Puzzled about Diffraction

Discussion in 'Exposure Discussion' started by T42, Mar 18, 2011.

  1. T42

    T42 Member

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    Hello Forum.

    Is diffraction categorically a problem at f/22, no matter what the actual, physical diameter of the open aperture is? If I am using a 200mm telephoto at f/22, is diffraction any more or less serious an issue than if I were using a 50mm lens, or a 24mm lens at f/22?

    Until now, I have been of the opinion that diffraction becomes an issue when the physical diameter of the aperture opening becomes small enough to be within the range of a few wavelengths of the light passing through it. One source I remember from the past said that somewhere around 1mm aperture and smaller, diffraction is going to be a problem.

    Lately I have read that one should not shoot at f/16 if he does not want diffraction as an issue. And I have also seen some things that suggest that diffraction has nothing to do with physical aperture size, but that it does have to do with f number and wavelength.

    Now, I would have guessed that f/16 would be big trouble with those new micro-format digi-cameras which have a 12mm normal lens or some such. That would be an aperture on the order of 0.75mm. In fact, I'm thinking that such cameras do not even have f/16 because of that. On the other hand, a 200mm lens at f16 would have an approximate/effective aperture diameter of 12.5mm, well over that 1mm threshold that I remember from the past.

    That's confusing to me how that f/16 or f/22 is categorically asking for diffraction issues, regardless of focal length. Can anyone here help me sort out this misunderstanding I seem to have?

    Thanks a million for reading this.

    :smile:
     
  2. jp498

    jp498 Member

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    My understanding is that diffraction is always present when an aperture is used, but as you stop down, the ratio of beamed light versus diffracted light decreases, so it become more of an issue. not an expert.
     
  3. Kevin Kehler

    Kevin Kehler Member

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    I am sure he might reply but in Way Beyond Monochrome, Ralph Lambrecht discusses what the reasonable diffraction limit is per format; if I recall correctly, for 35mm film it is between f/8 and f/11; for 6x6 it is f/16. Since diffraction is only truly visible in enlargements, larger formats suffer from the same diffraction effect but it is less visible since one does not normally enlarge a negative to the same extent which is what allows large format photographers to use such extreme f-stops (f/128 anyone?). If you enlarged a 35mm to 4x6 and a 4x5 to 20x25, they would exhibit the same diffraction effects. So having a different focal length of lens will not make a difference but since wider lens' tend to give "wider" portraits which objects are not close enough to see the diffraction, they would appear to not suffer as much.

    Very generally speaking, a picture with a 200mm lens @ f/22 will show little subject appear matter but make the subject larger thus showing any softness due to diffraction; a 24mm lens @ f/22 will show much subject matter (assuming the same focal length) but smaller and thus hiding softness due to diffraction. If you enlarged the 24mm shot, you would see the same effects of diffraction. It's a little difficult to explain - last postcard round I shot some leaves/flowers at f/32 on a 6x6 format, trying to get sufficient depth of field which printed too soft for my taste. I reshot it at f/16 with camera farther back (to get more depth of field) and enlarged to a greater extent in the darkroom to keep the same print size - much more sharp to my eyes.
     
  4. ic-racer

    ic-racer Member

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    Without getting into the technical details, just remember that, in general (with some caveats) your diffraction will be the same at the same linear aperture for each format. That is an absolute aperture size in millimeters, not F-stop numbers. The smallest I like to go is around 6mm.
     
  5. RalphLambrecht

    RalphLambrecht Subscriber

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    As Kevin said: May this graphic will help to illustrate the influence of diffraction:
     

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  6. polyglot

    polyglot Member

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    Ralph's chart is good because it shows you the effects of diffraction (coloured lines), also combined with (I assume) typical lens limitations for each format, though I don't know what lenses he tested with.

    Anyway, the point is that the size of your diffraction circles of confusion on the film is constant with respect to f-stop, independent[1] of the lens' focal length. The reason is that the ANGLE the light diffracts at is a function of the aperture's absolute size but with longer lenses, the light has further to travel between aperture and film. Say you use f/8 at 50mm for a 6.25mm aperture, the light diverges at some angle theta for 50mm, resulting in a circle of confusion diameter of x50=theta*50. Say you shoot the next frame at 200mm f/8; the aperture is now 25mm. The light diverges at an angle of 0.25*theta (because the aperture is 4x larger) but travels 200mm (4x as far), so you get x200 = (0.25 * theta) * 200 = x50.

    jp498: no, that is not correct. Yes there is always diffraction, but it actually spreads out as you squeeze the aperture - think of a spray nozzle on a hose as you squeeze the handle. It's not like there's a core of undiffracted light plus the diffracted light, one of which is changing in quantity. There is a just a single blur of light (gaussian distribution) and it changes size with aperture.

    [1] to within the small-angle approximation of sinx = x and making both thin-lens and unity-pupil-magnification assumptions.
     
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  7. JBrunner

    JBrunner Moderator Staff Member Moderator

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    Far more of my negs are soft because I suck than because of diffraction. YMMV.
     
  8. brucemuir

    brucemuir Member

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    Exactly, Mr Brunner.
    I was taught in school that it did exist but it wasn't a huge deal and my experiences bear me out on 6x6
    When I was learning my hasselblad I routinely would stop way down to f/32 when trying to shoot architecture with the 150 Sonnar.


    I noticed a 50mm Nikkor I had only went to f/16 so I figured if it was available it couldn't be too awfully detrimental.

    I did know that lenses had a sweet spot but it never stopped me from using smaller apertures.

    Now I shoot people mostly so I'm usually at almost wide open most of the time.
     
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  9. RalphLambrecht

    RalphLambrecht Subscriber

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    Lens manufacturers design aperture limits while taking diffraction into account. That's why a 35mm lens is limited to larger aperture settings than a large-format lens. The influence of diffraction also depends on print enlargement. If one routinely makes 8x10 prints of medium-format negatives, diffraction will never be an issue. But, a medium-format lens, such as the 150mm Sonnar, has twice the resolution at f/8, compared to f/32. Given a substantial enlargement, that will be clearly visible.

    Having said all of that, one should not hesitate to use all available aperture settings unless critical resolution cannot be sacrificed. Aperture settings are a photographic tool to optimize image composition, and should be used as such. Diffraction concerns are secondary. As you explained, apertures are chosen depending on subject matter not to minimize diffraction.

    There is nothing worse than a sharp picture of a fuzzy concept!
    Ansel Adams
     
  10. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    I have to agree with Ralph that people tend to worry much more about diffraction than they need to. Getting the right exposure and the appropriate amount of depth of field for the image you want to make are much more important than avoiding diffraction in most cases.

    One situation where diffraction can become a serious issue is macro photography at high magnification, when the effective f:stop is considerably smaller than the setting marked on the lens, and the gain in depth of field from stopping down the aperture is likely to be negated by the increase in diffraction and potentially other factors like camera/subject vibration with long or multiple-strobe exposures.
     
  11. Steve Smith

    Steve Smith Subscriber

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    I think that sums up my negatives too!


    Steve.
     
  12. polyglot

    polyglot Member

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    Ralph: do you not mean 4x as much (linear) resolution at f/8 than f/32? I understood that at the diffraction limit, the diameter of the Airy disc to the first null is d=2.44*wavelength*Fnumber. Therefore at f/32, the airy disc is 4x the diameter than at f/8.
     
  13. RalphLambrecht

    RalphLambrecht Subscriber

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    No, I said and meant 2x. I wasn't comparing theoretical diffraction limits (in which case you would be correct). I was comparing actual lens performance between these apertures as shown in my graph in post #5.
     
  14. polyglot

    polyglot Member

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  15. RMD

    RMD Member

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