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Tuesday, October 13, 2009

Comparing Photosites

In the race for higher resolution, there are trade-offs with image quality. It all comes down to the limits of the image sensors and the individual photosites on those sensors.

This Article Features Photo Zoom

Of course, a digital camera’s photosites are just part of the image-quality equation. Besides improved pixels, newer cameras feature a number of other technological improvements that result in better image quality for a given pixel count or pixel size. For one thing, there’s better onboard noise-reduction technology, both for long exposures and for high ISO settings. Image processors are more powerful and thus able to apply better processing algorithms. Many D-SLRs now use 14-bit A/D conversion, which provides four times as many tones or colors as 12-bit for smoother gradations and more accurate colors (many medium-format D-SLRs use 16-bit conversion for another fourfold increase in gradations). Newer amplifier designs reduce image noise. Gapless microlenses over each photosite increase light-gathering area and capability. Better low-pass filters and even Bayer RGB filters also improve image quality with newer cameras.

An example of newer being better is Nikon’s top-of-the-line D3X. It features a 24.5-megapixel, full-frame (35.9x24.0mm) sensor with 6048x4032 photosites, which translates to a pixel size of 5.94 microns—slightly larger than the D300’s 5.5 microns, but considerably smaller than the 8.45-micron photosites of the D3’s 12.1-megapixel, full-frame sensor. Based on pixel size alone, we’d expect the D3X’s image quality to be somewhere between the D3’s and the D300’s. However, the D3X produces the best image quality of the trio. (In fact, the D3X’s RAW image quality is second only to that of a $30,000, 60.5-megapixel medium-format D-SLR in DxO Labs’ sensor-performance ratings.) Two things primarily account for this: 1) the D3X’s newer technology produces better image quality than the older technology in the other two cameras; and 2) the D3X has twice as many pixels as the D3 and D300. When making a print, the D3X’s image can be downsampled to match the D3’s resolution, resulting in reduced image noise.

Likewise, the Canon EOS 5D Mark II uses essentially the same sensor as the older (and more costly) EOS-1Ds Mark III, yet produces better image quality and higher ISO capability (along with HD video capability). The EOS 5D Mark II’s pixels also are somewhat smaller than those of the 10.1-megapixel EOS-1D Mark III (6.4 microns vs. 7.2), but the 5D Mark II has newer technology and so many more pixels that it delivers better-looking prints of any given size, especially at sizes beyond the EOS-1D Mark III’s 10.1-megapixel limit.

Bottom Line
If you need to make huge blowups, you want lots of pixels. If you need to shoot in dim light at high ISOs, you want big pixels, but D-SLRs with higher pixel counts tend to produce a greater dynamic range and higher bit depth than D-SLRs with lower pixel counts. In any event, newer D-SLRs produce better image quality than older ones, due to improvements in sensors, image processors and A/D conversion.

RAW Converters
JPEG images are processed by the camera and come out “ready to use.” RAW images are just data until you process them using RAW conversion software. This software demosaics the data (converts it to color), maps the RAW colors to sRGB or Adobe RGB color space and applies (at your selection) sharpening, tone curve and other processing. Just as today’s digital cameras are better than their predecessors, so are today’s RAW converters. It’s worth it in terms of image quality to have the latest RAW converter, be it the camera manufacturer’s or a third-party option, such as DxO Optics Pro, Adobe Camera Raw or Phase One Capture One, to get the best out of your RAW images.


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