Aperture is the size of the hole and shutter speed is how long it stays open. Between them they decide how much light reaches the sensor, and when there is not enough light in the room for either of them to do the job, there is a third dial. ISO does not collect more light. It turns up the volume on the light that arrived, and the hiss it turns up with it is called grain.
ISO is a volume knob, not a window
The number is a leftover from film, where it described how sensitive an emulsion genuinely was: a roll of ISO 400 film had larger silver crystals than a roll of ISO 100, caught light faster, and looked grainier because the crystals themselves were bigger. You bought the sensitivity, physically, in the box.
A digital sensor has no such choice. Its pixels are the size they are, and they catch what they catch. What ISO changes is how hard the signal coming off them is amplified before it is written into a file. Double the ISO and the picture comes out twice as bright from the same light, which is exactly what one stop means, so ISO sits on the same ladder as the other two controls and trades against them freely.
The catch is in the word amplified. A sensor produces a signal and it produces a small amount of random error alongside it, and turning the amplifier up raises both together. The photograph gets brighter. So does the mess.
What grain actually looks like
Drag the dial through the range and the change is not linear to the eye. The first few stops look like nothing at all. Somewhere in the middle a texture appears that you have to lean in to see. Then, quite suddenly, it stops being texture and starts being the subject of the photograph.
Notice what goes first. It is not the moon and it is not the ridge line. It is the fine texture on the near hill, because noise and fine detail are the same size, and a camera cannot tell them apart. This is why a heavily denoised file looks like wax: the software removed the speckle, and the speckle and the detail were made of the same thing.
It is also why the honest question is never how high your camera can go. It is what the photograph is for. Grain that ruins a hundred percent crop on a monitor is invisible in a 40 by 60 print seen from two metres away, and a print is where most photographs end up.
Grain is a shadow problem
Noise is a roughly constant amount of error. Against a bright tone that error is a rounding difference nobody will ever see. Against a dark tone it is a large fraction of everything there is, so it shows. Every high ISO photograph you have taken was noisiest in the shadows, and the sky, the wall and the face were mostly fine.
Two useful things follow. The first is that a bright scene shot at ISO 6400 looks far cleaner than a dark scene shot at ISO 6400, so the number on its own predicts nothing. The second is that if you must underexpose, underexpose a scene that has somewhere bright in it, and let the shadows go properly black rather than sitting just above it, where the grain is loudest.
What you are actually buying
Photographers talk about ISO as though it were a cost, which it is, but nobody pays a cost for nothing. Every stop of ISO buys a stop of something else, and it is almost always shutter speed.
Here is the trade in one sentence. In a church at f/4, ISO 400 might give you 1/30 of a second, which is slower than your own hands and much slower than a person shifting their weight. ISO 1600 gives you 1/125, which is a photograph. ISO 6400 gives you 1/500, which is a photograph of a child running. Nothing about the room changed. You simply chose to accept some grain instead of accepting a blur.
Put that way, the choice is easy and it goes one direction. A grainy sharp photograph is a photograph. A clean blurred one is not. Raise the ISO and stop apologising for it.
every time
Raise it now or lift it later?
A tempting idea: shoot at a low ISO, keep the file clean, and brighten it in the editor afterwards. It does not work, and it is worth knowing why, because the reason is the same amplifier.
Amplifying in the camera happens before the signal is turned into numbers, while the noise is still small and fine. Amplifying in software happens after, so you are multiplying numbers that already lost their smallest values to rounding. The result is not more noise exactly. It is worse noise: blotchy rather than fine, arriving in patches, and dragging the colour around with it. Most cameras also have a second amplifier that switches in somewhere around ISO 640 or 800, and a file shot above that switch can be cleaner than the same scene shot below it and lifted.
So expose the photograph properly at the ISO it needs. Underexposing to protect a number is protecting the wrong thing.
Base ISO, and the one place low really is better
Every sensor has a base ISO, the setting where the amplifier is doing the least work. On most cameras it is 100, on some it is 64 or 200. That is where the file holds the most tonal range, the deepest shadow detail and the cleanest colour, and it is the reason a tripod is worth carrying: on a tripod, in a landscape, with nothing moving, you can stay at base ISO and let the shutter stay open for as long as it likes.
Ignore the extended settings printed at either end of the range, the ones written as L or H, or as ISO 50 and ISO 102400. Those are not real amplifier steps. They are the nearest real setting with a brightness adjustment applied on top, and they cost you highlight range at the bottom end and give you nothing at the top.
Set it once and stop thinking about it
Auto ISO has a bad reputation it stopped deserving about a decade ago. Used properly it is not the camera guessing; it is the camera doing arithmetic you would otherwise do badly under pressure. Set three things and it becomes the most useful control on the camera.
- A minimum shutter speed, which is the whole point. Set it to what your subject needs, not what your hands need: 1/125 for people, 1/1000 for a bird.
- A maximum ISO, set to the highest number you are actually willing to print. For most modern cameras that is 6400 or 12800, and it is higher than you think.
- A base of 100, so it drops back down the moment the light allows.
Then work in aperture priority, choose the depth of field the photograph is about, and let the camera hold your shutter speed by spending ISO. It will spend as little as it can, which is exactly what you would do, only faster.
How high can you go?
There is no single answer, because it depends on the sensor, on how bright the scene was, and on how large the photograph will be seen. But there is a rough shape to it, and it is more generous than most people assume.
| Situation | ISO | What it costs | Verdict |
|---|---|---|---|
| Landscape, tripod | 100 | nothing | Base |
| Daylight, handheld | 100 to 400 | nothing visible | Free |
| Overcast, or open shade | 400 to 800 | nothing in a print | Free |
| Indoors by a window | 800 to 1600 | a trace in the shadows | Take it |
| Zoo house, aquarium | 1600 to 6400 | visible grain, full detail | Take it |
| Evening street | 3200 to 6400 | grain, and it suits the subject | Take it |
| Concert, indoor sport | 6400 to 12800 | fine texture going | Worth it |
| Candlelight, no flash | 12800 to 25600 | loud, colour drifting | Last resort |
| Stars, tripod | 1600 to 6400 | grain in a black sky | Necessary |
One line travels with every row. If the choice is between a grainy photograph and no photograph, it is not a choice.
Grain is not a defect
Half a century of photographs people love were taken on film that was grainier than anything in the table above. Grain reads as texture, as atmosphere, as evidence that the light was low and the moment was real. What modern eyes dislike is not grain, it is the smeared, waxy, colour-blotched look of a file that was underexposed and then rescued, which is a different thing entirely and is avoidable by exposing properly in the first place.
Denoising software has also become very good, and the current generation reconstructs detail rather than blurring it away. Use it gently and late, after the exposure and the colour are settled, and stop before the skin goes plastic. A photograph with a little grain left in it looks like a photograph.
The triangle, closed
That completes the three. Aperture sets how much of the scene is sharp. Shutter speed sets whether movement is frozen or smeared. ISO sets how clean the file is. All three move in stops, all three trade against each other exactly, and every photograph is one arrangement of the three that you chose on purpose or the camera chose for you.
The order that works is almost always the same. Decide what the photograph is about and set that control first. Set the other of the two to whatever the subject cannot do without, usually a shutter speed fast enough to hold it still. Then let ISO absorb whatever is left over, without arguing about it.
The short version
- ISO amplifies the light you caught. It does not catch more of it.
- One stop of ISO is double the brightness and double the noise, same as the others.
- Noise lives in the shadows, so a bright scene takes a high ISO far better.
- Raising it in the camera beats lifting it in the editor, and it is not close.
- Base ISO on a tripod. Auto ISO with a shutter floor everywhere else.
- A grainy sharp photograph beats a clean blurred one, every single time.