Tutorial – Diffraction, that little known factor in image sharpness
by Jonathan Bourla (www.jonathanbourla.com)
When I was a teenager, shooting 35mm colour slides, I didn’t take diffraction into consideration. In my later school years I had heard of diffraction in my Physics class, but for some reason didn’t think how this applied to my photography. It was only in my early thirties, using a large format camera, that I learned again of diffraction, and that I should certainly take it into account when seeking to achieve as sharp photographs as possible. This was at a workshop I attended run by the brilliant American photographer Howard Bond.
Lenses don’t perform equally well at all apertures. At large apertures, lenses are typically affected by aberrations, which degrade the optical performance. There are different types of aberrations, but in general their effect is reduced as the lens’ aperture is stopped down. At the other end of the aperture scale, at small apertures, the lens’ performance is adversely affected by diffraction.
So, the consequence of this is that somewhere in the middle of the aperture range the lens will perform at its best. Typically this is at f8 or f11, and this explains why the lens manufacturers quote their lenses’ performance at these apertures. Photography is a balancing act, with aperture, shutter speed, depth of field, etc all needing to be assessed, and coming to what the photographer considers the best compromise. Now I’m saying to be mindful of the optimum aperture as well.
For large format cameras, like the one I use, the lenses typically have maximum apertures of f5.6 or f8. So the optimum aperture is close to the maximum. However, the demands of achieving focus throughout the image, which tends to be a convention amongst large format photographers, often results in using smaller apertures than the ideal. This is sometimes called being diffraction limited.
Now for a small format camera lens, such as 35mm (film or digital), this optimum is some way from the lens’ maximum aperture, and along with achieving the necessary depth of field should be considered when selecting the aperture.