stinkin corners.
Take down this video, Tony. Everything you say is false. It's about time to establish quality control.
Tony - diffraction occurs for any wave that travels through a slit no matter the size, given by a specific math formula. For a circular (or close to it in terms of relative differences) aperture this creates a diffraction pattern known as an Airy disc and described by the approximate formula sin(x) = 1.22*(wavelength/diameter) where x is the angle of first minimum, and is often used as the size of the disc (in reality a bit more complicated). The reason softness occurs due to diffraction is if the Airy disc is larger than the pixels receiving the information, because it can no longer distinguish two point sources cleanly. The aperture at which this occurs is known as the diffraction limited aperture, which is the largest aperture at which the Airy disc is so much bigger than the pixels size of the sensor that two point sources start overlapping the pixels that record them. Usually we use 2-3 (the popular online calculator uses 2.5) pixel sizes for which there will be softening. In the camera world the previous formula can be further approximated by x = 2 * 1.22 * wavelength * Fnumber. Using 525nm (green, assuming everything is corrected to this wavelength as a simplication) and f16 you get 20 microns. A 5d4 has a pixel size of 5.36 microns and a 7d2 has a pixel size of 4.1 microns so both would see the soften effects of diffraction. In reality with Bayer sensors you have more green pixels so they would technically hit DLA sooner. Note this formula assumes 100% pixel peeing at typical computer distances. As that is relaxed the DLA increases.
Thanks for a clearly expressed method to calculate diffraction... more useful to photographers than the underlying theory - which at some level always becomes "because it does" to anyone.
I AM a physicist. And the explanations below about Airy circles are correct. The f/stop diffraction has nothing to do with sides of aperture. The best way i can explain it to the photographers is that passing light through an aperture turns that light from hard to soft. The effect is allot more pronounced when size of aperture is close to wavelength of light; but the effect is there for all apertures and all wavelengths. Explanation by Antonio Sanchez in the comments is correct. Also, making smaller pixels does not make diffraction worse; it enables you to see blur from diffraction better.
A physicist huh? We are all super impressed. You should use that big ole brain that perfectly understands diffraction to figure out the difference between a lot and allot.
@@kronk358 are you being a Dick?
Because you kind of sound like a Dick. I am happy that he took the time to explain this mo better
@@kronk358 That post was uncalled for. The ability to spell has nothing to do with intelligence and a knowledge of physics.
@@nyobunknown6983 oh relax Francis, it was funny.
While the practical meaning of your video is right, the science or your guesses are quite off. Diffraction has long been understood, it is just damn complex math if you want all to be right, similar to semiconductor theory, a mess if you want to calculate stuff. Some other commenters already pointed out useful resources for the correct info. Love the community!
Physicist here:
Single slits also produce diffraction, of the same kind as the double slit experiment. It will produce two things, first of all an interference pattern, where you see concentric circles, dark and light. Second, the light will be diffused from the straight path and will travel in all directions, although most of it will be concentrated along the original path.
There is a characteristic angle that measures the size of these effects, and it is given by the wavelength of the light (around 500 nm) divided by the size of the slit (around 2mm), so about 0.00025 radians. This is a very small angle, but it is about angular size of the pixels of your camera as seen from the diafragm of your lens. So you will see these effects on your camera. I dont do optics, but it should possible to put concrete numbers on these effects (e.g. the angle to the first destructive interference, or the angle that contains 90% of the energy)
It seems like if you're willing to make the sensor larger, you could keep your apertures larger, and it would improve the *angular* effect.
Is the slit creating a bandlimited impulse or somesuch like you'd see in a 1d bandlimited signal? Would a (spatial) low pass filter prevent some of the interference (just as a low pass filter prevents aliasing in a bandlimited signal)?
@@microcolonel But then you would decrease your DOF, I am not sure if there is some optical trick to overcome diffraction and obtain a photo with the same DOF. Maybe it is possible to remove most of it computationally. In any case, the advantages of having most of your subject in focus for macro photography outweighs the resolution loss due to diffraction.
I am a Physics student at Purdue University (not a grad student or a professor, but I am graduating soon). Stopping down the lens still uses all the lens. The difference is that light coming in at more extreme angles (through the edges of the lens) are removed by the aperture. That removal of high incident angle light makes the focus point sharper because the centers of lenses are generally free of aberrations (the edges are much harder to get perfect). In laser laboratories, it is not uncommon to shoot the laser through the center of a large and cheap lens rather than through a small and expensive achromatic lens (the centers of lenses generally don't produce the same amount of chromatic aberrations). The reason that I say that the whole lens is still used is because the whole scene is still visible. Using less of the overall glass would be the same as masking (note that mirror lenses behave differently to masking, there it is more like an aperture (personal experience)). The blur from high f-numbers is most similar to the "knife edge diffraction." The light does interact with itself and the pattern can be seen in water (Huygens's principle).
I'm not a physicist myself, but I think this is all well-understood. And it is not to do with electrons in the iris attracting photons. It is just a result of the wave nature of light. You can see the same effect on waves on the surface of water, for example. The light bends around apertures as would any wave. Light at the aperture acts as a wave front, resulting in the light spreading out.
So what Tony says at 7 minutes then?
@@wallcrawler62 ... is TOTALLY incorrect
oh, no, Tony is wrong? again? and again ..
Thanks Tony. I have been following your channel for many years and love these videos that you put together. You explain them so well. I also love the depth of field that you are using to shoot these clips with in your kitchen, it is so sharp. Keep up the great work and thank you for all that you and Chelsea do.
Wow - who knew you had so many physicists tuning in Tony? As just a simple photographer I like to have reasonable understanding of the more technical aspects, but when you need 25 physicists debating theories I decide it's enough for me to know that diffraction occurs and I therefore endeavor to shoot as near my lens' sweet spot as possible. Love all that physics tho! :)
Rule of thumb - if your lens goes to F22 dont go over (halfway) F11 or if lens maximum is F16 dont go over F8. It would be a good test whether ND filter is worse image quality than lens diffraction for long exposures
Great topic. I’m an electrical engineer and astrophotographer (with telescopes), not an optical physicist. But the math for diffraction is pretty darn simple on a basic level. Instead of talking about aperture as a focal ratio, it is simpler to talk about it as a diameter or radius, r, of the physical aperture. Light near the edge of the aperture is bent. Light away from the edges in not. So just look at the ratio of the aperture edge (bent light) to aperture area (unbent light). Edge = Circumference is C=2*pi*r. Area is A=pi*r^2. C/A = (2*pi*r)/(pi*r*r) = 2/r. So diffraction is inversely proportional to aperture radius. Also, not sure about those waves - maybe a secondary effect? But as a first effect, some photons are nearer to edge than others, so they get bent more. Some are in middle of aperture and don’t get bent. Others are at the opposite edge of aperture, and get bent the opposite way. That is why the light scatters and an image point becomes distributed (i.e. blurry). We can call this the airy disk. This is essentially the single-slit experiment. If the single slit (aperture) becomes small enough to approach the wavelength of light, the diffraction rings show more strongly. If we have a longer focal length, then we magnify it more and it is easier to see (or it spans more pixels so the sensor can resolve it). Another point on shooting through glass (especially thick aquarium glass): a big aperture collect light that passes through a bigger area of the glass, so you see the variation across the glass. Telescopes through windows are very bad. My Fuji APS with 56mm f/1.2 is very bad at the aquarium. My iPhone with tiny sensor and tiny aperture radius is very good through the aquarium glass because the thick glass doesn’t vary much across the tiny couple of millimeters of aperture. Essentially, cheap glass is the same issue as lens aberration, only worse.
I love this stuff, from my past life it's what I spent a lot of time engaged in. On a fundamental level it really does come down to Heisenberg's Uncertainty principle. In short the more you squeeze a particle/wave (Wave-particle duality), the more it tends to spread out. It can be very counter intuitive. Fundamentally it comes down to Field Theory (It's the Waves that do the talking). Reach out and try and grab some waves, they won't be tamed. Photography really comes down to painting with light and the digital technology available today and where it's moving. The future of this technology opens up creativity on a whole new level. Tony, I really appreciate these interludes into the more technical aspects. Your wife is pretty cool to. Between the two of you, it open up an array of topics. Thanks again.
K man...
Nerdhole alert, apologies in advance (I think that Tony & Chelsea's is the best photography channel on youtube - great scope, depth, balanced, generous, candid, honest)....
The basic point that diffraction effects can degrade images when the aperture is small enough is right. So why would I ramble on? Too much time on my hands, that's why! (And I should probably have read for others who beat me to this.)
I'm curious about who is saying that there is some confusion over diffraction. There is certainly interest in surface effects - what happens at the boundary between two materials, but that isn't the issue here. This is a problem of classical optics, much of which has been well understood for over 200 years. It cannot simply be electrons interfering with photons. Photons are massless and carry no charge, and the mass of the blades of the aperture is too small for mass-energy equivalence to play an observable role. But the real giveaway is that you can make a pinhole or slit out of any number of materials and see the same diffraction effects (assuming the pinhole or slit is of a constant size and quality and the materials are of equal thickness). In classical optics, diffraction is a product of the interference of waves. Classical optics is an approximation, but more than good enough at macroscopic scales. (Much the same phenomenon is at work with sound, which is why you can hear someone around a corner even when there is no surface for the sound to bounce off.) In quantum mechanics, particles also have wavelike properties, so you also see diffraction in quantum mechanics.
One more nerdhole needle: There is a lot to be said for photographers understanding a little physics. For example, people used to go on and on about the superiority of semi-matte or matte finish displays over glossy. (I don't see much of this anymore, but maybe I'm just not paying attention.) The matte finish on a display is, in effect, a diffusion filter. It kills reflections, but it also degrades image quality. A little physics might also put to rest the cult of curved displays for HDTVs. Great for an imax or huge 70mm screen at a theater. Largely pointless even with a large screen at home (except to the extent that it also mitigates reflections).
So, no matter what my camera or lens is, the first thing to do as a landscape shooter is to stack in a steady environment could be inside or out outside and take a series of photos to each F/STOP and check where the lens sweet sharp F/STOP is and I will do this on infinity focus because that's what I do most of the time.
The slit experiments and the wave propagation examples are the same as this phenomenon.
This is a well understood phenomena. The comparison of light as wave is correct as opposed to the explation. The amount of light bending near edge is due to wave property. Eventhough the aprature is 5000 times the wavelength. Because the bending and bluring due to defraction is less as few pixel. Which is v less bending but is enough to make the image soft.
When I first started product photography I had only a little knowledge of photography. I had to use a high F-stop to get larger products in full focus. So I had a huge issue with diffraction and aberration which led to a lot of time in Photoshop. Once I learned about focus stacking my images improved considerably. Thanks to Tony and Chelsea, Jared Polin, and other gurus on youtube, I have learned so much. Thanks, Tony & Chelsea.
Technically, diffraction happens at all scales with all waves, and it's just that the significance it has is smaller as the size of the aperture gets larger relative to the wavelength. The thing about photons traveling as waves is a bit off, though you mention phase differences, there are also wavelength differences. Even so, diffraction still happens with coherent monochromatic light -- you can diffract research-grade lasers, for instance. The behavior at the photon level is largely statistical, and the wave behavior is the statistical distribution that follows the pattern of that wavefront. The general fit for the behavior at the aggregate scale is basically a Fourier Transform of the aperture shape. This is the model we also use to simulate not just lens aperture bokeh, but even things like glare as perceived through one's own eye. Of course, in practice, this is compounded by the fact that you anyway have some variance caused by passing through glass on the way to the aperture, and then more again after.
To be exact, the Fourier Transform is basically equivalent to Fraunhofer diffraction, and technically Fresnel diffraction is more correct (and appropriate for things like glare through the eye), but the significance of the Fresnel component at far field distances -- and the distance between a lens aperture and the camera sensor would qualify as "far" in this scope -- is basically zero, so it just reduces to Fraunhofer diffraction.
This was a great vid Tony....Thanks for that..... Way more useful and more educational than most photo vids out there....
Thanks for educating us. Love the video and learning heaps from this and other videos like it.
Hi Tony. Physicist building instrument for astrophysics. Your scientific description is unfortunately all wrong. I am not sure where you get these informations, I have the feeling that it was a simple guesses:
- "Photon attracted by electrons on the border" ?????? Where did you read such thing ???
- "Photon traveling not straight but going up and down in a wave pattern" ??? This is a total confusion of the so called wave-particle duality which is more a representation problem and has been solved by quantum mechanics since 70 years. What you can say is that their is no mentally, or by draws, satisfying way to represent what is a photon (or any particle in the macro world), some time it is better to represent light as particles bullets some time better to represent it as waves, like water waves (depend of what you want to show). But they are just representation or mathematical tricks. The best way to approach the problem, closest to reality, is to talk about probability. But no photon are not traveling as you drawn, not at all.
- There is no meanings in what you drawn, each possible part of the object you photograph is emitting light in any direction and you cannot avoid to place an optical element (or a pin-hole) to form an image.
They are many approach you could start with. And of course all are impossible to resume on a youtube comment. Maybe the best approach is to interview or ask a physicist, not to guess things. I am open for that or can orient you to any appropriate person in my lab with better communication skill than I have.
Really love all the comments. Tony I think you have unwittingly parachuted straight into the fundamentals of quantum physics. In "reality " you are attempting to describe probability graphically (which is impossible). I do think the more knowledgeable commenters should have cut you some slack..... your approach is extremely common and taught to our kids (eg graphic of an atom). This issue is well understood, just impossible to communicate in a non mathematical way. Besides your message is still 100% relevant from a photography perspective. Nice try mate!
Snapography this video indeed surprised me. So far I saw mainly simplifications from Tony contents which are totally fine even necessary. Sometime with mistakes perfectly forgivable. This video is kind of an invention presented as if it was a deep research of how light work. Quite disappointing and a bit scary when coming from somebody with millions of subscriber. Of course that is not so important here but what scares me is that I probably digest a lot of bs from this kind of channels i can trust on topics I do no know.
Sylvian I know what you mean. I've been doing aerial photography for years and watching Tony's videos on that subject is complete bs. It's a pity because you don't need to understand to get the photograhy message; no idea why tony didn't just acknowledge that and move on instead of dreaming up his own explanation
Yes. Fun fact: Diffraction in optical system is actually a "real-life" application of Heisenberg's uncertainty principle. The opening of a lens defines the position of a photon, which causes the an corresponding inaccuracy of the momentum of the photons and hence their direction, causing a blur.
As much as I like your videos on photography, the physics teacher inside me has to say this:
we know perfectly why diffraction happens. By the way, there is no "attraction" between the photos and the electrons in the diaphragm.
Interesting and well-explained, thanks! I knew this phenomenon occurred from results from my photography, but I didn't know exactly why...I just kept away from higher f stops because photographers who mentored me over the years said the sharpest results are found around the "middle" of a given lens (maybe 5.6 or so)
Very nice info. Lots of people forget, or just don't know what is actually going on inside the camera to come up and resolve into the image you get.
You really should look up how the angular resolution work.
Many people don’t understand that a given photon enters the camera through the whole aperture
thank you Tony, this explained a lot to me, keep these coming
There is an absolute inverse relationship between the diameter of the aperture and diffraction. So, with a smaller sensor, diffraction is a problem at lower f numbers. This is because the final viewed image is more magnified from the image on the sensor. Also, with smaller, high megapixel sensors, the cells are smaller and the diffraction is relatively larger. With my 4/3 sensor diffraction becomes noticeable at f8.
I love geeking out on the techie side of photography. Please make as many videos like this as you can. Good stuff.
Diffraction can be explained by Hygens principal because light behaves as both a wave and a particle. Check out the video about light by crash course.
Toby Bass Videos about quantum physics are helpful too. It sounds really unintuitive that the photons “know” the gap is small and have to have a less precise position, but that’s just how it works at that scale.
@@Ty4ons You don't really need QM for this. The math was done in the mid to late 1600s. The only tricky part is understanding the limits for diffraction to occur. He seems to think that light is a particle in his explanation (he acknowledged the wave nature but doesn't quite internalize it). The best explanation is the diffraction caused by a razor blade.
If you think huygens principle actually explains why diffraction occurs, you are missing the point. No one actually knows.
@@markevely1583 what is the point im missing? Seems pretty believable to me
@@tobylukebass6234 it is not about 'belief' it is about the physics and understanding a very simple point, no one actually understands why diffraction happens. Lookup the double split experiment , plenty of TH-cam other material. Huygens is just away of thinking about it not actually what happens. Or read QED by Richard Feynman then say you understand the subject.
Love these types of videos. It's great to see the other side of photography in order to bring it to a new level.
Tony, I love this video. A great science communicator Brian Cox did his PHD in difractive scattering. While I am not a physicist I have always had a great yearning to know more about high level physics and came across his work a few years ago. It goes into a bit of depth about what you are talking a with photons interacting with electrons.
In my physics lecture it was quiet a bit different, but yeah... Why did'nt you just ask a physics youtuber for help/collaboration?
or attended a physics lectures in school))
This has been explained many times. Thanks for another perspective that's easily understood
Diffraction is actually a fairly fundamental phenomena shown by light and can easily be understood using the wave theory of light. Basically in order to understand it, think of light as a wave. Imagine a wave created by a pebble dropping into water, travelling through the same slit. The wave will spread out from the ends of the slit. Light shows similar behaviour. Read about Huygen's principles to learn more
Watch the video; I address this
Yeah but your claim that scientists don't understand it is incorrect, it was understood in the early twentieth century
Enjoyed the video. The nitty gritty of knowing why diffraction happens is less important than knowing that it does, lets go take some pictures! Just finished a backyard bench test of my new Samyang AF 85mm f1.4 and I can clearly see flare and diffraction in action :o)
@Otto Danby II So....what's the sweet spot and how does it compare to wide open?
Tony, really enjoying your "nerdy" videos - interesting stuff! More please!
Tony, you need to read more. Photons have no charge and no mass. They can however be influenced by gravity, which in short is a product of mass. Photons contrary to common sense operate like particles and like waves. Here's a suggestion. Short story: The macro scale analogy involves taking a bucket, fill it with water, toss a pebble into it and you'll waves. Now place a board with slits toss a pebble into said bucket and you'll see waves forming interference patterns, i.e. defraction, that is to say waves cancel and reinforce one another, as they pass through the slits. Abstract to small aperture photography light probably (after all it is quantum physics) forms similar interference patterns as it passes through a small aperture.
As we used to say if you want to prove that, you'll need a bigger grant; if you want me to prove it get out your checkbook. If I explain more I'll have to add footnotes and a bibliography. One last remark, notice I did not bring the issue of wavelength, which for photography can be summarized as the ROYGBIV phenomenon.
I can't say what the physics are, but I thank you for bringing up this topic and giving us a nice simple mental picture that is sufficient to convey the effect and what photographers should do about it.
I join you in asking those who know a more correct physics explanation to explain it as simply as you did. Just saying "Airy disk" fails the Fenyman test in that it provides knowledge of words without providing actual knowledge of the subject. So we don't need 500 people commenting with that, we just need the one who can explain the thing to a 6 year old (or an undergraduate). Until then I'll use your mental picture.
Thanks for the video and please more such on technical subjects and practical photography techniques.
Also, thanks to you and Skillshare for the 2 months free.
It takes supermassive objects to bend light...
...or the aperture of a DSLR.
Tony knows! xDDD
Love your videos Mr.Northrup
Love the nerdy stuff. Thank you!
As far as I understand physics, diffraction is perfectly explained by quantum mechanics. It's exactly the same type of "self interference" as in double slit experiment, but instead of two slits you have a superposition of "infinite number of slits" with decreasing magnitudes the further you get from the central path of the photon.
Came to say this, but also that since light is a wave, it bends around corners (double slit experiment) and thus more bending when the hole is small
Came to say this, but also that since light is a wave, it bends around corners (double slit experiment) and thus more bending when the hole is small
Came to say this, but also that since light is a wave, it bends around corners (double slit experiment) and thus more bending when the hole is small
Came to say this, but also that since light is a wave, it bends around corners (double slit experiment) and thus more bending when the hole is small
Came to say this, but also that since light is a wave, it bends around corners (double slit experiment) and thus more bending when the hole is small
It depends on if they're Nikon or Canon electrons 🤹♂️🤣
The most creative comment hahaha!!!
😎👍😂
them canon electrons are no damn good i tell ya
Excellent video. Thank You.
I was wondering why the pictures were coming out blurry in the background- when I decreased the aperture to f/20-22... Lens: Canon 85mm f/1.8. Mystery solved.
Is the thickness of the apperture blade a factor with diffraction? Could thinner physical apperture blades reduce it?
My Nikon Z6 has a Diffraction Compensation (on/off) setting option. For both Photo and video modes. Any thoughts on this. Use or don't use? Wonder when it's ON if it's always doing something or just at higher aperture when the issues is most apparent as you mentioned in your video. Will test this out, but wondering what others think. Thanks!
I usually don't shoot wide open, and if I accept that it might not be the most perfect. Whatever is needed for the situation right? Nor do I shoot at F22, unless yes there is Macro.
Is there a loss of sharpness at F22? Yeah even with a nice big sensor and nice big "pixels", there are still limits to how things work.
But a slightly blurry but artistically good looking shot, or a super sharp shot but only a little slice? That is the question.
Thank you Tony! This is must-have knowledge. Something that has been plaguing me for many years. I am going to find the sweet spot for all my lenses.
Nicely done Tony..... makes a lot of sense, particularly how you described trying to find that that sweet spot. Liked it...
Just because you can't understand the physics doesn't mean that everyone doesn't understand the science. QED is one of the most detailed and well tested sciences that there is. Your attempt to use a photon as a particle, but photons have a duality, they are also waves. Diffraction is more of a wave effect. You have also not fully grasped the statistical nature of quantum effects. All together you just can't simplify these complex effects with your simplistic everyday experiences. If you want to understand what is going on do the math!
You should watch the video first
Skillshare online learning is great, but problem with online catalogues and memberships is that they can be taken away at any time. Let's say I comment on a video and YT doesnt like my comment. They have the ability to wipe my history out, blacklist me, ghost my account etc. Same with Amazon, FB and the lot. I like physical books because you don't have 'big brother' watching your every move.
Is there any formula to know what's the sweet spot of your lens? Does also cropped sensors change the formula?
Love the nerdy videos , many thanks.
Tony and Chelsea, I love your videos talking about various cameras. But, I feel that the best investment that a new photographer can make is purchasing your books. Beginners are not going to be happy with their photos until they learn the basics in my opinion. You two tell it like it is, and give invaluable advice. Thank you for all that you do!
Well done again, great learning video!
OK Tony, how do you focus stack when your lens suffers from focus breathing as most seem to do to at least some extent.
Crop.
Does the software adjust for focus breathing while focus stacking?
What happened to 'curved sensors' would that help. Also I think that the total image has to go through the aperture(f22) and that create a kind of bottleneck for the light and it scatters more?
Excellent, thanks Tony.
That's silly, Tony. A photon has no electrical charge. Electrons should have no "pull" on a photon. Just imagine if we could bend light with magnets or static electricity!
I think, lack a clear physical explanation, Tony was employing a metaphor. He's a knowledgeable guy and understands the basics of electrodynamics.
@@meta4101 You must be very intimate with Tony.
@@eurobum2012 Just an intelligent listener like your ...
@@meta4101 it's the basis of quantum mechanics that is needed here, which means dealing with wave equations when talking of photons and electrons. That's a whole lot different to electrodynamics, which is classical pre-quantisation model and really isn't appropriate at the quantum level.
For that, you need quantum electrodynamics (QED) and it's clear that Tony is not familiar with that. It's not surprising, as that requires degree-level physics.
@@TheEulerID In a sense, all physics is qm. That said, diffraction is more easily explained classically than the 2-slit (self) interference of qm and was well understood in the 19th century as a wave property of light well before qm was devised in the 1920s.
Very well made video. Thank you Tony. A humble request:
Please make a video on WHAT IS FOCUSING? What it means for an object to be in focus? How lenses change focus? And what exactly changes in lenses in order to change focus from one point to the other?
I have searched for this a lot but still don't have a good answer. I would be grateful if you could explain it.
PS Viewers also please reply if you have an explanation.
@@verybigheart What do you mean? I didn't get it.
Light oscillates in terms of the electric and magnetic field strengths, however, this is not actually a change in position transverse to the direction of the photons. Therefore, I doubt your explanation for the distribution of photons due to diffraction is correct. Note, I do agree the photons do spread out; I just don't agree with your reasoning for why.
More likely is that pixel photo-sites on modern sensors are so small that the diffraction (normally only noticed when the aperture is of a comparative order of magnitude to the wavelength of light) is still visible even with the difference in size of the wavelengths and the aperture of cameras.
Oscillations in the electric and magnetic fields could certainly cause differences in how the photons interact with nearby particles that they pass. The phase of the fields would determine how much and in what direction the photon will be deflected. Differences in wave phase between photons of the same frequency and on the same path would cause the smearing that Tony is talking about.
Thanks Tony , well appreciated!
Watch out ... all of the Sheldon Coopers are going to have a field day on this. ;)
When you use small aperture, some light may be reflected by the aperture blade itself and make some disturbance in the are between lens and aperture blade. Different color of light have diffrent wave length and they bended differently when passing throgh the glass.
While the main point of Tony's video is valid. "Smaller apertures do increase the effects of diffraction" The explanation is kinda off. I think a some of the confusion is caused by confusing the physical phenomenons diffraction and interference. Diffraction is the bending of light at the edges of an obstacle (the aperture blades in this case) Interference is when light (or any other wave) can cancels itself out because of being out of phase with another other light. When people think of an diffraction pattern its actually an interference pattern that is caused by the diffraction of light allowing the light to interfere with itself at the plane of the sensor. Diffraction is is very well understood in wave mechanics via Huygens principle. The quantum mechanical description produces the same results so its generally not required to get into the quantum mechanical description.
I think just the reflection from the edges. Also a small aperture amplifies the errors in the glass surface that would be otherwise canceled out with open aperture. Also there are lenses that have very minimal diffraction even on F22
First of all I'm no expert in this subject but here's what I understand of light and diffraction and why it does what it does.
Light is both a particle and a wave depending on what qualities are tested. Diffraction is a wave property and therefore light being bent has absolutely nothing to do with electrons in. This can be demonstrated with water also. You can think of parts of wave being the own beginning of a wave. This means that to keep a wave-front straight you have to have constant waves on either side to keep it moving in the desired direction.
You can demonstrate this by filling your sink with water and poking the surface with a finger. You'll see that a spherical wave forms from your finger's movement. Now if you stick your hand in with 4 fingers you'll see that you get two straight waves parallel to your fingers. This happens because the energy from the 4 fingers cancels out the wave's movement in all but the waves' mutually agreed direction. So the energy moving from your index finger to your middle finger cancels out by the energy from your middle finger moving towards your index finger. If you were to remove some of those balancing forces it would mean that the energy carrying on sideways in the wave becomes free to move sideways causing the wave to "bend" or "spread" in the direction it wishes. This is what happens near the aperture blade and causes the curvature in light making the images not as sharp. This phenomenon is magnified at smaller apertures due to the fact that a bigger portion of the light comes from near an edge where this "counter-balancing" is cut off by the blades and the wave is allowed to bend freely in the direction it wishes.
Folks below are waaaaay too caught up on the specifics required to understand this. I think your explanation for understanding the reasoning behind this is fine. An interesting way to prove the concept of waves bending would be to either watch a video on why the centre of a circular shadow is the brightest point (it's well trodden territory) or if you're at a large body of water just watch how waves interact with a solid pier or jetty.
Glad I watched this either way as I hadn't put much thought into why diffraction occurs, this has helped a lot with picturing it in my mind and cemented it.
Thanks Tony :)
Thanks, Tony. I was shooting a picture of a historical building in my town and I had this happen. The building had two towers like you would see in medieval times. I must have used F11 and the top of those towers curved inward. If that wasn't diffraction then it was caused for a different reason. I will try and go back to the site so I can shoot several at F8 and lower, then use F10 and up. Tried correcting this in my processing software to no avail. I will also try changing the focus point and see if it still happens.
Is is due to perspective as well as your lens distortion. Nothing to do with the aperture.
You can correct this in some advanced retouch software such as DxO.
Great article. What you are missing is the diffraction is affected differently by the medium that the particle is traveling through. The denser the medium, the more extreme the effect. Water is denser than glass, which is denser than air. So the light waves travel through the air and are slowed down by the glass, which causes bend. This bend is also affected by the tighter aperture, which applies more resistance or impendence to the light waves, which causes an acceleration effect due to the increases pressure.
If you've addressed this previously, I'd love a link, but this video made me think of something I deal with at times - chromatic aberrations. I shoot A LOT in the winter (in Alaska ~8 mos a year) and have found ways to deal with most issues. But I do like to use a prime lens at times (i.e. 35mm or 50mm @f1.8 or f1.4) when it is dark to get the most light possible when shooting things such as sled dog teams. Flash is prohibited in most of these situations. Anything you can contribute to getting the best images, of moving subjects, in near total darkness, without (or minimizing) chromatic aberrations, would be fantastic.
Always thought that diffraction, through any medium at any scale was a result of Huygen's principle. Looking forward to a follow up. Thanks for all the great videos!
Huygen’s principle is one of correct ways to look at it. Tony’s explanation in this video is extremely wrong.
Awesome! I work in fiber optics and love the relationship principles between my lens and fiber.
How does this affect leaf shutter cameras?
keep it coming tony
The interaction that causes Airy circles is a complex dance between the electromagnetic waves (photons) and the electric fields of the atoms and molecules that make up the material at the edges of the aperture. This interaction at the boundary is what leads to the diffraction of light. So you are correct in that regard.
Tony , whilst I agree that diffraction ruins sharpness, your explanation may be misleading. There is no value in considering how photons are 'attracted' to the edge of the aperture, since diffraction will occur regardless of the properties of the material used. You could electrically charge the aperture to no avail. Diffraction is absolutely only dependent on the geometry of the aperture and is the result of the destructive and constructive interference of the light wave as it passes and bends through the circular diffraction slit, which in this case is the aperture. Diffraction can only be understood by considering the wave model of light, not the photon model, and that wave-particle duality of light is the conundrum of quantum mechanics. The diffraction pattern of the circular aperture is called the Airy disk, mathematically proven way back in 1835. Consider a point source of light in the object plane that has a single wavelength, this presents a wavefront to the aperture that results in a corresponding point on the image plane, but that imaged point will exhibit an Airy disk, albeit very small but bigger than the original point light source. The width of this Airy disk X is a function of the wavelength L, aperture f-number N, where X = 1.22 LN.
NOW, consider that any scene we want to photograph is made up of essentially an array of point light sources, all of which produce an image at the image plane. We can consider these light sources as object pixels, so that each 'pixel' in the object plane corresponds to a pixel on the sensor image plane. The light's wavefront from each object pixel hits the whole aperture and results in an Airy disk at each sensor pixel in the image plane. At a certain aperture f-number and wavelength, the width X of the Airy disk can become bigger that the sensor pixel pitch, and hence you start getting smearing across pixels and a less sharp image. For example, at N=f/11 and L=420nm, X=4.6um, which can be bigger than the pixel pitch of many sensors. Given that the light from most scenes is a mix of colors and hence wavelengths, these wavelengths produce Airy disks of different diameters (similar to chromatic aberration), which produces additional smearing.
Conventional optical approaches cannot get around diffraction, but I live in hope that the nonlinear approaches used in high-precision photolithography, used in making integrated circuits, will be adaptable to camera optics to minimise diffraction effects.
Long time viewer, thanks so much for your videos! This video was interesting.
The focusing was a little distracting.
Another cool video, thanks!
Tony great video. When light from a subject arrives at the sensor after passing through a thin slit (the closed down aperture) it does so half a wavelength out of phase with the light leaving the middle of the slit. These two rays interfere destructively. In other words, the light from one half of the opening cancels out the light from the other half. The rays are half a wavelength out of phase because of the extra path length traveled by one ray... I think.
tony, your physical explanation is scientifically wrong.
Please read up on diffraction again. There is NO ATTRACTION of the photons by the electrons at the edge of the iris. I don't know where you get that from.
... my background: optic/laser physicist ;-)
the phenomenological explanation of it is alright though... for photographers. My recommendation is: Please don't seed fake news, it's better to simplily instead of coming up with non-truths :)
randfee how about explaining than why and how it’s happening? Or post us, please, a link at least where to find scientific explanation
I agree, perhaps Tony should replace "attraction" by interaction. I wonder if the wave lights closer to the diaphragm experience a combination of mirage plus pinhole effects. I mean, some partial refraction trough the surface of the diaphragm with a low-pass filter.
are you telling me the Coanda effect doesn't apply to photons!?
Daniel Clarke: I think the Coanda effect is more like water tension, like the way water rolls around a soft radial edge ( like a rain gutter hood) until it’s mass causes it to fall. But then what do I know.🥴
Douglas Michel yeah, that’s the joke
I still get the impression my canon 17-40 mm f4 is at sharp at f14, or am I wrong ? The DOF is quite high that's why I use it. I don't want to combine 5 pics with different focus
Yes! More and more technical videos like this are welcome.
This is cool! Thanks!
this should be easy to verify: get a monochromatic and coherent light source and check the diffraction
Thanks so much, Tony. Love your work. Question: So defraction at f11 on micro four thirds is the same as f22 on full frame?
The size of the diffraction spot on a focal plan dépend more or less of the f-number, the larger the f-number the larger is the diffraction pattern (google airy disk). A f/22 lens will have a diffraction spot twice larger than a f/11. However it’s proportion to the picture (the sensor) will be the same when comparing a f/22 on a ff and a f/11 on a M43.
Sylvain Guieu Thanks so much Sylvain. Got it
How does this affect a crop sensor vs full frame?
hahaha funny as someone slipping in a pile of shit
fullframe would make it worse as it captures more light oh er photons...
The light doesn't cross at the middle of the blades but at the middle of the lens. Some photons actually hit the side of the blades at angles and bounce of it. At the size of a photon the slice (side) of a blade is not thin at all. When the blades are wide open (f1.4) the angles of bouncing photons are more different and doesn't really affect the image at a fast shutter speed. they are more distributed on the sensor. But at f22 the angle of bounce is thinner and at a lowest speed it affect the image.
Great video!
You should also mention diffraction limit vs pixel size.
Thank you for a good explanation. I remember with our small-sensor video cameras around 2010, using the built-in ND filters are so necessary. Without it, not just soft - so soft the footage was useless.
Quantum physics has a principle called the Heisenberg uncertainty principle, which states that for any subatomic particle the more precisely you measure the change in position (I.E. the path a photon takes from the subject to the sensor) the less precise you can determine it’s momentum (I.e. to what degree the aperture blade will deflect the photon from a direct path straight to the sensor)
Well, it might mean that sensor size is not that important for landscape, since we would be limiting the amount of light on bigger sensors to a greater degree anyway, applying crop factor to aperture.
Would there be less divergence on a mirrorless camera due to the reduced focal plane distance? Resulting in less noticeable diffraction? That is, assuming a native lens is used.
No. the focal plane distance is decided by the focal length, not the flange distance. A 50mm lens still has to be 50mm away from the film to bring an object at infinity in focus, regardless of the actual flange distance.
It's over my head, Tony , but every lens I have owned even lens of the same make a model has its own "sweet spot" for sharpness. If it's not the iris then what is it? All my lens have an increase is diffraction at higher f/stops.
knew this quite early in my hobby, made the tests myself for my lenses, to know what i am relying on... i just hate i've forgot how i got to find this out.
Most people believe that their lenses are sharpest around f8-f11 but most lenses are sharpest around f4-5.6. I think people believe this because shots taken at f8 are more likely to be relatively in-focus edge-to-edge and it's easy to confuse something that is slightly out of focus with something that isn't sharp.
There are two theories of light and both are correct even though one does not explain the other. The quantum theory and the classical wave theory. The quantum theory applies to light when it strikes an object, e.g. when light strikes the sensor. Quantum theory of light does not explain diffraction and diffraction does not explain quantum theory. The wave theory explains diffraction and is used where light travels from its source to another object as a wave, these waves explain diffraction.
Diffraction is where the light waves of similar wavelength when passing through a small aperture will interact with each other, some waves will add to other waves and some waves will subtract at the aperture, what leaves the aperture will be something different. If you throw two or more stones into a pool at sufficient distance apart you will see the concentric circles interact with each other causing diffraction.
Photons are electrically neutral so how would a electron attract a photon?
Thank you for a great video
A proposed subject for a tech-oriented explanatory video: Consider the actual measurement of the diameter of the aperture of fixed aperture zoom lenses in contrast to the actual diameters of the apertures of variable aperture zoom lenses.
The definition of f-stop as f (which equals focal length of the lens) divided by the diameter of its opening (aperture) suggests that when a fixed aperture zoom lens is zoomed the actual measured diameter of the lens' aperture must be increased if the fixed value f-stop is to be maintained. Some definitions include mention of optics inside the lens modifying the effect of the actual measurable physical diameter of the aperture to produce a nominal equivalent diameter .
It would be interesting to see just how much the measured diameter of the aperture of something like a 70-200mm f/2.8 lens changes when the focal length changes throughout its range, and to contrast that range of actual diameters with the changes in aperture diameter of a variable aperture zoom lens such as a 55-250mm f/4-5.6. If I understand correctly, the measurement of effective aperture changes of the fixed aperture zoom lens might actually be greater than those of the variable aperture zoom lens.