Diffraction and sharpness 00

Diffraction and its Impact on Sharpness – By Tony Warren

I recently had some disappointing results while trying out a colour negative film in a Retina IIc. I was shooting the urban art (and some graffiti) that has been created here in Dunedin, and some shots of the rhododendrons in the Botanic Gardens. Many of the frames are distinctly soft in places whilst others are bitingly sharp, despite careful focussing with a well adjusted, rangefinder camera.

Tests of lenses often warn about using smaller apertures because of the effects of diffraction, especially beyond f11 which seems to be the smallest recommended opening, with 35mm film and equivalent formats, at least. There is a trend with many modern camera lenses to design them to have ever larger maximum apertures, so much emphasis being placed on bokeh these days. Resolution will inevitably suffer at smaller apertures, both from diffraction but also to a degree because the optical design is optimised for the larger apertures.

So what causes diffraction?

Diffraction and sharpness 01
How diffraction affects fine detail rendition. Little effect at a large aperture.

When light passes through the iris opening in a lens, the rays close to the edge of the blades are ‘bent’ slightly, causing blurring. With a larger aperture this has little or no effect, the blurred area being only a small fraction of the overall image. With a small aperture, f/16 and smaller, the blurring affects a much larger proportion of the opening on most smaller format lenses.

Diffraction and sharpness 02
Bold images not containing fine detail appear sharp at small apertures as here, f16.

If there is no important fine detail needing to be recorded, architectural subjects for example, small apertures can enhance the apparent sharpness overall. But where fine detail is important, this must be taken into account.

Diffraction and sharpness 03
Even with a smaller image, the less fine elements surrounding the wall art remain visibly sharper despite small f16 aperture.
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At f8 the fine detail remains as sharp as its surroundings.
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A typical shot relying on fine detail rendering – f5.6 unsharpened.

Using a Nikon F801 SLR I usually work in aperture priority auto, the camera’s shutter speed range (up to 1/8000) accommodating the brightest light at quite large apertures. As a result, I am able to keep my aperture settings to the optimum. My Retina in comparison only goes to 1/500, four stops slower. This top speed is probably a little slow with a camera of this age too, even though CLA’d, and so to be as close to the marked speed as possible I tend to use 1/250. The largest aperture I could use in similar conditions would be f/11 at 1/500, so at 1/250 I have to use f/16, exactly where diffraction can become a problem.

Diffraction and sharpness 06
Compilation of screen shots of unadjusted raw files in Affinity Photo. Fine targets suffer more than the bolder parts.
Diffraction and sharpness 07
Target used for compilation – central card used in each case.

I tested this by shooting the set up shown at all the apertures on my 55mm Micro Nikkor. The fine spacings start to soften at f16 and are really soft by f32. The larger patterns aren’t too badly affected and could be sharpened in post whereas the finer ones couldn’t be brought back much at all. The composite is from screen shots of raw files in Affinity from the central target without any processing.

Whilst aperture has most effect, lens design may also have a part to play, especially in my case. The Xenon lens on the IIc is the same 6-element design as the f2 of the Retina IIIc but with a reduced maximum aperture of f2.8. Having been designed as an f2 lens, it may be computed to favour better resolution at the larger apertures, a feature of such lenses. After all, there is no point in paying a higher price for a fast lens if it doesn’t perform well at the largest apertures.

And finally, the film grain will impose a limit on detail resolution, in turn affected by contrast in the image.

All things to bear in mind when approaching your subject.

This is particularly applicable to smaller formats where shorter focal lengths are used and higher magnification of the image. Diffraction, and grain too, is less of a problem as the camera format size increases. Medium and large formats suffer less from these effects, their lenses and aperture openings being physically larger than 35mm or smaller digital formats and results needing less enlargement. So if you are working in fields requiring the finest detail, size really does matter. For most of us, though, simply matching aperture choice to subject will suffice. And, of course, with digital, focus stacking is always an option for static subjects such as record or landscape when any aperture can be used but maximum sharpness and depth of field retained.

Diffraction and sharpness 08
Single exposure at f22 on left, 5 exposures focus stacked in Affinity Photo on right.

With a camera like the Retina or any camera/lens with small iris openings, using slower emulsions and possibly neutral density filters are essential for some subjects to avoid stopping down too far. Extended depth of field looks to be ruled out in some cases too. All part of the challenge I guess.

(Digitised copies of negatives are produced with a Sony A3000 with an AI’d 55mm Micro Nikkor and adapters processed in Affinity Photo.)

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13 thoughts on “Diffraction and its Impact on Sharpness – By Tony Warren”

  1. I’d love to see the effect on film, which has a very different rendition of fine details than digital.
    Note that in digital, some sharpening is often enough to mitigate most of the effects of diffraction.

    1. Yes, digital has made some things easier but film gives an image something extra. The effect of diffraction for me is quite frustrating, just not being able to pick out the finest details and is the same on film as well.

  2. Very important posting. Thank you for your hard work on this. But you showed that the old dictum of “nothing below f/11” is incorrect and should be “nothing below f/16”.
    Thanks again.

  3. Thank you for this interesting article. You raise a question in my mind, which I have asked others several times before without getting a definitive answer. Refraction of light, as you say, takes place when light passes through a small hole. With a 50 mm lens, f16 gives an aperture of 3.1 mm diameter, and f16 is often regarded as the minimum aperture one should use with this lens. However f16 with a 35 mm lens gives a diameter of 2.2 mm, 1.8 mm on a 28 mm lens, 1.5 mm on a 24 mm lens and a staggering 0.9 mm on a 15 mm lens. If we assume that 3 mm is the limiting aperture diameter for minimising diffraction then we should not use anything smaller than f8 on a 24 mm lens and f5 on a 15 mm lens – and my 15 mm lens has a maximum aperture of f4.5! Your comments would be appreciated.

    1. That’s really interesting. I hadn’t looked at it in that way. Some recent interiors, shot with my 17mm were a little soft overall where I had gone down to f11 so I should really limit it to f5.6 or maybe f8 for that kind of work.

  4. Thank you for the discussion on this. I had not thought about the actual mechanics of diffraction, so this was a nice explanation. I tend to view film not as a way to capture detail but as a way to explore form and light. I just spent a week in Laguna Beach, Calif., and shot only film on Minolta cameras, and I had to keep reminding myself about this, because the weather was generally sunny, meaning apertures were up around f11, f16!

    1. An enviable location Gil. I agree that the final image is the important thing and your comments bring to mind the colour image of a bull fighter at one extreme and a monochrome portrait of a woman behind a screen door at the other. The former a long exposure with nothing sharp and the latter depending entirely on absolute sharpness across the frame. Knowing what effect your settings will have on the desired result is what really matters.

      1. As a postcsript Gil, I had to look up the photographer of the bullfighter, who was the great Ernst Haas. The Screen door portrait was by Ansel Adams, Portfolio V, plate 8. Senior’s moment.

  5. Ibraar Hussain

    Thanks for the detailed explanation!
    And the chart comparing the levels of diffraction for given F stops is excellent!

    I once read an article by ken Rockwell about this and since then I haven’t shot anything smaller than f11 that’s when I want max dof but often enough f8 is sharper and does the job.

  6. On film, the negative effect of diffraction is a lot less apparent compared to high resolution digital cameras in my experience. Probably the level of pixel-peeping is due to that. 100% magnified scan of a negative will show grain first and formost. The digital grid of pixels will show the diffraction in its full glory.
    On my DSLR, I avoid going beyond f/8 and try to keep f/5.6 as the go-to aperture; on my film cameras (including a well working rangefinder with bitingly sharp Voigtlander Ultron 35mm) I do frequently use f/11.
    Since I never really felt that “having everything sharp” helped images look any nicer, f/16 was never the place to be for me. But for macro work, diffraction can really get in the way.
    Ultimately, as nice as it is to get everything technically as good as possible, the first thing is always getting the image right – the composition and desired look of the result comes first. Even if that means using a less-desirable aperture.

    1. Couldn’t agree more, Wouter. Knowing what is needed for a given result is the lesson all this taught me.

  7. Thanks Ibraar – most of film photography is compromise. Digital has freed us in many ways but physics is physics I guess.

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