Fujifilm X-Trans CMOS II sensor

Fujifilm has a long history in designing its own unique sensors which don't use conventional Bayer-pattern color filter arrays. The X-Trans CMOS is the latest design to emerge from its out-of-the-box thinking, and while it uses a conventional square-grid pixel layout (unlike the company's EXR compacts), the color filter array over the pixels has been completely redesigned.

The result, according to Fujifilm, is minimal susceptibility to color moiré, which in turn allows the company to dispense with the anti-aliasing filter that's used by most other cameras. In principle, this means the X100S should be able to resolve more detail than Bayer-array cameras with a similar pixel count. The 'II' version of the sensor adds another trick; on-chip phase detection autofocus.

The color filter array

Almost all digital cameras use what is called a Bayer color filter array, named after the Kodak engineer who developed the pattern. Over the years it's proved to be an excellent way of capturing both color and detail in a scene. Essentially, it consists of a simple repeating pattern of four pixels, two of which are sensitive to green light, one to red and one to blue, in a square RGBG layout.

However, one problem with the Bayer array is its susceptibility to false color artefacts when faced with an image that contains finely-repeating patterns (such as textiles), caused by interference between these patterns and the regular grid of photosites. The result is unsightly bands of color, which in most digital cameras is suppressed by the addition of an optical low pass (or 'anti-aliasing') filter in front of the sensor that blurs away the finest image detail. This reduces any moiré patterns, but with an inevitable loss of resolution.

Film never showed an analogous effect due to its random grain structure, and Fujifilm's engineers reasoned that modifying the sensors' color filter array to make it look more irregular would have a similar effect. The result is the X-Trans CMOS's 6x6 color filter array, with red, green and blue photosites on each row and column (diagrams courtesy of Fujifilm):

The common 2x2 'Bayer' pattern used in most digital cameras The 6x6 color filter array pattern of Fujifilm's X-Trans CMOS sensor

Use of an unconventional CFA is not without its complications, though; most obviously, it demands a completely different demosaicing algorithm for Raw conversion. This has proven to be a problem for third-party raw conversion software, as we documented in our review of the X-Pro1. Fortunately, support has improved a lot since the X-Pro 1 was released, with Adobe's Camera Raw support getting noticeably more robust over a couple of recent iterations of the plugin, as well as solid support from Capture One.

Even if you never process Raw files on a computer, Fujifilm's superb out-of-camera JPEGs and flexible in-camera raw conversion are genuine selling points of the X100S and indeed the X-system as a whole.

On-chip phase detection autofocus

Almost all compact and mirrorless cameras use contrast-detection autofocus - in its simplest form, this racks the lens through its range of focus distances and picks the one that delivers the highest contrast at the selected AF point. Once upon a time this was painfully slow, but focusing algorithms and lens designs have improved substantially over the past few years, and it can now be extremely fast and accurate. But it struggles in some situations - most notably with moving subjects.

This cross section of the X-Trans CMOS sensor shows the following:

1) Microlenses
2) X-Trans color filter
3) Left/Right light interception filter
4) Phase detection sensor / green filter pixel
5) Photodiode

Diagram courtesy of Fujifilm

One solution to this is to use phase-detection autofocus. In short, this is able to tell from a single measurement exactly how to adjust the lens to achieve an in-focus image. All SLRs by necessity use this approach, with a dedicated autofocus sensor. But it's becoming increasingly common on mirrorless cameras too, using a system that's integrated onto the main imaging sensor. It's employed most successfully by Nikon's 1 System cameras, but other manufacturers such as Sony and Canon use similar technology, with varying degrees of success.

Fujifilm was in fact the first manufacturer to bring on-chip PDAF to market, with its F300EXR in 2010. But it's appeared curiously reluctant to exploit the technology in higher-end models until now. Both the X100S and the smaller-sensored X20 use the system, which delivers much-improved focus speeds over their predecessors. The PDAF sensors are localized towards the center of the frame, and use pixels that are masked to receive light from the left- and right-sides of the lens's exit pupil. The difference between the images coming from these two paths allows the focus distance to be determined. We'll be examining how well the system works in the performance page of this review.