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?
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).
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.