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| p.1 #7 · Canon autofocus information |
When you half-press the shutter release (or the * button, if you've used the custom function to move focusing control there), the activated AF sensor "looks" at the image projected by the lens from two different directions (each line of pixels in the array looks from the opposite direction of the other) and identifies the phase difference of the light from each direction. In one "look," it calculates the distance and direction the lens must be moved to cancel the phase differences. It then commands the lens to move the appropriate distance and direction and stops. It does not "hunt"...Show more →
I've read before that it works something like this, but this is all much more detailed than what I've seen before, so thank you very much for this info. What I've never understood, however, about the bit above is that I would expect the camera to need to know the current distance the lens is focused at, as surely the phase difference gives rise to a different change in focus distance depending on the current focus distance. Or perhaps the camera just tells the lens what phase difference to adjust for, and that calculation is internal to the lens, in fact I can't think of any other explanation given that we know not all lenses pass distance information to the camera. This is another possible source of miscalibration, if the lens has an incorrect measurement of the distance it's focused at when it performs this internal calculation, and this is why a lens can front or back focus on a perfect body, as well as bodies front or back focusing with perfect lenses (due to the AF mirror positioning as you mentioned).