# Autofocus working conditions



## Pancho (Dec 6, 2012)

I have a candid question about autofocus functionality.

Majority of Autofocus are working with a lense at f:5.6.
Some (very) high cost DSLR can work with lenses at f:8.

What is the limiting factor?

Is it the AF system illumination? I'm not sure, because if the AF system can work with a f:5.6 lense at 0EV, it would be able to work with a f:8 lense with 1EV illumination...

Is it a question of Deth Of Field? At f:8 we have more DoF than at f:5.6 and then less precise error measurement inside the AF system.
But if the AF system is able to work with a 18mm f:5.6 at 15m distance, why wouldn't it be able to work with a 400m f:8 at 3m distance?

Thanks for your help.


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## neuroanatomist (Dec 6, 2012)

It's a question of the 'baseline' - the narrower the max aperture of the lens (or TC combo), the less the phase difference between two points in an image that the phase-detect AF system needs to determine the focus calculation. It's not a hard line, there is some flexibility (obviously, else Canon couldn't change from f/5.6 to f/8 with a firmware update, nor would the Kenko TC or pin taping tricks work). But the issue is reliability and speed at the edges of performance.

The wider the baseline, the more accurate the focus calculation - that's an advantage for the Canon f/2.8-sensitive AF points.

See http://www.the-digital-picture.com/Photography-Tips/Canon-EOS-DSLR-Autofocus-Explained.aspx.


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## Pancho (Dec 7, 2012)

Thanks Neuro for your answer. 
I didn't really catch yet the 'baseline' concept, I need to work more to well understand.


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## neuroanatomist (Dec 7, 2012)

The baseline concept comes from the idea that each AF line pair is acting as a simple rangefinder, that determines the distance to the subject by triangulation. The phase difference between the adjacent points of incoming light, separated by small prisms, is the 'base' of that triangle. A narrower aperture means less separation for the phase difference the system needs to measure in order to accurately subject distance.


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