ISO and noise
"Raise the ISO and you get noise" is true, but it has cause and effect backwards. ISO does not change the light hitting the sensor; it amplifies the signal after readout. The noise is there because the scene is dark, and the ISO setting just makes it visible. So the question to ask is how much light you have. ISO only makes up the difference.
1. What ISO does
Only two things decide how much light arrives: aperture and shutter. ISO comes after that. The sensor's charge is read out, amplified according to the ISO setting, and mapped to image brightness.
Noise has three sources, and it helps to keep them apart.
| Source | What it is | When it dominates |
|---|---|---|
| Shot noise | Photons arrive at random; the fewer of them, the larger that randomness looms | Almost always |
| Read noise | A roughly fixed amount added by reading and amplifying | Deep shadows, very short exposures |
| Thermal noise | Generated as the sensor warms during a long exposure | Exposures of tens of seconds and up |
The first row is the one that matters. Noise comes from a shortage of light, not from a number in a menu. Shoot a dark scene at ISO 100 and brighten it later, and you won't get less noise; often you get more.
2. Base ISO and dual gain
- Base ISO is where the sensor adds no extra gain, and usually where dynamic range is widest. On most bodies that's ISO 100 or 200, but it varies, so check your manual or the manufacturer's spec sheet.
- Extended ISO, the settings marked L or H or shown in brackets, isn't extra sensitivity, just a digital stretch. Going lower often costs highlight headroom; going higher is basically the camera brightening the file for you.
- Some sensors have two readout gain circuits, sold as dual native ISO. At a certain setting they switch to the second circuit and read noise drops again, so one stop up can look cleaner than the stop below it. Where that switch happens depends on the model; take it from the manual, not from someone else's camera.
3. Exposing to the right, and when to underexpose
If noise comes from too little light, then gathering more light without clipping improves the signal-to-noise ratio. That's all "expose to the right" means: push the histogram as far right as it'll go, then pull the brightness back down in editing.
The margin for error is thin, and sometimes you should underexpose on purpose:
- The highlights are the subject. Stage lights, neon, the sky around a setting sun. Once they clip, no RAW file brings them back.
- The scene has more contrast than the sensor can hold. Shooting from inside a room out through a window, something has to give. Protect the highlights and leave the shadows for editing.
- The shutter is already as slow as you can hold. Dragging it further for ETTR swaps a certain failure for an uncertain gain; see safe shutter speeds.
Lifting shadows costs you noise. Clipping highlights costs you the highlights. That asymmetry tells you which way to lean.
4. The order to decide in
ISO isn't the first setting you choose. It's whatever's left over:
- Shutter first. Fast enough for your hands and for whatever is moving. This is the hard constraint.
- Aperture second. As much depth of field as the picture needs: wide open for a single portrait, stopped down for a group or a landscape.
- Whatever brightness is still missing, ISO makes up.
In practice, auto ISO with a ceiling beats setting it by hand every frame. Put the ceiling near the usable limit you measured, and the minimum shutter speed at your safe speed. The camera follows the light; you decide how far quality is allowed to drop.
5. What denoising can do
- Colour noise reduction is nearly free. Removing the red and green speckles costs almost no detail.
- Luminance noise reduction always costs detail. It trades directly against sharpness: hair, foliage and fabric go waxy first. Leave a bit of grain. Grain looks like film; smearing looks like plastic.
- Averaging frames works best. Shoot several frames from a fixed position, then align and average them: random noise cancels and the detail stays. Four frames roughly double the signal-to-noise ratio, the same as two extra stops of light. Worth it for night scenes and astro.
- AI denoising generates new detail. The results can be startling, but what it fills in isn't necessarily what you photographed. Check hair at 100%, distant signs and foliage edges, and take extra care with documentary work.
More light beats any fix afterwards. A faster lens, a tripod and a slower shutter, or just waiting for better light will each do more than the noise reduction slider.
6. FAQ
Is a lower ISO always better?
With enough light, yes: base ISO gives the widest dynamic range. But dragging the ISO down until the shutter is too slow to hold, or shooting well underexposed and lifting it later, usually ends up worse than just using a higher ISO.
Should I use extended ISO?
You can, as long as you know it isn't real sensitivity. Extending up is the camera brightening the file, which you could do in editing. Extending down often gives up highlight headroom, which hurts exactly when you're trying to hold a sky.
Where should I set the auto ISO ceiling?
There's no universal number. It depends on your body, your output size and how much grain you'll accept. Shoot the ladder from section 2, find the highest setting you still like at the size you actually print or post, and put the ceiling there.
At high ISO, RAW or JPEG?
RAW. In-camera noise reduction is baked into a JPEG and can't be undone. RAW lets you set the balance between detail and grain yourself and leaves room for stacking. Full comparison in RAW or JPEG.