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| p.1 #10 · ? minimum practical subject size vs image size for pro cameras |
Just out of curiosity Alan, have you checked EXIF data to see what focusing distance the lens is reporting? If I'm reading you correctly, I've seen some similar things from all sorts of camera and lens combinations, and it usually seems to manifest itself when most of the detail in the image is approximately the same distance away (and, I'm presuming from the EXIF data I've seen, the AF chooses to front focus for no apparent reason).
Not that it's definitive, but it might help you at least identify some patterns in AF behavior. I've often wondered if AF arrays can be influenced by interference patterns of complex subjects (branches in trees, for example, which we often assume to be a good, high contrast subject that AF 'should' be able to lock onto).
Regardless, I'm with you...
Hi Jeff. Long time no contact.
What you suggest makes sense but I'll have to put in some effort to do the tests properly. That cannot happen with my Canon for at least several weeks because once again I'm on the opposite side of the country from my gear. I've got some Nikon gear with me that has much the same performance limitations but I have no tripod with me.
I have not yet observed distance as a significant factor but maybe I was not looking carefully enough. It seems to be more related to subject detail size vs AF sensor size.
I'm certain that some contrast patterns will upset any AF system. It would be useful if Canon and Nikon would tell us how big their AF pixels are so that we could know what size of details are invisible to AF sensors and hence which patterns are more likely to be problematic. It's also relevant that some patterns that are visible to linear AF sensors are not so obvious to us when we see an image area rather than image strips.