If shooting at f/22, is a fast 1.8 lens still considered fast?

Started Apr 17, 2011 | Discussions thread
noirdesir Forum Pro • Posts: 13,493
AF systems only sees light from the f/5.6 'ring'. Full stop.

Leonard Shepherd wrote:

noirdesir wrote:

Anything faster than f/5.6 (or probably f/5) is lost on the phase-detect AF (except on some Canon bodies which have a few special f/2.8 focus points). The phase-detect AF sensor in all Nikon bodies only sees light coming from the 'f/5.6 ring'.

I am sure this is an internet myth.

And I am sure you have come across lots of detailed explanations as to why what I said is absolutely correct but somehow your subconscious prevents you from accepting them.
Have a look again at this detailed description how AF systems work:

(you can skip to where the AF part starts, pay particular attention to Figure 14). To simply things further, I made you another very basic drawing:

Nikon do not mention a limit. Those of us with a range of Nikon lenses find in general f4 focuses faster than f5.6, f2.8 focuses faster than f4, and f2 focuses faster than f2.8.

This could not happen if phase detect limited AF speed to no faster than an f5.6 lens

Just because any given faster lens focusses faster than a slower lens certainly is no proof at all that the f-stop is causing this, simply a stronger, faster AF motor can explain this as well. Correlation is not causation.

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