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:

Andre Affleck wrote:

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.

Thanks - I suspected that they were designed for focus with f/2.8 or slower lenses.

I assume that this means that accurate AF with fast lenses is not technically possible?

Not unless the manufacturers add the sensors and associated optics for those wider apertures, which is completely possible, just not practical unless the need justifies it.

Nikon does not publish their AF performance specifications, but Canon does, and they claim that for f2.8 lenses, the camera should achieve within 1/3 DOF. If Nikon is similar, then you are still achieving 2/3 DOF for f1.4 lenses which is still pretty good.

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