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Suppose I am shooting in low light with a 20 mp (compact) camera.

If I change the image size to say 5 mp, can I then use a faster shutter speed / smaller ISO setting?

In other words, is there a trade-off between the image size captured, and exposure? Here, four pixels on the sensor will gather light for each pixel on the image.

Edit:
I have an old Canon A720 8 mp compact camera. When I fix the ISO and aperture settings, changing the image size does not change the (automatic) shutter speed. So it appears that there is a 1:1 correspondence between the sensor pixels and the image pixels. Is this the same with all cameras? Do they just crop the image?

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So it appears that there is a 1:1 correspondence between the sensor pixels and the image pixels. Is this the same with all cameras?

Pretty much, yes. The image pipeline in the camera will always generate a full-size image internally. When you save it as a smaller image, it is downscaled at that point, not when it is read from the sensor; this is why RAW files are always 1:1 with the sensor pixels.

There may be a small image quality advantage in getting the camera to save the 5 MP JPEG rather than a 20 MP JPEG and then downscaling that outside the camera due to the information that will be lost when saving and reloading the 20 MP JPEG but this will probably not be a significant advantage.

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  • \$\begingroup\$ The sequence of events is interesting thank you. What lies behind the question is the choice of a replacement compact camera. \$\endgroup\$ Oct 8, 2022 at 18:09
  • \$\begingroup\$ Honestly, there are no replacement compact cameras. Phones killed the market 10 years ago. \$\endgroup\$
    – Philip Kendall
    Oct 8, 2022 at 18:10
  • \$\begingroup\$ I get that, and "the best camera is the one you have with you". Except for optical zoom. I am looking at high-end compact cameras, because I don't want to carry anything big or heavy or easily damaged. \$\endgroup\$ Oct 8, 2022 at 18:34
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In terms of exposure (light density/noise/etc) everything revolves around light/area, not light/pixel. If you have more pixels in an area it just means each gets less light, but the total remains the same.

Many make the mistake and correlate that larger pixels perform better in low light with less noise. On its' own, that is true. But it is equally true to say that smaller pixels provide greater resolution of the noise, making it more noticeable at greater magnification.

The real difference is whether the larger pixels are larger because they are on a larger sensor of the same resolution... then there will generally be an increase in light/area and low light performance. But there will still be no difference in exposure settings (Ap/SS/ISO).

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  • \$\begingroup\$ I wonder if any cameras apply more aggressive low-frequency filtering in low-light conditions than in brighter conditions? In low-light conditions, much of the higher-frequency content will be noise which is simultaneously hard to compress but lacking in useful informational content. Using a lower resolution setting will inherently apply a low-pass filter, but at the expense of reducing detail in brighter parts of the image. \$\endgroup\$
    – supercat
    Oct 9, 2022 at 18:45
  • \$\begingroup\$ @supercat, most cameras will apply heavier filtering in lower light, based on the use of higher ISO. This is typically only done digitally for camera generated jpegs, but some cameras have also been known to filter raw data for noise as well. \$\endgroup\$ Oct 10, 2022 at 12:55
  • \$\begingroup\$ The main purpose of filtering would be to minimize the portion of JPEG bandwidth used to capture noise. It's been ages since I've bought any camera beyond the ones included in cheap phones, so maybe low-light noise levels have been improved. \$\endgroup\$
    – supercat
    Oct 10, 2022 at 14:35
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Exposure is always about the amount of light collected per unit area. Thus, if a sensor is four times larger in area (two times larger in linear measurements) than another, it has to collect four times as much light to accomplish the same exposure.

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