You would find this under:
C.Fn. III-8 "Expansion with selected point."
The reason I'm asking this question is that I'm assuming, since this isn't the default camera setting, that there is some sort of trade off using "focus expansion" as opposed to using straight single point focus. I'm wondering if the expanded points either (1) Help focus on the selected focus point or (2) Is and average of the surrounding focus points in relation to the selected focus point?
If (1), then why would anyone want this setting off?
Thanks very much.
There is a 'gotcha' to look out for with this custom function setting. Basically, if you are using single point focus and have C.Fn. III-8 Expansion with selected point turned on then the camera will automatically pass off focus to the surrounding points if it cant focus on the one point selected.
Many times this may be beneficial - but I have found it occassionaly problematic - sometimes I want focus on the eye and only the eye - in which case the camera passing off focus to another point is detrimental.
Edit - the expanded focus points dont help focus on the main point - they are instead of the main point and will only come into use should the main point be unable to achieve focus lock.
This is covered very well in this document :
Tailoring the 1DS MK3I do use this setting - but for fast moving kids who wont stay still and that I have trouble tracking.
