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Why is it that lenses come with an AF option when the camera has the same? Which controls the AF? One would think that they contend with each other for control. Why have two entities doing the same single task? A photographer friend of mine said to set the lens to Manual. If every photographer does why offer lenses with this setting?

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Some cameras can use lenses that don't have a built-in autofocus motor like the Nikkor AF series of lenses. Those cameras have an internal motor with an AF-coupling. So you have a switch on the camera, which switches not just the coupling but also the whole AF-feature.

Newer cameras also enable the use of older lenses like e.g. Nikon D300/500. And many newer lenses have their own AF-motors. So now you have both: A switch on the camera and one on the lens.

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Although not all cameras and lenses work the same, there is a common difference:

  1. Lenses have an AF/MF which that determines if it acts upon focus instructions from the camera. In lenses with a mechanical coupling, this disconnects the link. In ones with electronic control, it tells it to disregard input that controls focus.
  2. Cameras usually have a three-way switch. This selects between single-shot focus, continuous autofocus or manual focus. This controls what signals, either mechanical or electronics, that the camera sends to the lens.

These do not content with each other because it is a chain. When the lens in set to MF, it will simply not autofocus regardless of the switch on the camera since it is ignoring focusing input. When the lens is set to AF, it would accept focusing control but if the camera is set to MF, it will not send any focus control and so the lens will not automatically focus.

There is an evolution which brought things to this point and that depends on the brand and mount but if things were done from scratch, then possibly there would be no switch on lenses since the camera is capable of offering superset of what the lens can offer. Then again, some entry-level mirrorless have no switch on the camera, so it is cumbersome to enter the menu system to switch between AF and MF modes.

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