Why do higher ISO settings often look worse even if my measurements suggest better SNR?
Asked 3/11/2014
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I’ve measured read noise, thermal noise, gain, and related sensor behavior on cameras like the Canon 70D and 6D, and I’m trying to reconcile theory with real-world results. My testing suggests that as ISO increases, noise rises but the signal-to-noise ratio can also improve under some conditions. Yet in practice, higher-ISO images often look worse after processing, with more unnatural grain and less pleasing results, especially for time-lapse where stacking and calibration frames are impractical. Why doesn’t a measured SNR improvement at higher ISO reliably translate into a better-looking image? Is this mainly about dynamic range, exposure choices, output size/noise reduction, or am I likely misinterpreting what ISO changes actually do?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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I believe it may be a flaw in your testing. Raising ISO should not be getting you a higher signal to noise ratio (or usable dynamic range).
You are raising the noise floor, without any corresponding increase in dynamic range at the bright end. Thus you should be left with overall lower dynamic range. It would not be possible to go the other way.
Is it possible that your testing did not test full dynamic range (between noise floor and lightest point without clipping) for each ISO, or that you made false assumptions such as not compensating for the gain caused by raising ISO? For instance, taking a photo with higher ISO but with exactly the same lighting, shutter and aperture will raise the intensity of the image by nominally the same amount as the intensity of the noise which may make it appear that you have lost no detail to noise, yet at the other end it will also mean the brightest intensities that were not clipped are now clipped, because the overall exposure is higher as well.
In a real world situation when you decide to raise ISO you'll effectively then be able to use a faster shutter speed in order to equalise the exposure. When doing so, ie when keeping the final exposure the same, you'll find that you lose detail in the noise floor while preserving the same clipping cut off at the top end, resulting in a net decrease to usable dynamic range.
Originally by user3422. Source · Licensed CC BY-SA 4.0
user3422
12y ago
0
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Higher ISO does not magically create more light; it mostly amplifies the sensor signal earlier in the pipeline. Whether that helps depends on what changes along with ISO.
If shutter speed and aperture stay the same, raising ISO mainly brightens both signal and noise. It can improve results when read noise is a limiting factor, because analog amplification can lift the signal above downstream electronics noise. But it does not increase the sensor’s full dynamic range, and often reduces usable highlight headroom.
If you raise ISO in aperture or shutter priority and the camera shortens exposure, less light reaches the sensor, so photon shot noise dominates and image quality gets worse.
That’s why measured SNR improvements don’t always match perceived image quality. “Better SNR” in part of the tonal range can still come with less dynamic range, more visible grain, clipped highlights, or a harsher noise pattern after processing. In practice, very high ISO images often look acceptable only after downsizing or strong noise reduction.
So the real answer is: higher ISO can help in low light when exposure time is constrained and read noise matters, but it usually won’t beat giving the sensor more actual light. For timelapse especially, the look of noise and reduced dynamic range often matter more than a narrow SNR gain.
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