How to correctly use magnification for focus assist on FujiFilm X-E1?

Question

I have a FujiFilm X-E1 mirrorless digital camera that has a magnification feature designed to assist with manual focusing. The documentation tells me the following:

Manual Focusing Methods

In the viewfinder, the distance index bar is displayed together with the depth of field scale and aperture value, providing helpful guides for focusing.

For pinpoint precision in manual focus mode, just press the Command Dial and confirm sharpness with a zoom view of the focus point.

I'm trying to figure out what this feature actually does, why it appears the way it does, and whether I'm using it in the optimal way.

My guess is that it does the following. It takes the middle 1/9 of the field of view (1/3 horizontally and 1/3 vertically) and displays it with a digital zoom, so that each pixel on the sensor is blown up into a 3x3 block of pixels, big enough so that I can see individual pixels. Does this sound right?

Often, especially when I have bright light and the focus is very nearly correct, I will see a sprinkling of pixels that are completely white. As I then play with the focus to try to optimize it, I see these pixels dance around. The best focus seems to be close to the point at which I maximize the number of these dancing white pixels. Is this a good way to use this feature?

My interpretation of the dancing white pixels is as follows. I think when the focus is good, some pixels really are especially bright compared to their neighbors, and they max out. I think the set of pixels that are like this is probably determined by some combination of (a) focus, (b) EV, (c) motion of the camera if shooting hand-held, and (d) intrinsic differences between the sensitivities of different pixels on the camera. Does this interpretation sound right?

When I'm trying to shoot long exposures with a small aperture, often the view through the viewfinder doesn't work well enough to focus. In this situation, I focus using a wider aperture, then stop down for the actual shot. Does this seem like the right thing to do? It won't let me see the actual depth of field, since the aperture is wrong. I guess I could also take a test shot and then examine the test shot at high zoom, if I could find a way to do that with my camera's user interface.

Answer 1

I cannot speak about the look and feel of the viewfinder view. However I can tell you that it is common practice to open the aperture wide on a SLR when focusing. Your opening the aperture on your mirrorless echoes this procedure. Opening the lens to max aperture reduces the depth-of-field to zilch. This aids the user to reach hard focus for a specific subject distance. Once achieved, we stop down to the working aperture for the shot. The stopping down act expands the zone of DOF so that it likely expands to cover the desired near and far distances.

Side-bar – Photographers working with large format cameras focus by observing the optically projected image on a ground glass screen. It is common practice to use a loop magnifier as an aid to guarantee best possible focus. The magnifying viewfinder mode on your electronic viewfinder is the counterpart of this procedure.

Answer 2

My interpretation of the dancing white pixels is as follows. I think when the focus is good, some pixels really are especially bright compared to their neighbors, and they max out. I think the set of pixels that are like this is probably determined by some combination of (a) focus, (b) EV, (c) motion of the camera if shooting hand-held, and (d) intrinsic differences between the sensitivities of different pixels on the camera. Does this interpretation sound right?

No, this is not right. The white highlights are not pixels which are "maxed out", but rather an overlay on the LCD screen showing areas of maximum contrast — this is a feature known as "focus peaking".

(More later — I just wanted to start by pointing this out.)