An explanation of Fujifilm’s Super CCD EXR sensor

A look at Fujifilm’s innovative EXR sensor, the latest iteration of its flagship Super CCD sensor, along with some analysis of images from production cameras. Admittedly this would have been more interesting as a speculative piece a year ago, but better late than never

tl;dr: Fujifilm’s EXR sensor is extraordinary, mostly for its dynamic range. If you’re after the best non-DSLR image quality around, your choices start at the Fuji F200EXR, F70EXR, S200EXR, and end there.

Fujifilm has long been a leader in revolutionary sensor technology, particularly at the smaller scale sensor market where the majority of manufacturers have long been content pumping out traditional, vanilla CCD sensors with square grid-based Bayer Filter Arrays.

In September of 2008, announced plans for their latest sensor: the Super CCD EXR, which combines the unique color filter array (CFA) and pixel binning features of various previous sensors into a single “switchable” sensor that can be optimized in one of several areas (which are typically mutually exclusive when designing a sensor): high resolution, high dynamic range, and low noise.

High resolution

High resolution mode is the default mode, which utilizes the full set of photosites on the sensor and produces an image with a corresponding pixel on each photosite – nothing too special here, though Fuji claims the diagonal layout of photosites (as opposed to simple square grid) helps to improve resolution.

High sensitivity

A comparison of a typical Bayer CFA (left) and the CFA on Fujifilm's new EXR sensor (right)

The second mode of operation for the EXR sensor is a high-sensitivity mode which Fuji calls “Pixel Fusion Technology”, which is fancy marketspeak for pixel-binning (combining reading from adjacent pixels together to produce a better signal). With the EXR’s pair-based CFA layout, Fujifilm claims that interpolation (and thus color resolution) will be more accurate because the binned pixels are closer together (e.g. the pair blue pixels are pretty much in the same location, while they’re separated by two pixel lengths in a standard square-grid Bayer array. I don’t know that I buy this argument particularly well – it’s true that same-color pixel values will be more accurate since they’re closer, but you can’t get something for nothing: for example, the average distance from red-to-blue is going to be increased, which lowers accuracy for interpolating blue values at red pixels.

Regardless of whether their CFA and photosite layout nets them better interpolation, the key element here is the combination of pixel readings to generate a stronger signal, thus decreasing the proportion of noise. Using microlenses to patch up the fill factor (area of the sensor which is actually responsive to light) and various optimizations to lower read noise will get the high sensitivity mode EXR sensor closer to the noise level of a natively lower resolution sensor.

Wide Dynamic Range

A diagram detailing the two exposures captured by the EXR sensor when operating in large dynamic range mode

A diagram detailing the two exposures captured by the EXR sensor when operating in large dynamic range mode

The third mode of operation for the EXR sensor uses variable photosite sensitivity to greatly extend dynamic range.  The concept is taken from some of Fuji’s older generation SuperCCD SR sensors – at a given pixel location there are in fact two photosites, one operating at a lower sensitivity and one operating at a higher sensitivity. This essentially produces two images for any particular shot, one at low sensitivity that is underexposed (capturing highlight detail, such as a bright sky), and one at high sensitivity that is overexposed (capturing shadow detail, such as a shaded building face). These images are combined, much like HDR combination is done, to create a single image which captures a much larger dynamic range than a single exposure could.

Edit: dpreview seems to report that the EXR sensor actually achieves this by operating one image at a shorter exposure time (shutter speed) than the other, rather than actually varying the sensitivity. If so this would be even better, as you’d have lower noise due to operating both sets of photosites at the same lower sensitivity.

As with pixel binning for greater sensitivity, the pixel count in the resulting image will have to halve as well.

There are some notable improvements compared to Fuji’s older SR sensors. For starters, the low and high sensitivity photosites are now of equal size, which Fuji claims will allow for a greater dynamic range extension (the SR sensors consisted of mostly “regular” photosites with tiny “low sensitivity” photosites sandwiched in). Furthermore, based on most of the image samples that can be found, the recombination method used for EXR is a bit closer to HDR blending, which doesn’t map values linearly on the same tone curve – this produces a punchier photo with better contrast that still looks natural upon viewing (due to the way human vision judges brightness in relative terms rather than absolute), even if its not quite pixel-accurate. This seems to address one of the complaints about Fuji’s older SR sensors, which provided a large dynamic range but ended up squashing it linearly to the same 12-bit RAWs or 8-bit JPEG images that all other cameras provide – the results were images that did have more highlight detail but looked “flat” and lacked contrast (because that 0.5-1 stop of highlight detail at the top is squashed into a small 250-255 pixel value range).

The EXR sensor has a big advantage over conventional HDR as well (i.e. taking multiple exposures and blending them): it captures an extended range image in a single instance, making it usable for moving subjects (HDR sports photos, yay!).

The Results

The first EXR sensor, the Fujifilm F200EXR, debuted in February 2009, and was followed up not long afterwards by the S200EXR bridge camera and the ultracompact ultrazoom F70EXR, giving us a chance to see some hard results.

Imaging-resource, as always, has perhaps the most comprehensive test bed of images, and samples from their express review of the F200EXR can be found here:

Their site isn’t the most comparison-friendly however (though you can give their comparator a shot) so I’ll link to dcresource’s reviews of the F200EXR and F70EXR as well and reference these.

The first thing to note is that Fujifilm hasn’t lost a step in the noise race – in both the standard high resolution (no binning) and high sensitivity (binning, lower resolution) modes, the EXR sensor simply wipes the floor with every camera on the market this side of a full-fledged DSLR.  In the F200EXR review there is a side-by-side comparison (search for the text “Again, things look great through ISO 400” – it’s right above this) between the 6MP high-sensitivity mode image, and a 12MP high-resolution mode image that is downsized to 6MP – essentially doing the same as pixel binning but off-camera, and digitally, rather than in-camera and analog.  The result is a slightly crisper image but noticeably more noise, though the effect isn’t dramatic.

What’s interesting is that the side-by-side comparison in the F70EXR review shows that the high-resolution mode, downsized to the same resolution as the high-sensitivity mode, actually produces better results – the same amount of noise but much crisper detail. This seems to punch a hole in the effectiveness of the EXR’s in-camera pixel-binning: if the digital data (full of rounding errors, and compressed to 8-bit jpeg) can be averaged and produce more effective results than binning the analog data (the raw readings from the sensor), then we can surmise that having more accurate data on the location of brightness values (i.e. more pixels) helps us produce more accurate images overall than having slightly more accuracy on the actual brightness values.

Further down on the F200EXR review (search “so the two would be the same (6MP) resolution” – right below this), you’ll see a direct comparison using the camera’s wide dynamic range mode. As opposed to the high sensitivity mode, here we can see real, significant benefits – highlight detail that is hopelessly blown out in the left image is very much visible in the wide dynamic range image. For those of you too lazy to navigate the admittedly long and cumbersome dcresource review pages, here’s a marketing image from Fujifilm that gives you the general idea:

Standard dynamic range (left) and wide dynamic range (right) - probably exaggerated a bit

Standard dynamic range (left) and wide dynamic range (right) - probably exaggerated a bit

This image gives you a general idea of the difference, though I wouldn’t take it at face value. The image on the right is probably a fair representation of what you’ll get using the wide DR mode (and you can compare this with shaded interior/sunlit exterior/sky photos you’ve probably taken), but the image on the left has way more contrast (and less DR) than any typical camera would, on its default settings at least.

The Triumph of EXR – Dynamic Range

So is Fuji’s EXR sensor a success? It depends on what you’re after. Many diehard Fujifilm Super CCD fans fell in love with the low-resolution F10/11 and F30/31 ultracompacts, both of which came in at just 6MP and absolutely wiped the floor with the competition in terms of noise performance.  And while subsequent SuperCCD iterations have maintained a clear advantage over competitors in this area (and this newest EXR sensor does to it better than its predecessor), the fact is that the high 12MP or so resolutions found on today’s sensors still compromise noise performance, despite any fancy “Pixel Fusion Technology” that Fujifilm tries to market.

The true triumph of the EXR sensor is in its dynamic range capability, and its separate pixel design (it essentially operates two sensors) works not only better than any of its competitors, but far better than even a natively lower resolution sensor.  While a larger photosite does afford more highlight headroom, halving the pixels (doubling the area) affords at most 1 stop. The EXR’s method, which essentially captures two independent exposures, is in theory capable of capturing dynamic range that is infinitely far apart, though for most scenes they’ll likely need to overlap to avoid gaps in coverage, which based on the settings allowed on current cameras is 3 stops.

According to dpreview’s dynamic range test of the F200EXR, the EXR can deliver nearly 11 EV (stops) of dynamic range.  Not only does that far outclass any compact (or even the bulky SLR-like bridges that use the same small sensors) on the market, but exceeds the dynamic range of DSLRs like the Canon 7D, Nikon D300, et. al, which all range around 8 EV for their jpegs. With a bit of tweaking with RAW files in Adobe Camera Raw, the DSLRs just about manage 10EV.

It’s simply remarkable that jpegs from a camera with a pint-sized sensor can beat out RAW images from the highest-end DSLRs, but that’s what innovative technology can do for you over hammering away with sheer physical size and trying small refinements from there (which is how most of the rest of the sensor industry has been operating for years). I can’t begin to fathom how much the Super CCD would change the landscape of photography if Fujifilm ever scaled up the sensor to DSLR size.

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