Super CCD EXR
The F200 EXR is, as the name suggests, built around Fujifilm's newly developed Super CCD EXR technology. It's a continuation of the Super CCD designs that the company has been developing over the past couple of years but optimized for small-sensor cameras.
The EXR system differs from conventional sensors (the vast majority of which are based around the same underlying idea), in two ways. The first change is in the pattern of different colored photosites (a re-arranged color filter array, if you want to be precise about it). The new pattern means that, unlike conventional sensors, there are always two adjacent photosites detecting the same colour.
|The common 'Bayer' pattern used in most digital cameras||The color filter array pattern of Fujifilm's Super CCD EXR sensor|
The advantage of this layout becomes apparent when you think about what happens when you start 'pixel-binning,' the process of combining information from four or more pixels into a single 'super pixel' to increase sensitivity. Doing so with a Bayer sensor results in a huge loss of resolution (due in part to the need to blur away the color artefacts introduced by you combining photosites detecting different colors). EXR gets over this by combining two pixels (rather than four or more) of the same color, so the increase in sensitivity is more modest, but the loss of resolution is considerably lower too.
|With the Bayer pattern, the nearest similarly colored photosite is always a pixel width away||The EXR arrangement means that there is always an immediately neighboring photosite of the same color|
The EXR arrangement means there are always adjacent photosites of the same color, which means that data from these adjacent pixels can be combined without producing the strong color artefacts (moiré) introduced when binning with Bayer sensors. It is the need to remove these fringes (using blurring) that causes the huge loss of resolution seen with binned output from Bayer sensors. This arrangement, along with some changes to the way the underlying sensor is used, allows the F200 EXR to capture light in three distinct modes:
In most situations, the F200's uses its high resolution 'HR' setting in which it uses the full resolution output from its sensor much like any other camera. This is also how the camera behaves when it's not placed in EXR mode. When the mode dial is set to EXR, it will also use HR mode when it determines that there is enough light for it to offer the best quality result.
In low light situations, the camera can use its SN mode, which makes use of the designed-for-binning color filter array. Because most noise that can build up in an image occurs randomly, combining the results from two photosites helps to average-out that noise. As such, the SN mode aims to improve the signal-to-noise ratio by producing a pixel-binned six megapixel image from the twelve megapixel sensor.
The EXR sensor does more than pixel-binning, though. The underlying circuitry has two read-out channels that take their information from alternate rows of the sensor. The result is that it can act like two interleaved sensors, with different exposure times for each half of the photosites. This means that half of the photosites can be intentionally underexposed so that they fully capture the brighter areas of the scene. This retained highlight information can then be blended in with the output from the other half of the sensor that is recording a 'full' exposure, again making use of the close spacing of similarly colored photosites.
The EXR sensor is able to 'underexpose' half of its photosites by reading out data from them before the end of the full exposure.
Again the arrangement of the color filter array means pairs of pixels can be easily combined.
These techniques clearly promise a lot in terms of addressing some of the shortcomings that can hold back cameras with small sensors. However, these additional sensor modes do mean that some unusual image processing is required, prompting Fujifilm not to provide the RAW sensor data output capability some potential users have asked for.
All diagrams are schematic and do not precisely represent the spacing between photosites. They are also copyright dpreview.com 2009 and cannot be reproduced without written permission.
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