Digital split image appears to use phase-detect pixels in the center of the viewfinder. Align the bands for gross focus. Look for the "shimmer" to fine-tune focus.
When active, the focus area is always visible in the center of the display. On older cameras, the focusing area is monochrome. On newer cameras, color is available.
There are no focus points to select, so it is necessary to focus and recompose.
This mode works better with larger apertures. The split is smaller and more difficult to see with smaller apertures. With manual lenses, it's helpful to open the aperture for focusing. Some adapters can open and close the aperture without changing the setting on the lens.
Lenses with spherical glow or too much contrast may be used with split image, even though they may not work well with peak highlight.
Although film simulation does alter the appearance of the display, it does not affect my ability to focus with split image.
Start turning the focusing ring with large, rapid movements. Each time you reverse direction, move it more carefully and slowly.
The focusing region is divided into four horizontal bands. Aligning high contrast edges that cross from one band to another achieves focus. The bands are easier for me to see and align in monochrome mode. The larger the aperture, the more accurate focus will be, but for most lenses, only gross focus can be achieved consistently.
Press the focus assist button to magnify the focusing area. Only one magnification level is available. If you have enabled a 6x4 grid, the magnified area corresponds with the central 2x2 region on 24mp sensors. (It is smaller on previous generation sensors.) It is divided into four bands that can be aligned as before. You may check their alignment if you wish.
For fine focus, look at an area with fine detail and contrast that you wish to focus. As you turn the focusing ring, you should notice a "shimmer" come and go. It is easier for me to see the shimmer in color. Turn the focusing ring back and forth until you find the point with maximum shimmer. The lens should now be focused.
Some subjects are easier to focus with digital split image. Others are easier to focus with peak highlighting. To switch focus modes, hold down the focus select button (thumb wheel). The modes cycle: standard → split image → peak highlight. The "shimmer" is visible in split image and peak highlight, but not in standard mode.
