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Author Topic: f/2.8 telephoto lenses have better AF than slower lenses?  (Read 760 times)

FarQinell

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f/2.8 telephoto lenses have better AF than slower lenses?
« on: May 01, 2012, 05:48:45 AM »
This I have culled from www. learn.usa.com (with thanks) about 7D auto focus:

"... the center AF point has a separate diagonal pair of high precision line sensors which provide even greater precision - automatically used with lenses f/2.8 or faster....."

When considering an investment in a big lens is this a consideration ie go for the f/2.8 lens rather than a longer f/4?
« Last Edit: May 01, 2012, 10:05:32 AM by FarQinell »

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f/2.8 telephoto lenses have better AF than slower lenses?
« on: May 01, 2012, 05:48:45 AM »

neuroanatomist

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Re: f/2.8 telephoto lenses have better AF than slower lenses?
« Reply #1 on: May 01, 2012, 10:44:40 AM »
IMO, it's definitely a factor to make one consider an f/2.8 lens over an f/4 lens, provided the trade offs (cost, size) are acceptable. 

The 5DIII and 1D X have five of those high-precision f/2.8 AF points.

If it helps, I wrote up a description of EOS AF Systems, which includes a discussion of the high-precision points, as well as the differences between cross, dual cross, what f/2.8 vs. f/5.6 'sensitivity' really means, etc. 
EOS 1D X, EOS M, and lots of lenses
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FarQinell

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Re: f/2.8 telephoto lenses have better AF than slower lenses?
« Reply #2 on: May 01, 2012, 12:08:06 PM »
IMO, it's definitely a factor to make one consider an f/2.8 lens over an f/4 lens, provided the trade offs (cost, size) are acceptable. 

The 5DIII and 1D X have five of those high-precision f/2.8 AF points.

If it helps, I wrote up a description of EOS AF Systems, which includes a discussion of the high-precision points, as well as the differences between cross, dual cross, what f/2.8 vs. f/5.6 'sensitivity' really means, etc.

Thanks for the very useful background information - worth reading for anyone contemplating a big lens investment - f/2.8 better than an f/4?




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Re: f/2.8 telephoto lenses have better AF than slower lenses?
« Reply #2 on: May 01, 2012, 12:08:06 PM »