Why is autofocus slower in live view?

S

semajha

Well-known member
Just recently I discovered when using the viewfinder to snap pics, the autofocus is MUCH quicker. However, viewing it in live view, the autofocus takes awhile to focus. Can someone explain to this noob why is this?
 
When you autofocus through the viewfinder, the camera uses its main autofocus sensor which is located under the mirror of the SLR. This sensor can determine focus very fast. When in live view, it is no longer possible to use this main AF sensor because the mirror is in the up position. In Live View, AF is performed on the main image sensor using pixel contrast which is much slower.
 
Since autofocus isn't continuous on the T2i, I focus my shots 'before' pressing record using the camera's AFQuick center selection point and fine tune with manual focus. AFQuick uses the same 9 sensors as when taking pics and is much faster than AFLive mode. One thing to be aware of is some cheaper lenses don't have full time manual focus, which means you have to flip a switch every time between AF and MF.

The AFQuick center selection point uses a cross type that gives high-precision focusing with f/1.0 - f/2.8 lenses. The mirror does flip down during AFQuick so you lose the image on the LCD for a moment. Still much better than all the hunting involved with AFLive.
 
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