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Thread: Please explain diffraction in lenses

  1. #1

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    Please explain diffraction in lenses

    I've been testing some of my large format lenses in the past few days. I've noticed that none of them are as sharp (in the center) at f32 as they are at f16.
    -This is across many different focal lengths and many different designs.
    i know that this is caused by diffraction at small apertures.

    But really, ive never had a good explanation of why this occurs, and why it only appears at small apertures.
    Could someone answer this? THX

  2. #2

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    Re: Please explain diffraction in lenses

    It's basic physics. If by "good" you mean "simple," then you probably never had a good explanation because the why isn't simple.

    Start at that link then at the bottom of that page are links to the math and deeper discussion about why the why is the why
    https://luminous-landscape.com/under...s-diffraction/

    The fast and furious explanation is that the edge is always messing with the light and it becomes proportionately more of the image as the central area inset from the edge becomes smaller in proportion to how much edge there is. i.e. less clean central light versus more messy edge light the smaller your aperture.

  3. #3

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    Re: Please explain diffraction in lenses

    Take a look at the following. Diffraction is a fundamental property of waves and their interaction with physical objects. It's a function of wavelength as well which is why systems using blue lasers, X-rays etc have higher resolution than (for example) sound waves or water waves. There's a reason they call it "Blu-Ray" and why it gets higher data density. Unfortunately even Sony technologists agree that digital optical data recording capacities crap out well before magnetic recording technologies.

    If you put a blue filter over your lens you might see less of an effect. Unfortunately, shorter wavelengths also exhibit more scattering in air so you'd also see more "noise" unless you photographed in a vacuum - or in drier air. Which is why telescopes are usually on top of mountains. Anyhow, start with the following and have fun.

    http://en.wikipedia.org/wiki/Diffraction

    http://www.cambridgeincolour.com/tut...hotography.htm

  4. #4

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    Re: Please explain diffraction in lenses

    The analogy I like is of a wave hitting a dock or breakwater sideways. As it goes past the dock, some of the wave is deflected into the area on the opposite side that should be in the "shadow" of the dock. This deflected wave isn't simple like the original wave, it's more of a mess of disturbed water.

    The dock is the diaphragm. If the lake (opening) is large, what is deflected is relatively small, by proportion to the rest of the wave in the lake and on the backside of the dock the wave is mainly intact. When the lake is very small and the dock relatively long, the deflected area, the disturbed water (or light) that isn't properly part of the original wave, represents a larger proportion of the water that's beyond the dock than the original wave, and the original wave (light that forms the the image) is less clearly defined, obscured by the large proportion of disturbed wave.

    An illustration: http://fphoto.photoshelter.com/image/I0000irq3tbIzF20

  5. #5
    fishbulb's Avatar
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    Re: Please explain diffraction in lenses

    That one would be my recommendation for a simple, easy to understand explanation. But yeah, complex topic.
    -Adam

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    Re: Please explain diffraction in lenses

    By the way, diffraction is not always a bad thing. A pattern of concentric rings (precisely calculated, of course!) will act as a lens because the rings are spaced so as to "interfere" at a known distance thus focusing an image. Check out "Zone plates". I have a pinhole set where the maker also provides a set of zone plates with different "focal" lengths. The zone plates are effectively at a much wider aperture than the pinhole.

    http://en.wikipedia.org/wiki/Zone_plate

  7. #7

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    Re: Please explain diffraction in lenses

    Thank you for the links. However they seem to explain the effects of diffraction rather than the cause.

    In the cambrige example, light is shown going straight through a large hole, and then diffracting through a small hole.
    It doesn't explain why the 3 light rays somehow 'know' they went through a small hole instead of a large one.

    Do they interact with the hole in any way?

    if they are interacting with each other, why dont they interact with each other when there are more of them going through a larger hole?

    Are they diffracted by the air, or the glass, or??

    What if I had a large aperture made of mostly small holes.. would their be alot of diffraction or not?

    What if there were only 3 rays of light, traveling closely together, close enough that they could pass through a f64 aperture, but in actuality they passed through a f1.4 aperture. Would they still diffract?

  8. #8

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    Re: Please explain diffraction in lenses

    "The fast and furious explanation is that the edge is always messing with the light and it becomes proportionately more of the image as the central area inset from the edge becomes smaller in proportion to how much edge there is. i.e. less clean central light versus more messy edge light the smaller your aperture. "

    Patrick - if diffraction is caused by irregularities in the edge of the aperture, would a thinner, better light absorbing, more regular aperture cause less diffraction?

  9. #9

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    Re: Please explain diffraction in lenses

    What causes diffraction is the fundamental nature of waves and the way waves interact with objects. Basically that's how waves (all waves) act when they encounter objects in their "path". You're asking a very good question - the problem is that answering the question of why waves act that way gets into the realm of very advanced PhD level physics. It's sort of like gravity. We all know how gravity acts and use the engineering level understanding to do things like put satellites in orbit, It took Einstein to figure out WHY gravity works the way it does.

    I'm afraid that in the realm of photography we have to be satisfied with the engineering level understanding of diffraction.

    And by the way - are you really sure that the loss of sharpness you're in the center isn't mostly caused by focus shift rather than diffraction?

    Re your specific questions

    Do they interact with the hole in any way?

    Yes - they interact with the EDGE of the hole.

    if they are interacting with each other, why dont they interact with each other when there are more of them going through a larger hole?

    Don't confuse waves with the nice straight lines we draw when tracing "rays"

    Are they diffracted by the air, or the glass, or??

    Diffraction is caused primarily by holes/things with edges - basically the aperture. What you get from air and glass is more appropriately called refraction. The sky is blue because of a phenomenon called Rayleigh Scattering so dust and moisture in the air causes some problems but I think we can ignore that for purposes of the present discussion.

    What if I had a large aperture made of mostly small holes.. would their be alot of diffraction or not?

    Yes - diffraction would be much more significant - depending of course on the diameter of the individual holes. If the holes were very very small you'd probably see something akin to the rainbow effect of the track spacing on a CD - you'd have a super diffraction grating.

    What if there were only 3 rays of light, traveling closely together, close enough that they could pass through a f64 aperture, but in actuality they passed through a f1.4 aperture. Would they still diffract?

    1) As above, don't confuse "rays" with "waves".

    2) There is diffraction everywhere a wave encounters an edge. As was pointed out by mdarnton, the amount of edge compared to the area of the hole increases as diameter is reduced. If r= the radius of the hole, the relationship is given by (2*pi*r)/(pi*(r*r)) = (2*r)/(r*r) = 2/r. As r gets bigger, edge effects are less important, as r gets smaller, edge effects dominate.

    By the way, thanks for asking these questions. I was a Chemistry & Physics major but it was so long ago that I've forgotten everything I once thought I knew.
    Last edited by Jim Andrada; 21-Apr-2015 at 03:12.

  10. #10

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    Re: Please explain diffraction in lenses

    One way to understand the mechanism or process is as follows. Consider a wavefront of light. The waves are in some sense balanced - one part of the wave is supported by the waves around it - that is why the light or wavefront is cohesive. When part of the wavefront is cut off by an aperture, the part that is on the periphery of the section that passed through the aperture is now imbalanced since the part supporting it on one side got stopped/eliminated by the aperture/obstruction. This imbalance results is a spread in the wave function. So there is really no mechanical way (thinner aperture etc.) to eliminate this effect. Does that help?

    Cheers, DJ

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