Canon® EOS Digital Photography Photo Workshop

5.6. DIFFRACTION

When light travels through lens elements and through your aperture opening, some small amounts of it are diffracted, meaning dispersed and essentially no longer a part of your image. Usually the amount of light diffracted is negligible under normal conditions; however, it can occur when you are shooting with an exceptionally small aperture setting, such as f/32, which can result in a slightly softer or blurry image. If you're trying to achieve a very deep depth-of-field image and you take a photo with a very small aperture setting, it is possible, especially with some lesser-quality lenses, to experience this effect that is known as diffraction-limited optics. Basically, when you have a lot of light passing through a very small space, the rays can interfere with one another if the lens elements are not carefully engineered optically to optimize light's pathway to the image sensor.

You may not want to assume that your lens is experiencing diffraction-limited issues, however, unless you've eliminated all extraneous variables such as ensuring you're using a good-quality tripod, mirror-lockup (to avoid camera shake), and a very good-quality lens. Many photographers avoid shooting at the extreme, high limits of their apertures to avoid an potential softness—with any lens, even if it's a very high-quality model.