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Contrary to conventional wisdom, higher resolution actually compensates for noise

Sunday November 16 2008

Sensor Insight
Introduction | Modeling small pixels | Comparing the SNR of cameras with different resolutions | Real-life comparisons | Conclusion
Contrary to conventional wisdom, higher resolution actually compensates for noise

This Insight uses specific 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?

Consider, for example, the Canon EOS 350D, a model released in February 2005 and a very good camera for expert amateurs, and the Canon EOS 1Ds Mark III, a more recent (August 2007) professional DSLR camera, with a 21Mpix sensor. Both sensors have the same pixel pitch (6.4µm), and measurements of SNR on the RAW images under equivalent shooting conditions give similar results: 35.25dB for the 350D, 35.05dB for the 1Ds Mark III.

Of course, at the same focal length the 1Ds Mark III can see either smaller details or have a wider field of view (the sensor is full frame 36x24mm, while the EOS 350D is 22.2x14.8mm). Moreover, the measurement above corresponds to 1:1 viewing conditions on a screen. Except for image retouching or examining details, this is probably not the most useful way to view the image. A fit-to-screen viewing or a print on a given-size paper format is a more likely use case.

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 of about 3dB to the 1Ds Mark III.