5D-III focus spot size

Started Jun 7, 2012 | Discussions thread
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tony field
Veteran MemberPosts: 7,417
5D-III focus spot size
Jun 7, 2012

Page 75 of the user's manual indicates that the single point AF can have two different sizes - a larger or a smaller point can actually be selected to "refine" the exact area from which focus information is derived.

To get a handle on this, I photographed a white lawn chair with a shadow background. Using AI-SERVO mode and the AF-ON button depressed, I first focused on the white plastic chair. Subsequently, I carefully moved the focus point so that focus was acquired on the background and immediately took an exposure.

I did this for both the small and the large AF point. In DPP and turning on the focus point view, you can see the approximate size differential:

Although this is not a perfect measurement, it does give a reasonable working indication of the approximate size of the focus points.

I also noticed that the outer focus points, doing the same test, have slightly different apparent sizes - which makes sense from a simple geometry point of view.

Would be nice if somebody to come up with a better test - possibly using a single LED on a dark background, possibly moving from the dark background to the LED - something to do when I get around to it.

Canon EOS 5D
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