Kerry Pierce
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As of now, only 2 people, other than myself, voted no, which is about what I expected. Until now, I've always believed that faster glass was better for AF than slow glass, in low light. I suppose it is a holdover from my MF days, where fast glass certainly made a difference to the prism and to the viewfinder in general.
I recently did some tests, on static subjects, with the d200, d300 and d700, using f/5.6 glass and several other lenses, down to f/1.4 glass, in very dark conditions. In no case, did AF fail with f/5.6 that also failed with any other lens. In every case, AF worked at f/5.6 when it worked with faster glass. This included cross type and single type sensors, so that also didn't matter.
I was really surprised to see the f/5.6 lens, the 24-120, lock on targets that I could barely, and I mean barely, see in the viewfinder.
The only difference that I could discern, in about 2 hours of testing, was that the viewfinder was brighter with the faster glass.
My conclusion is that the AF module sensor is very, very sensitive and doesn't need much light at all, to work and work reliably. All of my tests were done in a large room lighted by a single, shaded 60 watt bulb and I deliberately targeted very low contrast, low light objects far from the lamp. The testing conditions were such that no "normal" photographer would even consider trying to shoot without benefit of flash, focus assist or other added light source, (long exposure night landscape scenes excepted).
I did not test TCs, which I tend to think goof AF more than some others seem to think, nor did I do any tracking tests.
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? What body are you referring to? I believe that is a Canon thing unless i am mistaken. All of my af points work perfectly weather I have a slow lens or a fast lens on. I have never noticed a difference..
That is to say, that you get fully functioning AF at f/5.6