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Topics - Normalnorm

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Lenses / AF questions
« on: August 23, 2013, 03:50:56 PM »
I am calling upon the collective wisdom here to help me understand something that has bedeviled me for some time

I generally use Canon lenses though I do own the Sigma 50 1.4 and in the past had an assortment of Tamron and Sigma lenses.
I am puzzled as to why some lenses hunt and others do not. For instance, a Tamron 24-75 and a Canon 28-105 that I used to own would hunt mercilessly.
If the AF module is in the camera and it sends its instructions to the lens why would the lens hunt? It seems to me that if the AF module sees OOF images it sends a correcting command to the lens and the lens should stop. Many comments I have seen seem to imply that the lens is making some calculations of its own but I can hardly imagine that to be the case as the light just passes through the lens.

For CDAF I understand it is an iterative process whereby the lens overshoots perfect focus and then backtracks to eventually settle at correct focus but what is the case for PDAF?

TIA

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