Does FL affects AF speed?

Started Feb 9, 2013 | Discussions thread
dpyy
Contributing MemberPosts: 777
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Re: Does FL affects AF speed?
In reply to EinsteinsGhost, Feb 14, 2013

EinsteinsGhost wrote:

dpyy wrote:

EinsteinsGhost wrote:

dpyy wrote:

Does FL affects AF speed? Why does my Z24 AF so much faster than E50 in the same light? Even though now both supports PDAF and I'm using the exact same aperture etc (1.8)?

Assuming everything else to be equal, AF speed can also be determined by DoF. The E50 has shallower DoF than Z24, so the focusing system has to cope up with that. With greater DoF, the focusing would be faster. So yes, FL can affect AF speed.

But I'm using center focus though, shouldn't it be the same and not affected by dof?

It will be affected. Let us assume 10 ft focusing distance with both lenses at f/1.8. The respective DoF:

Z24: 46 inch

E50: 10 inch

The area in reasonable focus for the shorter focal length (and consequently greater DoF) is over four times better whereas with the E50, the camera has little room for error.

This is why small sensor cameras can get away with CDAF but large sensor cameras cannot. In fact, as an extreme case, when I use my Rokinon 8mm lens, I use it in "set f/5.6 and forget" mode. I don't even have to set focus, as everything already is due to its immense DoF. If it were an AF lens, the camera would have no trouble acquiring focus. OTOH, this is a 135mm f/2.8 lens on NEX, from a reasonable distance... the DoF is tiny. The camera's AF system (not applicable in this case) would have to work hard to be within that little space barely the length of a fruit fly.

So according to your explanation, the smaller the DoF the harder the camera has to work? Because it needs to be more accurate? If so, I should stick with shorter FL if I'm looking for the quickest focus right? Because I'm planning to buy a fast AF lens to go with my EA2 and so it would be counter productive to say use it 135 lens vs something below 50? Not to mention SSM would be a good idea?

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