Will averaging several high-ISO shots match the noise of one longer low-ISO exposure?
Asked 2/2/2011
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On a tripod with a perfectly still, dark scene, how does noise compare between:
- several short exposures at high ISO that are averaged together, and
- one longer exposure at low ISO that is brightened to match?
For example, compare multiple 1-second frames at high ISO with one 5-second frame at ISO 100. If the short exposures are averaged to reduce random noise, will the final result have the same noise as the single long exposure after adjusting brightness?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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Provided your ISO100 image was not underexposed I wouldn't expect a noticeable reduction in noise (except maybe in the deep shadows) with the 5 1 second ISO1600 images blended together.
In the infamous other thread I demonstrated that a 1/30s ISO100 will contain more noise (lower signal to noise ratio) than a 1/30s ISO1600 image. Same amount if light but the higher ISO had less noise.
The reason for this was that the read noise is proportionally greater in the ISO100 image (as readout happens after amplification). In a "correctly" exposed ISO100 the read noise is so small compared to the signal that any reduction in read noise is probably not noticeable.
edit: just did the experiment
I shot one photo at ISO100 16 seconds, and 16 shots at ISO1600 but only 1 second. All images were well exposed. What follows are two crops, the top row is a single ISO1600 image, and the bottom two are the 16 ISO1600 images averaged in Photoshop, and the ISO100 image. I won't tell you which way round the bottom two are, to see if anyone can actually tell the difference - I certainly can't!
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
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Averaging multiple frames does reduce random noise, which is why stacking works. But it usually will not beat, and often will not quite match, a single properly exposed longer low-ISO frame of the same still scene.
Why: a longer exposure collects more total photons per frame, so the signal-to-noise ratio is better from the start. With many short frames, each image includes its own read noise, and stacking combines that too. Averaging helps random noise roughly by the square root of the number of frames, but it does not eliminate the penalty of repeated readouts.
In practice, if the ISO 100 long exposure is not underexposed, it should generally look at least as good, and often better, especially in detail and shadows. Higher-ISO stacks may appear cleaner in some cases because of how a particular sensor handles read noise and amplification, but that is sensor-dependent.
So: stacking high-ISO shots can greatly reduce noise, but for a perfectly static scene, one longer low-ISO exposure is usually the better baseline when you can make it without clipping highlights or introducing other issues.
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