5D-III focus spot size
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.
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