LensRentals Nikon AF test just posted

Started Sep 14, 2012 | Discussions thread
Andre Affleck Senior Member • Posts: 2,362
Re: Unfortunately, no info on fast lenses wide open

inasir1971 wrote:

While their finding is interesting, I think some of it was expected - that the Nikon PDAF is better than the previous generation of Canon AF with lenses f/2.8 or slower.

The one thing that they don't consider and which is relevant is the accuracy with fast lenses wide open. I believe that they only test at f/2.8 and what I see is a lack of accuracy with the Nikon system on the D800 and D4 with lenses faster than that, particularly 1.4's, whereas some reviewers (as I don't have any Canon) seem to be reporting extremely accurate AF even with the Canon f/1.2 lenses with the new 1DX.

Essentially the AF sensors only receive light from regions at the f2.8 diameter even if the lens aperture is larger than f2.8. With faster lenses, light from wider apertures beyond f2.8 do not reach the AF sensors, so the accuracy at f1.4 is no better than f2.8. However, since the DOF is 1/4 the size, the resulting accuracy is reduced by 1/4.

So one can always expect to get worse results shooting wide open with faster than f2.8 lenses, and the reduction in accuracy will be a straight ratio of DOFs. There's really no point in testing AF at 1.4 since it will be the same results as the f2.8 data, divided by 4.

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