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.
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.