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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.

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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.

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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.)

  • I agree with you about the pixels not being "maxed out", but it's not clear in the question which focus modes OP is using. I use red as my highlight color, and the "dancing" seems different from the highlight. I also see them in split image mode, but not in standard mode. – xiota Jun 11 at 3:05
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I have a FujiFilm X-E1 mirrorless digital camera...

I have not used an X-E1. However, I do have an X-E2 and X-T20. Much of my response is based on my experience with these later models.

... that has a magnification feature designed to assist with manual focusing.

The X-Series cameras have three focus-assist modes. They all allow the focus area to be magnified.

  • Standard: Magnification only.

  • Focus Peak Highlight: Areas of high contrast are highlighted with a selected color: white, red, or blue.

  • Digital Split Image: Central region of the display is divided into bands that can be aligned for focus. Appears to use phase detect pixels to form the bands.

... the distance index bar is displayed together with the depth of field scale and aperture value...

Display of these additional information require the use of XF/XC lenses, along with appropriate display settings. They do not appear with adapted manual or electronic lenses (EF). Based on one of your other questions, it seems you are using K-mount lenses with adapter. (Troubleshooting a shutter that doesn't actuate)

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...

Based on this description, I would guess you are using the digital split image mode. It is the only mode limited to the center of the viewfinder. The other modes allow a focus point to be selected and magnified.

... 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.

Using specs of the X-E1, the sensor has 16mp (4896 × 3264). The EVF has 2.36M dots. If we divide the sensor dimensions by 2.6, we get 1883 × 1255 = 2363165. So it seems the EVF resolution is good enough to display one dot per pixel (after demosaicking). Since these are "dots", 2/3 of color per pixel is dropped.

I will see a sprinkling of pixels that are completely white... 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.

The pixels that "dance around" are visible to me in both peak highlight and digital split image, but not standard mode. I describe them as a "shimmer". I too have found that best focus often corresponds with maximizing the visibility of these pixels.

My interpretation of the dancing white pixels is ... some pixels really are especially bright compared to their neighbors, and they max out.

I doubt any pixels "max out", which would indicate complete loss of information (also described as "blown out"). I suspect their appearance is related to noise and moire. The shimmer is present with fine, high-contrast details (moire), and low light seems to fill the entire screen with dancing pixels (noise).

I suspect they appear in peak highlight mode because of contrast enhancement. They appear in digital split image because of the use of phase detect pixels. But they do not appear in standard mode because neither condition is present.

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.

This is especially true of digital split image because large aperture is needed to show the phase difference. Focus peak highlight may work better. However, the larger depth of field is, the more difficult it would be to find the "point" of maximum focus. Also, too little light in any mode makes focusing difficult.

I focus using a wider aperture, then stop down for the actual shot.

This is reasonable to do with manual lenses, as Alan Marcus points out. It is what would happen automatically if you were using native XF/XC lenses. Some adapters (FD) have a switch to control the aperture without changing the setting on the lens.

It won't let me see the actual depth of field, since the aperture is wrong.

Depth of field always increases when you stop down from wide open. So there is little need to see the "actual" depth of field while focusing. You can trust that if you have difficulty nailing "perfect" focus, increased depth of field will likely improve it.

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