The Fujifilm X100S is the latest in a recent rush of cameras to include phase-detection elements on its imaging sensor, giving an AF system that is a hybrid of contrast and phase-detection methods. However, Fujifilm also uses this system to provide a unique and incredibly clever manual focus aid - which could finally allow digital cameras to offer the speed and convenience enjoyed by manual-focus SLR and rangefinder users. Fujifilm UK has posted a video showing 'Digital Split Image' focusing and Japanese camera site DCWatch has published details that allow us to show how it works.

What Digital Split Image focusing offers

The X100S's Digital Split Image system splits the central section of the camera's live view into four black-and-white stripes of the scene. These stripes line-up when the camera is in focus - in a similar way to a split prism viewfinder on a manual-focus film SLR. This gives a method of achieving manual focus while retaining a view of the entire scene, making it possible to assess focus and framing at the same time. This speed and convenience is what has helped focus peaking (a feature the X100S also offers) become a highly desirable feature in mirrorless cameras.

How it works

The best way of understanding how the system works is to understand how on-sensor phase detection works. Because the Digital Split Image focusing system is essentially presenting the camera's phase detection information visually.

Fujifilm was the first company to offer on-sensor phase detection in one of its cameras, when it launched its F300 EXR and Z800 EXR compact cameras. When they were launched, we explained how the system works. The key thing is that the light entering the left-hand-side of the lens will only give the same image as the light from the right-hand-side when the lens is in focus. So if you can 'look at' the left- and right-hand sides separately, you can compare the two images and calculate how much you need to move the lens to get them to match up.

Roll-over the buttons below to show what happens as you focus the lens:

Back Focus
In Focus
Front Focus
In this simplified schematic, you can see what happens to the image cast by the light passing through the left (blue dotted line) and right (red dotted lines) sides of the lens.

When in focus, the light from both sides of the lens converges to create a focused image. However, when not in focus, the images projected by two sides of the lens do not overlap (they are out of phase with one another).

Of course this is a massively simplified diagram with a single, vertical straight line as the subject (and no inversion of the image as it passes through the lens). The point is that we can derive information about focus if we can separately view light coming from opposite sides of the lens.
 
How does a phase detection sensor 'see'?

And we don't need the whole image to do this. Think about a strip of pixels taken from the sensor in the previous diagram. If you could make one such strip that receives light only from the left hand side of the lens and another that 'looks' only to the right-hand side of the lens, then you have enough information to find focus.

By comparing images from just these two strips it's possible to work out not only how far but also in which direction the lens needs to be moved to bring them into phase.
Back Focus
In Focus
Front Focus

As the slide published by DCWatch shows, the Digital Split Image system works by showing these left and right-looking images in the camera's live view - allowing you to visualize what the phase-detection system usually 'sees.' The first and third stripe shown in the viewfinder shows the image being gathered by the left-facing pixels while the second and fourth stripe show what the right-facing pixels are showing. Matching them up (bringing them into phase), brings the image into focus.

Fujifilm explanation of Digital Split Image focusing - from DCWatch

The focus region is black-and-white because the left- and right-facing pixels are all 'green' sensor elements, so aren't capturing any information about any of the other colors in the scene.

If the feature works in practice (and our brief hands-on experience is positive), it's a feature we expect to see in an increasing number of cameras - particularly mirrorless models, many of which still lack a quick method for assessing manual focus. Sadly, because they lack phase-detection elements, it's not a feature Fujifilm can add to its existing X-Pro1 and X-E1 models.