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| p.2 #8 · p.2 #8 · TDP Review: Zeiss Otus 55mm f/1.4 Distagon T* |
Mikael Risedal wrote:
please explain then, what is the accuracy of the AF point? and what is minor variations
The reference lens Canon use to tune AF in the camera body is a locked special 50mm 2,8 lens
Sensitivity relates to the required aperture needed for the baseline phase detect.
Accuracy/Precision is a totally different property and is usually the depth of field of the lens's max aperture unless you are using a high precision af point then it is 1/3rd the depth.
"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. "
"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. "