Why does the background look grainy at ISO 1600, and how can I reduce it?
Asked 9/21/2021
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2 answers
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I shot a photo on a Sony a6000 at 210mm, f/6.3, 1/1000s, ISO 1600. The background looks grainy/noisy. Why does this happen, and what can I do in-camera to reduce it?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
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It's called "photon shot noise."
For a given intensity of light there is also a given amount of noise; and the noise component is equal to √(photons/time/area). So, with more light intensity/availability there is also more noise; but also a better SNR (signal to noise ratio), which results in a less noisy image (because the noise is buried under the signal).
E.g. 40,000 photons/time/area has a noise component of 200; and an SNR of 40000:200 (200:1). Whereas 1,000,000 photons/time/area has a noise component of 1000 and an SNR of 1,000,000:10000 (1000:1).
Many think that the ISO causes the noise, but it doesn't... using a higher ISO either reduces the noise (increases recorded SNR), or it makes little/no difference (ISO invariant cameras). In fact, you can record an image at a higher ISO in bright/strong light and end up with less noise than recording the same image in weaker light at a lower ISO (or no-worse at least).
There are many ways to fix it after the fact; but like most problems, it's best to avoid it in the first place if possible; and that means collecting more photons (light).
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
4y ago
0
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The grain is mostly image noise from limited light, not simply because the ISO number is high. At 1/1000s and f/6.3, the sensor collected relatively little light, especially in the background, so the signal-to-noise ratio is lower and noise becomes more visible.
To reduce it, you need more light per shot:
- open the aperture wider if depth of field allows
- use a slower shutter speed if subject motion and camera shake allow
- add light with flash or a reflector
- lower ISO only when you can compensate with more light
For the settings you listed, something like a wider aperture and/or slower shutter could give the same brightness with less visible noise because the sensor gathers more total light.
Also, noise often looks worse when zooming in heavily on the image. At normal viewing sizes, it may look fine.
So the practical fix is: collect more light, then use the lowest ISO that still gives the shutter speed and aperture you need.
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