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Next PZ lens test report: Sigma 24mm f/1.4 DG HSM | A ("Art")
Quote:Field curvature plays no roll for PDAF.  It is literally impossible for the AF sensor to sample a different image than the sensor would.  If the lens' focal plane is shifted 300 microns at the edge focus point and you focus there, it will focus accurately for that position on the sensor. 


AF adjustments correct error on the lens' mount location.  While all lenses stop at the same position on the barrel at infinity or past or wherever the stop is, the focus may not reach the sensor correctly.  The lens reports it is at position "1" and the camera will tell it to move to position "3800" to focus properly.  If this is miscalibrated, 3800 may not truly be in focus.  The AF adjustment is to ensure that "3800" is "3800."


Some older models have closer to 24 focus positions.  Newer ones have thousands.
Cameras do not tell lenses to move from position 1 to position 3800. It is not a 1 look, one movement kind of thing. PD AF goes in steps also, and inaccuracy and adjustment of it is a bit more complicated...


2 very simple examples to illustrate this.


For a camera to be able to determine that a lens has to move from 1 to 3800, the camera must have a pretty precise idea of the focal length of the lens. However, as you know, the camera can't really know the focal length of a lens. For instance, the lens may be reporting it is 200mm, but reality may be that it is only 135mm. Or, one can use a non-reporting teleconverter (like my Soligor 1.7x TC). Neither lens nor camera will know that 200mm (or at least 200mm the lens thinks it is) is actually 340mm (or al least what we think of as 340mm going by the numbers). Yet AF still gets its thing done.


Or another experiment... If you tel the camera "hi, I am an autofocus lens of X focal length and I am in AF mode" but in fact you are a manual focus lens without any electronics to determine position or what not, the camera will fire a shot (when in a mode that is focus prioritised)  when things are... in focus. 


With the faster more silent lenses it is a bit difficult to observe the steps the AF procedure makes, but with slower and/or noisier lenses it is easier to see the process in action, sometimes with some trial and error movement when probably focus has been missed.


I do realize that newer camera models have more complex AF systems, which drive newer lenses differently than older (or third party) lenses, and that there is not just one way of AF protocol between lens end camera.
About Canon users being less of issue seekers... that is wrong. There are many more Nikon users who scoff at the idea of that there is something wrong with a Nikon camera/lens/whatever that then are Canon users that behave that way. Canon users tend to nitpick on every little detail of equipement more often. It is an interesting sociological thing to observe, on internet forums. 


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