A99 Poor backlight AF performance :(

Lets see if I get this straight. the AF point being used is being effected by the exposure?  I don't see how that can be. unless the system is programmed to always use input from the sensor based AF pits and that data is not available or something when odd (not odd but not usual like strong back-light)  setting are imposed on the sensor. I always thought modern sensors were "ISO less" so that seems unlikely too (they don't have to gain up/down sensors anymore).  So if the AF information is garnered from light redirected from the mirror how is the after-the-fact exposure settings affecting it's performance?

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http://www.daisyseyecancerfund.org/Files/Awareness/photographing_rb_poster.pdf