Fujifilm X100S Digital Split Image focus – how it works |
Digital Photography Review
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. 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……..
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