Understanding Lens Diffraction

Creating Iconic Locations by Dimitri Vasileiou
Understanding Lens Diffraction
Lens diffraction; what is it? What do we do about it? Which is the best way to avoid it? Dimitri Vasileiou explains a few things that, hopefully, will help you understand about diffraction

As landscape photographers, we are always desperate to achieve the greatest sharpness and depth of field; in fact, the best of both worlds. I am afraid though, that sometimes we have to compromise. Stopping down the lens to a small aperture (i.e. large f/stop number), in order to achieve as much depth of field as possible, does not always produce the best results.

All but the very best and expensive lenses will produce maximum quality images when stopped down by 1 or 2 stops from the widest aperture provided by each individual lens. So, a lens with a maximum wide aperture of f/5.6 will probably give best results between f/8 - f/11. From f/11 on, if the aperture size continues to be reduced to f/16 - f/22, diffraction will start and image quality will be reduced. Often I notice people with DSLR cameras taking landscape pictures using an aperture of f/22, and this concerns me.

When light passes through a lens, the diaphragm blades disperse and diffract the light. Even at large apertures the light passing through is diffracted, but only at a very small percentage which really does not affect image quality. As the aperture blades are closed down, that percentage is increased and eventually, at smaller apertures, the diffraction is at such a high percentage that light struggles to reach the sensor and image quality suffers.

The smaller the sensor, the poorer the results when it comes to image quality and that is why compact digital cameras usually come with an aperture no smaller than f/8; diffraction is one of the reasons, the other, depth of field: the smaller the sensor, the greater the depth of field; hence less need for a very small aperture. Here is a rough example. On a compact camera, in depth of field terms, f/8 aperture is the equivalent of f/22 on a full frame sensor. So, on a camera with an APS-C sensor, an f/11 aperture is the equivalent of f/16 on a full frame. These are rough estimates and used here as examples to make things simple to understand.

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What is the best solution? We need small apertures to cover depth of field adequately in landscape work, so here are some suggestions.

  • Check all your lenses indoors in a controlled environment to find out at what aperture diffraction is severe. Take the same picture at f/8, f/11, f/16 and f/22 and compare them at a view of 100%.
  • Make sure your focusing technique is correct for best depth of field results.
  • You can buy a tilt/shift lens (more on this in a future article).
  • Learn how to use your live view properly, if your camera offers it; this will improve your focusing technique.
  • Start with a larger aperture, say f/8, while focusing and keep checking the results; then stop down if you need to. If you are not sure, focus at different parts of the scene and experiment.

In a nutshell, I recommend using an aperture no smaller than f/13 when using a DSLR with a small (APS-C) sensor or f/16 when using a full frame sensor.

Read this article, and many more, in High Definition, inside Issue 18 of Landscape Photography Magazine.

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  1. While this article is nice, it is only the tip of the iceeberg and what is really important, and it concrns me just like it concers the author when people does landscape shots at f/22 (which is not a problem when your min f is 45 btw) is that the article is lacking of any example image of the effects of diffraction.

    How can someone who didn’t knew anything about diffractionn properly identify it’s issues without any kind of visual reference?

    I remember when i was studying optics and how difficult was to transport the abstract theory to the practical application, same goes here, those who first meet diffraction through this article have no visual references to identify it, not even an accurate description of what to look for.

    1. Here is a quick answer. If your lens allows you to use aperture f/45 (I assume you are talking about full frame and not large format), then take a picture at f/5.6 and the same picture at f/45. Now zoom in 100% at the point where you have focused and you will see exactly what diffraction does, the f/45 picture will be a lot less sharp than the f/5.6 one.

  2. Thanks for bringing me another "Aha" moment as I piece together my understanding of photography. My main interest is underwater photography, but your explanation of diffraction touches all specialties. Thank you again.

  3. I have only just learnt about diffraction today, and I'm glad I have. There are a lot of things to take in when starting out with photography.

    I have a burning desire to take landscape photos, and at the moment my equipment consists of a Canon 550D and a Canon NiftyFifty. I'm planning to buy a Tokina 11-16 UWA so I can get more serious about landscape photography.

    I'm one of those people who just naturally assumed f/22 would be the best – irrespective. I took some photos of a sunset last night and was quite disappointed with the results, and I think if I had taken this into account then the results may have vastly improved.

    I know what I'm going to be doing tonight haha.

    Thanks for this valuable information!

    Chris.

  4. Invaluable advice Dimitri, and not something that beginners are often aware of. I have some otherwise great shots of a cliffscape that I drove for hours to reach, at low tide, in the perfect evening glow, beautifully composed and exposed, but unfortunately they are soft as an overipe banana because in my ignorance I set my Nikon D200 to f22! ..I've never made the same mistake again.

    Whilst the plethora of slideshows and video clips LPM chuck at us are OK I suppose, it's simple, pratical tips and techniques like this, that are what a lot of us most appreciate I think.

    Keep up the good work!

    Andy

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