Understanding AF precision
An article explaining the AF system and precision (in this case a Canon) I wanted to share. The issue of AF precision has been raised and the possibility that there is an inverse relationship between compatibility with slower lenses and precision. This article sheds light on that.
"Precision is repeatability - if you take several shots of the same thing, how close will the focus of any one shot be to all the other shots? For EOS AF points, there are two levels of precision - 'normal' which is within one depth of focus for the attached lens at max aperture, and 'high precision' which is within 1/3 of the depth of focus for the attached lens at max aperture. Note that depth of focus is related to, but not the same as, depth of field, although the same factors influence both. "
"An f/2.8 sensor line is more accurate than an f/5.6 sensor line - the wider the aperture threshold, the wider the rangefinder baseline for triangulation and thus, the more accurate the measurement of focus. However, the wider the aperture, the fewer lenses that work with that aperture (and the more expensive those lenses are), and also, the detection range of f/2.8 sensors is narrower, meaning it may take longer for an f/2.8 line to achieve a focus lock when a subject is well out of focus. As a result, AF systems will usually focus in two steps when possible - 'coarse' focus with an f/5.6 line, then 'fine' focus with an f/2.8 line."
If anyone has a link to which sensors are which on the D800/D4 please post. Thanks.