Fujifilm X-Trans CMOS sensor
Fujifilm has along history in designing its own unique sensors which don't use conventional Bayer-pattern colour 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 colour filter array over the pixels has been completely redesigned. The result, according to Fujifilm, is minimal susceptibility to colour moiré, which in turn allows the company to dispense with the anti-aliasing filter that's used by almost all other cameras. In principle, this means the X-Pro1 should be able to resolve more detail than Bayer-array cameras with a similar pixel count.
The color filter array
Almost all digital cameras use what it 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 RGGB layout.
However, one problem with the Bayer array is its susceptibility to false colour artefacts when faced with an image that contains finely-repeating patterns (such as textiles), caused essentially by interference between these patterns and the regular grid of photosites. This results in unsightly bands of color, and 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' colour filter array to make it look more irregular would have a similar effect. The result is the X-Trans CMOS's 6x6 colour filter array, with red, green and blue photosites on each row and column (diagrams courtesy of Fujifilm):
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| The common 2x2 'Bayer' pattern used in most digital cameras | The 6x6 color filter array pattern of Fujifilm's X-Trans CMOS sensor |
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Use of an unconventional CFA is not without its complications, though; most obviously, it demands a completely different demosaicing algorithm for RAW conversion, which complicates third-party RAW support. The more-complex demosaicing also demands an upgraded in-camera processor, which Fujifilm calls the 'EXR Processor Pro'.
The elimination of the low-pass filter has knock-on benefits - it allows the shutter to be set closer to the sensor, which in turn enables more flexibility in lens design as the 'back focus' distance from the rear lens element to the sensor can be shorter. This is reflected in the lens mount specifications for the all-new 'X' mount, which allows a back focus distance of a mere 10.2mm.
All-electronic X mount
A new camera system requires a new lens mount, and Fujifilm has duly complied with the 'X' mount. It offers few surprises to anyone who's been following recent developments in mirrorless camera systems, being an all-electronic bayonet mount with ten contact pins for communication between the camera and lens. The lenses released alongside the X-Pro1 have traditional-looking aperture and focus rings, but these have no direct mechanical coupling, and do nothing when the lens is dismounted from the camera.
The X-mount's claim to fame, though, is an extremely short flange distance (from mount surface to sensor) of 17.7mm - shorter even than Sony's E-mount for its NEX system. The lenses themselves feature unusually short backfocus distances from the rear element to the sensor, and use large rear elements to maximise the illumination of the corners of the frame. The diagram below (again supplied by Fujifilm) illustrates this principle, here with the 18mm lens:
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