Nikon's new SLR leads the pack for sensor quality

It's not a surprise that the Nikon D3X, the company's brand new 24.9-megapixel SLR, tops DxO Labs' sensor performance test. What is a surprise is the margin by which it leads its rivals from Canon and Sony.

The top four SLRs in DxO Labs' current rankings. (Credit: DxO Labs)

When the French firm unveiled its DxOMark Sensor benchmark test last year, Nikon's D3 was the top scorer at 80.6, a composite number that represents various performance features. Very close on its heels were Nikon's D700 at 80.5, Canon's EOS-1Ds Mark III 80.3, and later Canon's 5D Mark II at 79 and Sony's Alpha A900 at 78.9.

All those cameras were close, but the D3X stands apart with a score of 88. The result shows how much ground Nikon has made up on Canon, which has dominated high-end digital SLR technology.

DxO Labs said cameras must be about 5 points apart for people to see a difference; a 15-point difference is equivalent to a full stop of improvement, meaning that a camera would have the same image quality as a rival with half the available light.

The test measures a sensor's colour depth, dynamic range, and low-light performance through a variety of detailed tests of the camera. No lens is mounted during the test of the SLRs, so differences from optics are minimised. Unlike other tests, DxOMark Sensor measures performance based on the cameras' raw-format images, which eliminates changes that the camera or software make when converting the raw data into a recognisable photo.

DxO has real chops in the testing business; the company makes a business selling analytical tools and services to camera makers and others. Don't confuse the DxOMark results with a full evaluation of a camera, though; other significant factors include autofocus, in-camera processing, durability, optics, interface, accessories, customer support, and availability of third-party and used lenses.

The Nikon D3X beats out the D3 and Canon's 1Ds Mark III in image sensor performance.
(Credit: DxO Labs)

Sony builds the D3X's sensor with Nikon dictating much of the design.

How exactly did the D3 fare better than its top rival, the 21.1-megapixel 1Ds Mark III, which currently goes for about AU$11,999? A notch better in colour depth, which measures the fineness of gradations between different hues, a big notch better in low-light performance, and a very big notch better in dynamic range, which measures how well the camera can accommodate details in bright and dark areas at the same time.

There's been a lot of teeth-gnashing over the price of the D3X, especially given how much it shares with the D3, which still beats out its higher-end sibling on low-light performance and shooting speed. But with most technology, top-end products sell in vastly smaller quantities, and the higher up the ladder you go, the more each step costs.

All the top-scoring cameras employ full-frame sensors, which are the size of a frame of 35mm film from days of yore. The attendant larger pixels and extra light-gathering ability allows much lower noise. Most SLRs sold use smaller, vastly cheaper sensors, of which 200 can be carved from an 8-inch silicon wafer compared to 20 many fewer full-frame chips, according to a Canon white paper on sensors (PDF) from August 2006 that's still relevant.

Compact cameras use yet-smaller sensors, which shows in part why the performance even of high-end models such as Canon's newer PowerShot G10 significantly trails even elderly, low-end SLRs.

The next big shake-up on the DxOMark list should come in the next few weeks, when DxO Labs adds results from medium-format digital cameras, whose sensors have about twice the area of full-frame SLRs. That will help show just how big a competitive threat these high-end, high-megapixel SLRs are for the studio photographers who are the biggest customers of the super-expensive medium-format models.

DxO Labs also added a new reviews section that describes cameras. So far the D3X is the only camera so featured.

Brace yourself for a big tangential digression. Illuminata analyst Gordon Haff in a chat with us pointed out that the Canon wafer comparisons from the white paper seem off base, and we agree.

Here's how the white paper expresses the figures comparing the smaller sensor size, called APS-C, with full-frame: "If the sensors are APS-C size, there are about 200 of them on the wafer, depending on layout and the design of the periphery of each sensor... Full-frame sensors? Just 20." Next to those words is an image of an 8-inch wafer.

The area of an 8-inch wafer, also known as a 200mm wafer, is about 31,416 square millimetres, which isn't enough for 95 even APS-C sensors, much less "about 200". Newer chip manufacturing processes use 300mm wafers with an area of about 70,686 square millimetres, and that would be big enough for 200 APS-C sensors, each of which has an area of about 330 square millimetres.

Indeed, when we asked him about it, Canon Technical Advisor Chuck Westfall told us, "The machine that Canon uses to manufacture its CMOS image sensors for EOS digital SLRs does in fact use a 300mm wafer ... Canon has had the capability to use 300mm wafers since at least 2006."

Westfall stood by the figures in the white paper, but in our view the comparison is misleading. It compares how many APS-C wafers fit on a 300mm wafer to how many full-frame sensors fit on a 200mm wafer. But certainly the same overall economics apply: more APS-C chips than full-frame will fit on a given size wafer.