Focussing area is different in live view than when viewed in viewfinder

Question

When I use the AF-S mode or any other mode through the view finder, I can see an area in which the focus point can be moved. It is relatively small. What if I want to focus out of that area or focus on a subject that is at the bottom left corner of my scene. However, in live view I can move the AF point anywhere on the scene. Is it by default or can I change that in the settings somewhere. I recently purchased a Nikon d850

Answer 1

When you focus using the focus points visible in the optical viewfinder, you are using phase-detect autofocus (PDAF). The sensors for are located on a secondary sensor that is not part of the imaging sensor, which receives light reflected off the mirror while it is down. These focusing points are fixed in location and cannot be moved (although they are typically a bit bigger than what's actually displayed in your viewfinder).

On the other hand, live view uses contrast detection autofocus (CDAF). This uses the actual imaging sensor, and works by analyzing the image that the sensor itself is receiving with the mirror up. Because it's using the imaging sensor itself, rather than a bunch of secondary sensors in a fixed grid, you have more flexibility in choosing your focus points.

This cannot really be changed, other than by switching to a camera body that has a denser pattern of PDAF sensors. Although, the D850, with 153 PDAF sensors (some with varying characteristics), you're already towards the top of the pile in that respect - some older PDAF sensors only had e.g. 9 or 15 elements.

Answer 2

The focusing system used by the camera when you are using the viewfinder is an independent system from the focusing system used when you are using Live View.

Each system is designed and operates completely separately from the other. One works when the mirror is flipped down and the imaging sensor is "blind". The other works when the mirror is flipped up and the dedicated PDAF (Phase Detection Auto Focus) sensor is "blind" along with the viewfinder.

Both systems have relative strengths and weaknesses compared to the other. You've discovered one of the strengths of Live View and one of the weaknesses of off-sensor based PDAF systems.

Most DSLRs, even high end ones, have relatively limited areas near the center of the frame from which to select AF points. This is governed by the shape and size of the hardware used to make the dedicated PDAF sensor array. In fact, the dedicated PDAF sensors in most larger sensored cameras are more limited to areas near the center of the frame than smaller sensored cameras. If one uses the same dedicated PDAF sensor array, the same angular amount of coverage will stretch over a larger percentage of a smaller imaging sensor than the same angular amount of coverage will spread over a larger sensor.

Cameras with imaging sensor based AF systems usually allow selecting focus areas that cover more of the total frame. This is allowed because the hardware used to detect contrast is embedded into the imaging sensor itself.