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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

Ever since the first digital cameras appeared, there has been a quest for more pixels. In large part, that’s because the more pixels an image contains, the finer detail it can present and the bigger we can blow it up before the pixels become visible to the eye. Today, we have compact digital cameras with nearly 15 megapixels, 35mm form-factor D-SLRs with 24-plus megapixels and medium-format D-SLRs with 60-plus megapixels.

Nikon Full-Frame Sensor

Let’s go over some terminology. “Pixel” is short for “picture element,” so the term really applies to the elements making up a digital image, not to the tiny “wells” on the image sensor that collect light and determine the sensor’s “pixel” count; those are photodiodes. Generally, with D-SLRs, the pixels making up our digital images correspond to the photodiodes on the image sensor: A 12-megapixel sensor containing 4256 photodiodes across by 2832 photodiodes down produces images consisting of 4256 pixels across by 2832 pixels down.

There’s also the term “photosite,” which generally refers to the light-capturing “pixels” on the image sensor. As long as we all know what we mean, we can use the terms “pixels” and “photosites” to mean both the light-sensitive photodiodes on an image sensor and the tiny squares that make up a digital image.

Medium-Format CCD Sensor

Okay. More pixels mean more detail and bigger prints. But as they say, here’s the rub: To get more pixels on a given-sized image sensor, we have to make them smaller. And the smaller the photosites, the less effectively they collect light. This limits the higher end of the camera’s ISO range. Smaller pixels also produce a lower signal-to-noise ratio because there’s less signal for a given amount of noise, and this can adversely affect image quality at all ISO settings, but especially at the higher ones. Another drawback to smaller pixels is that they can hold only so many photons—if more hit them, they’ll overflow, blowing out highlights and reducing dynamic range.

Photosites on image sensors come in a range of sizes, all quite tiny. In current D-SLRs, they range from 4.29 microns square in the 12.3-megapixel Four Thirds System cameras to 8.45 microns in Nikon’s 12.1-megapixel, “full-frame” D3 and D700. A micron is one-millionth of a meter; an average human hair is around 100 microns thick. Even the largest photosites are tiny.