AF phenomenon on the 70D: Can anyone explain?

Started Dec 18, 2013 | Discussions thread
RS_RS Senior Member • Posts: 1,788
Re: AF phenomenon on the 70D: Can anyone explain?

Very interesting and peculiar. I have never seen this behaviour with any of the many Canon SLRs and DSLRs that I have owned over the years; these do not include the 70D but they do include the 7D which has a closely related AF system, and my wife has one of those too.

Just a point for clarification. When you refer to aperture, I assume it is always the maximum aperture of the lens being tested, since PDAF is completed at maximum aperture before stopdown, so the phenomenon is presumably independent of the aperture used for exposure, yes?

I wonder if the problem is in some way connected with the fact that at the centre point there are two different sets of AF sensors, normal precision operating at f/5.6 and wider and high precision operating at f/2.8 and wider. If the HP sensors are operating intermittently and are out of spec, that could give rise to what you are seeing, with the correct shots being focused by the standard precision sensors when the HP sensors fail to operate, and the duds being focused incorrectly by the HP sensors when they do operate. Just a thought , may be totally off target.

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