Does lowering image size let you use a faster shutter speed or lower ISO?
Asked 10/8/2022
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If a camera offers smaller image sizes, does selecting a lower resolution (for example 5 MP instead of 20 MP) allow you to use a faster shutter speed or lower ISO in low light? Or does the camera still capture the full sensor image and only downscale afterward? I noticed on an older Canon compact that changing image size did not change the automatic shutter speed when ISO and aperture were fixed.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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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.
Originally by user11371. Source · Licensed CC BY-SA 4.0
user11371
3y ago
0
Generated from our catalog & community — verify before relying on it.
No. Changing the saved image size usually does not change exposure. The camera still reads the full sensor and makes a full-size image internally, then downsizes it when saving a smaller JPEG. That’s why RAW files normally match the sensor’s native resolution.
Exposure depends on the amount of light collected per unit area of the sensor, not on how many output pixels you choose to save. So selecting 5 MP instead of 20 MP will not normally let the camera use a faster shutter speed or lower ISO.
What can change low-light performance is sensor size, not just pixel count. Larger pixels can look cleaner at the pixel level, but smaller pixels simply show noise at finer detail. If two cameras have the same sensor size, reducing output resolution alone does not change the exposure settings needed.
In short: on most cameras, smaller image-size settings mean downsampling after capture, not combining sensor pixels during exposure.
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