Great post, Neuro. That illustration is great for describing the difference between precision and accuracy.


Quote Originally Posted by neuroanatomist
Chuck Westfall has told me that, "The AF precision for the standard precision sensors is within the depth of focus for the maximum aperture of the lens, while the AF precision for the high precision sensors is within 1/2 or 1/3 the depth of focus for the maximum aperture of the lens, depending on the camera model under discussion."

Chuck misspoke here slightly. The maximum aperture of the taking lens doesn't matter at all -- all that matters is the f-number rating of the activated AF sensor. (Aside from the obvious fact that which AF sensor will be activated does depend on the f-number of the taking lens, of course.) His statement implies that since an f/1.0 lens has much thinner depth of focus than an f/2.8 lens that it will have higher autofocus precision. In fact, the precision is exactly the same as the f/2.8 lens, because the autofocus sensor only sees a tiny sub-aperture from inside the annular ring corresponding to f/2.8. That's only the center point of course; most autofocus sensors on non-1D bodies are f/5.6, so the precision is much lower.