What happens inside a DSLR when you manually select a different autofocus point?
Asked 1/19/2017
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On a DSLR such as a Canon or Nikon, what changes internally when I manually choose a different AF point in the viewfinder? Since these cameras don’t use sensor-shift for focusing, is the camera simply using a different part of its autofocus system and then commanding the lens to refocus for whatever is under that point? Also, does this require an autofocus-compatible lens that can communicate with the camera body? For example, I use a Nikon D3200, which has no built-in focus motor, so some older lenses won’t autofocus on it.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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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.
Originally by user34900. Source · Licensed CC BY-SA 4.0
user34900
9y ago
0
Generated from our catalog & community — verify before relying on it.
Selecting a different AF point does not move the camera sensor. In a DSLR, the camera uses a dedicated autofocus sensor array for phase-detect AF. Each selectable AF point in the viewfinder corresponds to a particular area or pair of sensor lines in that AF system.
When you choose a new AF point, the camera starts evaluating that different AF area. If focus is off, it tells the lens to move its focusing elements. In some systems it iterates until focus is reached; in others it calculates how far to move the lens and commands that adjustment directly.
So yes: the thing that physically changes focus is the lens, not sensor-shift. The camera body decides which AF sensor area to use and sends focus instructions to the lens.
And yes, autofocus requires body/lens compatibility. The lens must be able to communicate with the camera, and on bodies like the Nikon D3200, lenses that need an in-body focus motor will not autofocus because the body doesn’t provide that motor. Compatible AF lenses with their own focus motor will autofocus normally.
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