Of pixels and millimeters

Reader Dave asks:

“I have a technical question about sensors and pixels… I understand that larger sensor sizes gather more light and thus are better in low light situations and that more pixels will help make a sharper picture… but why do too many pixels make noise and what is the perfect balance? I.e. how many pixels would be good to have on a 1.6 cropped frame sensor?”

Well, the perfect balance will be a challenge. But indeed, this is a balance. Make the sensor larger and you get less noise. Make it smaller with the same number of pixels, and you get [corrected:] more noise. It is not “too many pixels” that make noise. It is “too many pixels stuck together into a small place, meaning they are small pixels”. The smaller you make a phototransistor, the more noisy it gets. So the more pixels you stick in a given area, the more noise.

So to reduce noise you can increase the sensor size, or reduce the area.

That’s why

  • Canon have reduced the pixel count on the G11 compact camera from the G10. Fewer pixels in the same area sensor gives you less noise.
  • My 1Ds MkIII creates less noise than the 7D. They have about the same pixels, but a larger sensor (the 1Ds is full frame, the 7D is 1.6 times smaller) means less noise.

Does that make sense?

0 thoughts on “Of pixels and millimeters

  1. You said “Make it smaller with the same number of pixels, and you get less noise” – shouldn’t it be “more noise”?

    But otherwise, yes, that’s a very clear explanation.

    Have you written any books on photography? It seems like a logical next step to your teaching to reach a wider audience.

    Is there an objective measure of sensor noise? I am sure Canon et al must have ways to measure it for design and QC purposes but they likely keep the numbers to themselves.

  2. That should read “more” – oops! I’ll go back and edit it…

    Oh yes, there are objective measures of noise – the signal-to-noise-ratio is the bane of an engineer’s existence. But indeed the numbers are not published by Canon or anyone else.

    Bit like batteries. “Energizer bunny” and “emergency response forces trust Duracell”: content-free advertising, where they could just publish the number of mAh!

  3. Well on the G11 canon didn’t choose to make a new sensor that has low pixel density. They simply selected the new “high sensitivity” sensor from Sony. Which happens to have less megapixel than the G10. The thing is while there’s improvement at higher ISO it’s not night and day difference. I think if Sony makes a higher MP version of the sensor next year we’ll see a G12 with higher megapixel again.

  4. True they did not make a new sensor, but they chose to put in a lower pixel sensor – good decision imho, and a welcome one. If the next gen sensor is low noise with more pixels, all the better.

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