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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

ABOVE: The typical CMOS sensor diagrammed with the photosite and the attending local amplifier. The more photosites, the more amplifiers, and therefore more power consumption and heat. Heat creates noise in the image, so manufacturers work to reduce the power consumption, as well as use image processing to reduce noise.

Photosites on today’s medium-format D-SLRs come in a similar range of sizes: 6.0 to 9.0 microns. The medium-format cameras have much larger image sensors, so they can provide either more photosites than 35mm form-factor D-SLRs or larger ones for a given pixel count. Pixel counts for current medium-format D-SLRs range from 16 to 60.5 megapixels. Just for comparison, most compact consumer digital cameras have sensors about the size of a fingernail with commensurately sized pixels: 1.7 microns is a common size.

Sheer photosite size doesn’t necessarily mean better light-gathering ability, however. What’s important is the size of the light-gathering area. A photosite consists of the light-gathering “well,” plus the necessary electronic circuitry to operate it and move the data along the imaging pipeline. Manufacturers work hard to minimize the size of this circuitry so the light-gathering area can be larger for a given photosite size. From a practical standpoint, this means newer D-SLRs, with their greater “fill factors,” produce better image quality than earlier ones, even with the same, or slightly smaller, overall gross pixel size.

Cameras with sensors that have larger photosites tend to generate better color and higher dynamic range (because of a bigger, clearer source in relation to the background noise from the charged chip) and better performance at higher ISO. For example, the Nikon D700 and Nikon D3 rank #1 and #2 in DxO Labs’ high-ISO performance rankings, but #3 and #4 overall (www.dxomark.com). These full-frame (Nikon FX-format) cameras produce the best high-ISO performance, in part, because they have the biggest pixels among current models.

Heat is a major concern among camera manufacturers because heat on the sensor increases image noise. To mitigate the heat, manufacturers use several tactics. Ultimately, heat comes from power consumption. All other things being equal, a CMOS sensor will draw less power and generate less heat and noise than a CCD sensor. However, in practice, all things aren’t equal, and manufacturers work to reduce heat and image noise through several approaches. Choice of sensor technology is one approach, as are pixel spacing on the sensor and onboard image-processing technology.

Noise also affects bit depth, the number of tones from white to black (or number of shades of a color) a sensor can deliver. The camera’s A/D converter converts the analog data from the image sensor to digital data, which theoretically consists of 12-bit (4,096 possible tones) or 14-bit (16,384 tones) output per color channel (red, green and blue). But the noise level isn’t identical for each channel, so the actual bit depth of the image is less than the theoretical 12 or 14 bits. (And keep in mind that JPEG images are 8-bit, with just 256 tones.) Among today’s D-SLRs, those with the most megapixels tend to produce the highest actual bit depths; in DxO Labs’ RAW sensor tests, the 60.5-megapixel, medium-format Phase One P65+ produced the highest actual bit depth (in part because of its 16-bit A/D converter) followed by Nikon’s 24.5-megapixel D3X; the highest under-24-megapixel camera rated 10th on the list. (Again, it’s a balance between pixel count and sensor size; the highest-rated APS-C D-SLR ranked 14th.)

Phase One’s P+ Sensors So lots of pixels provide lots of detail and let you make big blowups, while big pixels provide good high-ISO performance with low noise. Wouldn’t it be nice to have the best of both worlds? Phase One’s P+ cameras and backs provide this. The 645-format P65+ provides a whopping 60.5 megapixels. But you can switch to Sensor+ mode, and the camera will combine the analog signals of four adjacent pixels into one “super pixel,” reducing resolution to “only” 15 megapixels, but increasing sensitivity and reducing noise greatly in the process.