Diffraction And ƒ-Numbers
Lens aberrations are generally strongest at wide apertures and are reduced as you stop the lens down. Stopping the lens down also increases depth of field. So you’d think that stopping way down for every shot would be a great idea. It isn’t, for at least a couple of reasons. First, sometimes you want to limit depth of field to isolate a subject from the background. Second, as you stop the lens down, diffraction reduces image quality.
Diffraction becomes a significant issue when the lens is stopped down to small apertures. We suggest you do your best to keep to ƒ/11 unless it’s absolutely necessary to go smaller.
Diffraction is the spreading of light rays as they pass through an opening. The smaller the opening, the more they spread, reducing sharpness. So, as you stop down, depth of field increases, but overall sharpness decreases. Each lens (and with a zoom, each focal length) has a sweet spot, where the balance between wide-aperture aberrations and small-aperture diffraction is optimal, and best image quality will occur at that aperture. Which aperture is it? You should test your gear to find out, but generally 1.5 to 2 stops down from wide open is pretty close.
In practice, use the aperture you need to provide the necessary depth of field, but only that; don’t stop down any farther than necessary.
The ƒ-number is the ratio between the diameter of the opening in the lens and the focal length. An 18mm lens at ƒ/16 has an effective diaphragm opening of 1/16 of 18mm, or 1.125mm. A 12mm lens at ƒ/16 has an effective diaphragm opening of 0.75mm; a 9mm lens at ƒ/16, an opening of 0.563mm. The smaller the effective opening, the more diffraction adversely affects image quality, so this is another reason why landscape photographers tend to favor larger formats. That said, however, many of our surveyed pro landscape shooters do work with smaller digital formats—using moderate apertures no smaller than ƒ/11.