Don Clary Offline Upload & Sell: On
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As Ron stated, the 20D requires a lens with f2.8 aperture, to allow the central precision cross sensor to function. It is not about focus speed, nor is it about the amount of light. It is about the off axis angle of light, that permits these sensors to operate. These sensors are 3 times as accurate as the vertical sensors. On the series 1 bodies and the EOS 3, the central precision cross sensor will work at f4.
Considering the extreme importance of sharpness to most photographers, having a lens fast enough to permit these sensors to operates is extremely important. With my extensive experience with lens testing and manual focus bracketing, I can clearly see how important accurate focusing is. A consumer lens manually focused to perfection, can easily exceed in sharpness an L lens that is not perfectly focused. A lens is only as good as its focus.
In my style of photography, I use a tripod about 98% of the time, and I always manually focus. Only on rare action shots and in portraits, do I use autofocus. YMMV.
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