ISO and noise
ISO buys brightness when there is not enough light, and pays for it with noise. Knowing when that trade is worth it is the skill.
iso describes how strongly the camera amplifies what the sensor collected. Doubling it — 100 to 200, 200 to 400 — buys one stop of brightness.
It is the only one of the three controls that does not change how much light actually arrives. It turns up the volume on what did.
Why that matters
Amplifying a signal amplifies whatever else is in it. The result is noise: a speckled texture that shows first and worst in the shadows, because that is where the signal was weakest.
This is why ISO is the control to reach for last. Aperture and shutter speed collect more light; ISO makes do with less.
When to raise it anyway
Constantly. The alternative to a noisy photograph is usually a blurred one or no photograph at all, and noise is by far the least bad of those.
The order that works:
- Open the aperture as far as the depth of field allows.
- Slow the shutter as far as the subject and your hands allow.
- Then raise the ISO to cover the rest.
If steps one and two are exhausted and the photograph is still dark, ISO is not a compromise — it is the answer.
Underexposing is not a workaround
A common instinct is to keep ISO low, accept a dark photograph, and brighten it afterwards.
This is worse. Brightening in software amplifies the signal in exactly the way raising ISO would have, and usually with less information to work with. Many cameras apply amplification before the noisiest part of their own electronics, which software cannot do.
Expose properly at a higher ISO instead.
Knowing your own camera
Sensors differ enormously, and published figures are less useful than your own eyes. Spend twenty minutes photographing the same dim scene at every ISO your camera offers, look at the results at the size you actually use, and find the point where the noise starts to bother you.
That number is worth more than any general advice, because it is about your camera and your tolerance.
Try it
Find the light
Window light only, and the shutter is already as slow as you can hold steady. Raise the sensor's sensitivity until the still life is properly exposed.
Loads an interactive 3D camera simulator.
Key takeaways
- ISO amplifies the signal rather than collecting more light.
- Amplification reveals noise, worst in the shadows.
- Use aperture and shutter first, then raise ISO to cover the remainder.
- Underexposing and brightening later is worse, not better, than a higher ISO.
Common questions
What is the highest ISO I should use?
Whatever gets you the photograph. Test your own camera in advance so you know where its noise becomes objectionable to you, then use up to that freely and beyond it when the alternative is no photograph.
Is it better to underexpose and brighten later?
Almost never. Brightening a dark file amplifies the same noise that raising ISO would have, and usually more of it. Getting the exposure right at the time is better than either.
Next steps
The exposure triangle
Three controls decide how bright a photograph is, and each one changes something else as well. That trade-off is the whole of exposure.
Shutter speed
How long the sensor is exposed decides whether movement is frozen or smeared. It is the most visible of the three controls.
Metering, and why it lies
Your camera's meter assumes every scene averages to mid-grey. Knowing when that assumption fails explains most automatic exposure mistakes.