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I cannot find a question answering the mechanism of auto-focus point selection and there seem to be no articles that describe it either-from my searching.

Canon and Nikon DSLRs to my knowledge don't incorporate sensor-shift as part of their focusing mechanism.

So is it only the lens that refocuses onto the newly selected point, in an already compose scene? This would make sense as different focus points lie on objects located at different distances.

Additionally does this imply that the prerequisite is that your lens can 'speak' to your body? (I use a d3200 that has no built in focus-motor, so it doesn't autofocus with the old Sigma 70mm that I use.)

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    Please clarify what you mean by "...auto-focus point selection..." Do you mean when an AF point is selected manually by the user (often well in advance of a particular scene being framed)? Or do you mean when the user enables automatic AF point selection and the camera selects an AF point based on the scene? – Michael C Jan 19 '17 at 8:17
  • Yes, the former, I will edit the post to clarify. – Chai Jan 19 '17 at 8:18
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The camera 'maps' the selected focus point to the focussing sensor, and uses that part of the focussing sensor to determine if the image is focussed or not. If not, it moves the lens until focus is achieved.

If you then select a different focus point, it will then examine this new part of the focussing sensor and the cycle is repeated.

Edit: As per Michael's comment, in some focussing systems the camera doesn't move the lens until focus is achieved, but instead calculates how far the lens should move to achieve focus and then moves the lens (instructs the lens to move) that far. Errors in this process or incompatibilities between the body and the lens can lead to front/back focussing problems.

  • PDAF doesn't really wait until focus is achieved. It measures how far off and in what direction the lens is and instructs the lens to move that much. At best it uses a position sensor in the lens to check how far the lens actually moved. For much more comprehensive information on how this all works, please see this answer and the links it contains. – Michael C Jan 19 '17 at 8:23
  • Good point - I guess otherwise you wouldn't have the problem of front or back focus and the requirement for micro-focus adjustment. Answer edited. – Steve Ives Jan 19 '17 at 8:29
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Phase detection autofocus systems with multiple points have an array with lines of sensors that correspond to each AF point in the viewfinder. Which area(s) of the sensor are active depends on which AF point(s) is(are) selected. The shape of the PDAF array don't necessarily match the arrangement of the AF points in the viewfinder. In fact, they usually don't match. The light entering the PDAF array is divided by a splitter and aimed by microlenses to the appropriate areas of the PDAF sensor.

When a particular AF point is selected the camera pays attention to the corresponding pair of lines on the PDAF sensor. Exactly how this is accomplished depends on the overall design of the specific camera and its AF system as well as the particular settings for that camera selected by the user.

In general, the camera uses the AF point selected to measure how far and in which direction the lens is out of focus. It then tells the lens to move that far in that direction and takes the picture. The camera will use the highest area of contrast within the active AF area. Please note that the actual area of sensitivity for a particular AF point may be much larger than the square in the viewfinder representing that AF point. You'll need to practice and experiment with your particular camera to learn what the areas of sensitivity for each AF point are.

How accurately the lens moves to where the camera instructs is dependent upon the design of the lens, whether the lens has a position sensor that communicates to the camera exactly how far it actually moved, how accurate that position sensor is, and whether or not the camera is designed to take advantage of the information provided by the lens. For a more comprehensive answer that details this entire process, please see this answer to: If the focal plane is curved, should the outer AF points work correctly or front-focus?

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