Tuesday, January 13, 2009

More resolution = less noise

Most photographers realize that the more resolution your camera captures, the better. How does that relate to noise in your image? Especially when it goes to print. DxO Labs has the answer. They did an independent complete analysis of signal-to-noise ratio (aka SNR) with two Canon model cameras. The results might surprise some people.

DxO is a free resource that offers non-biased in-depth technical analysis of RAW-based image quality. There is a wealth of data on their site. They are highly recommended by places like Popular Photography.

Contrary to conventional wisdom, higher resolution actually compensates for noise

This Insight uses currently-available DSLRs to demonstrate the technique for objectively comparing noise for cameras with different levels of resolution. Such comparisons conclusively show better results overall for high-resolution sensors, despite the increase in noise.

You like the look and feel of your old camera, but you want to upgrade it to a higher resolution. If you keep the same optics, the same sensor size (and therefore the same field of view), what will be the result? Basically, the same amount of light captured by the optics will be shared by more pixels, and since each individual pixel will receive less light, the signal-to-noise ratio (SNR) will be lower. In short, you will end up taking photos with more noise.

Two questions naturally arise:
  • How can the noise from two cameras with different levels of resolution be fairly compared?
  • Which is better, a low-resolution camera with less noise, or a high-resolution camera with more noise?

...Suppose now that the images have exactly the same field of view (by changing the lens focal length) and are printed on 30x20cm paper with the same 300dpi printer. Because of the very high resolution of the 1Ds Mark III, the printer will downsample the image and decrease the noise, giving a clear advantage (about 3dB) to the 1Ds Mark III.

Imagine a low-resolution sensor. A higher-resolution sensor can be obtained by cutting each pixel of the low-resolution sensor into four smaller pixels.

For a given exposure time, each smaller pixel receives four times less light than a large pixel-the equivalent of reducing exposure time by a factor of four. So to get the same sensor response, exposure time needs to be multiplied by four, which means that the ISO sensitivity of the high-resolution sensor is four times less than the low-resolution sensor.

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