Double cross AF Points for f2.8 and faster?

Started May 20, 2012 | Discussions thread
bobby1781
Regular MemberPosts: 230
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Re: Double cross AF Points for f2.8 and faster?
In reply to Dale Buhanan, May 21, 2012

That is interesting, but how was the test performed if you can remember?

Since different lenses are using different motors and a 2.8 lens will also have more weight to move than the smaller diameter glass of a 5.6 there could be a lot of factors I think.

I guess my thoughts go like this: and these numbers are COMPLETELY made up just for sake of explanation so I can better understand...

400 F5.6 focuses in 1 second
400 F2.8 focuses in 1.2 seconds

but...

200mm F2.8 focuses in .8 seconds

My theory is that the 200 F2.8 and the 400 5.6 would have the same diameter requirements and in a perfect world without a difference in number of elements would weigh the same or close.

So when looking at it like this maybe an F2.8 lens focuses faster than the same F2.8 lens on a camera without the extra AF sensors? Yet an F2.8 lens won't focus faster than a F5.6 lens due to the physical weight of the elements? For example the 85 1.2 that focuses slower that the 1.8, is this due to the weight of the glass or is it due to the DOF being thinner and perhaps the lens AF system is intended to sacrifice speed for accuracy, I don't know the answer to this, just that the 1.8 focuses much faster.

Although you say accuracy more than speed, so this I can't make sense out and eagerly await some more replies.

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