I often read discussions on forums and social media by someone who has “read somewhere” than a Nikon cannot autofocus a lens at an aperture smaller than f/5.6. Is this true? Absolutely not!

Fact: Older Nikons may not autofocus reliably with a lens having a maximum aperture smaller than f/5.6. That may sound bad, if you assume that it means the camera will not autofocus reliably when the aperture is closed down to f/5.6. That is not what Nikon is saying! Instead they are telling us that autofocus may not work reliably on older Nikons with a lens having a maximum aperture of f/5.6, with the emphasis on the word maximum.

Maximum (f/2.8) and Minimum (f/16) Aperture

“Maximum aperture” means the largest aperture (biggest hole) the lens can possibly have when it is wide open. A lens autofocuses with the aperture wide open and then stops the lens down (closes the aperture to a smaller hole) only at the very moment of exposure. If you are using a lens with a maximum aperture of f/5.6, the widest the lens can ever open is f/5.6, which does not provide enough light internally for many older Nikons to autofocus reliably.

It is critical you understand that I am not talking about using a standard Nikkor lens at f/5.6, f/8, f/11, f/16 or a smaller aperture opening. All AF Nikons will autofocus with an AF lens at any aperture the lens supports. They can do that, again, because autofocus happens at maximum aperture, which on even low-cost Nikon lenses is usually about f/3.5. Maximum apertures on many prime lenses are f/1.4 or f/1.8. And on pro zooms the maximum aperture is usually f/2.8 or f/4.

Therefore, even an older Nikon will autofocus on normal AF lenses all the way down to the smallest aperture because the actual autofocus happens at maximum aperture (largest hole), which on most lenses is bigger than f/5.6.

When you add a teleconverter to a slow telephoto lens, the maximum aperture can sometimes drop below the f/5.6 barrier, making autofocus difficult or impossible on older Nikons.

Newer Nikons, such as the D7200, D7100, D600, D610, D750, D800, D800E, D810, D4, and D4S, all support lenses having a maximum aperture of f/8 with a limited number of the 51 (or 39) available AF points (usually 11, 7, or 1 AF point). Newer Nikon cameras have a special arrangements of AF points that can autofocus on lenses with a maximum aperture all the way down to f/8. That makes your newer Nikon a special and valuable camera if you want to use telephoto lenses with teleconverters.