Question on minimum F-stop on AF

Started Apr 24, 2012 | Discussions thread
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James Hagan
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Question on minimum F-stop on AF
Apr 24, 2012

Hi, all

Why do DSLRs operating in Autofocus override the lens' F-stop? For instance, the A57 minimum AF F-stop is 3.5. Although this seems pretty common among DSLRs, I can't figure out why?

It makes little sense. For Sony to tout a new line of SLT cameras with movie mode, AF, and continual tracking that allow interchangeable lenses AND THEN override the lens' minimum aperture defeats the purpose of the whole thing -- I mean, this feature is designed for video -- for indie filmmakers who thrive on the DOF. With the obliteration of the AF f-stop, this camera no longer outperforms the Rebel or the Nikon 5100 or the GH2 because they ALL require a follow focus in order to really take advantage of a lens' DOF.

And this idea is not unprecedented. For example, the 1.03 firmware for the Sony A65 allowed a 1.8 f-stop in AF, but this feature was removed in the 1.04 update. I find it hard to believe that Sony removed this feature in the A65 1.04 update because it contributed to non-shut-off problems commonly reported in 1.03.

Anyone have experience in this respect?

Panasonic Lumix DMC-GH2 Sony SLT-A57 Sony SLT-A65
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