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I have a Nikon D7200. The autofocus does not work when using the optical viewfinder. When I use live view, the autofocus works fine. But when I use the optical viewfinder, the autofocus won't stop in focus so I can't take a photo. Why is this and how I can fix it?

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In DSLRs the autofocus is not done the same way between viewinder and liveview.

  • When you use the viewfinder a small mirror ① behind the main mirror ② reflects some of the light on a focus sensor ③ at the bottom of the camera body.
  • When you are in live view, the main mirror is up, the main sensor ④ is exposed, and its image is used to determine focus (usually by trying to maximize contrast).

enter image description here

So if you don't get focus in viewfinder mode this can be because the focus sensor is damaged/disconnected or the small mirror is stuck, and this doesn't prevent the other focus method from working.

Another possibility is that you are using a lens with a small aperture. The "phase detection system" used for focusing when using the viewfinder requires by design the lens to have a minimum aperture (usually, f/5.6 nominally, but there are many f/6.3 zoom lenses on the market, so there is obviously some margin). But if you are using a lens that that opens at f/8 max, your autofocus won't work. Also, if you use a lens with an extender/converter, this reduces its maximum aperture, so for instance an f/5.6 zoom with a 1.4x extender becomes an f/8 lens and the autofocus won't work if you use it with an extender.

The maximum contrast method used in live view has not such hard requirements. With a small aperture it could struggle but still work if there is enough light.

Original DSLR Image By Hanabi123, CC BY-SA 3.0

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  • \$\begingroup\$ You left out that the focus and capture sensors can be out of alignment, which is why DSLRs need AFMA, but mirrorless cameras don't. \$\endgroup\$
    – xiota
    Aug 30, 2022 at 3:50
  • \$\begingroup\$ "Autofocus won't stop" sounds like hunting to me... which is typically/predominately due to a lack of light/contrast where the camera is attempting to focus (even w/ PDAF). Live view was likely using a different focus area/method. \$\endgroup\$ Aug 30, 2022 at 12:08
  • \$\begingroup\$ TC's make a lens slower in terms of light/contrast, which is why PDAF points drop functionality/reliability; not because the AF points cannot see the required portion of the objective element like with an actual f/8-11 lens. That's why the Canon 1DX (mk 1) has center point AF (expanded) only at f/8 w/ TC. But the 1DX (mk 2) drops to 9 AF points at f/8 with the mk 1 or 2 TC's, and retains the full 61 AF points at f/8 with mk 3 TC. I have tested cameras to (PDAF) autofocus at f/11 with TC's, and the ability to AF with many more focus points than advertised at f/8 (Nikon D810). \$\endgroup\$ Aug 30, 2022 at 12:20
  • \$\begingroup\$ TC's change the size of the exit pupil, which is the "effective aperture" as viewed from behind the lens, just as the entrance pupil is the "effective aperture" as viewed from the front of the lens. When the exit pupil appears smaller from behind the lens, it does, in fact, reduce the angle of view that the "AF points can see". The fact that the firmware in a TC can "turn off" AF when it detects a lens with a narrower maximum aperture than what the designers want the camera's AF system's effectiveness to be judged at does not obviate the optical limitation. \$\endgroup\$
    – Michael C
    Sep 1, 2022 at 16:45
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The viewfinder is out of focus: Rotate the diopter adjustment control to adjust viewfinder focus. If adjusting viewfinder focus does not correct the problem, set the autofocus mode to AF‑S and the AF‑area mode to [Single-point AF].

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  • \$\begingroup\$ The diopter setting should have no bearing on whether or not the camera can achieve focus. Can you elaborate on why you think this might be connected? It sounds like the camera is hunting for focus and never stops. \$\endgroup\$
    – osullic
    Jan 4 at 21:24
  • \$\begingroup\$ The viewfinder diopter setting has nothing to do with the camera's autofocus (or even manual focus). They are two completely separate and different focus systems. \$\endgroup\$
    – scottbb
    Jan 5 at 0:57

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