Aperture is the size of the hole. Shutter speed is how long it stays open. 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 let in more light. It turns up the volume on the light that already arrived, and it turns up the hiss along with it. That hiss is grain.
ISO is a volume knob, not a window
The number is left over from film. Back then it really did describe how sensitive the film was. A roll of ISO 400 had bigger silver crystals than a roll of ISO 100. It caught light faster, and it looked grainier, because the crystals themselves were bigger. You bought the sensitivity in the box.
A digital sensor does not get that choice. Its pixels are the size they are and they catch what they catch. What ISO changes is how hard the camera amplifies the signal coming off them before saving it. Double the ISO and the picture comes out twice as bright from the same light. That is one stop, so ISO sits on the same ladder as the other two dials and trades against them.
The trouble is in that word, amplify. The sensor makes a signal, and it makes a little random mess alongside it. Turning up the amplifier raises both. The photograph gets brighter, and so does the mess.
What grain actually looks like
Drag the dial through the range. The change is not steady. The first few stops look like nothing at all. Somewhere in the middle a texture turns up that you have to lean in to see. Then quite suddenly it stops being texture and becomes the thing you notice first.
Watch what disappears first. Not the moon, not the ridge line. The fine texture on the near hill goes, because noise and fine detail are the same size and the camera cannot tell them apart. That is also why a heavily denoised photo looks like wax: the software wiped out the speckle, and the speckle and the detail were made of the same stuff.
So the real question is never how high your camera can go. It is what the photograph is for. Grain that looks awful when you zoom in on a screen 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
The amount of mess stays roughly the same everywhere in the picture. In a bright area it is a tiny fraction of what is there, so nobody ever sees it. In a dark area it is a big fraction of almost nothing, so it shows. Every high ISO photo you have taken was noisiest in the shadows, while the sky, the wall and the face were mostly fine.
Two handy things follow. A bright scene at ISO 6400 looks far cleaner than a dark scene at ISO 6400, so the number on its own tells you nothing. And if you have to shoot dark, let the shadows go properly black rather than sitting just above black, which is exactly where grain is loudest.
What you are actually buying
People talk about ISO as a cost, and it is one. But nobody pays a cost for nothing. Every stop of ISO buys you a stop of something else, and it is nearly always shutter speed.
Here is the trade. You are in a church at f/4. ISO 400 gives 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 just chose a bit of grain instead of a blur.
Put like that, the choice only goes one way. A grainy sharp photo is a photo. A clean blurry one is not. Raise the ISO and stop apologising for it.
every time
Raise it now, or brighten it later?
It is tempting to keep the ISO low, keep the file clean, and just brighten the photo on the computer afterwards. It does not work, and the reason is worth knowing.
The camera amplifies before it turns the signal into numbers, while the mess is still small and fine. Your editing software amplifies afterwards, so it is multiplying numbers that already lost their smallest details to rounding. What you get is not just more noise, it is uglier noise: blotchy instead of fine, arriving in patches, dragging odd colours with it. Most cameras also switch to a second amplifier somewhere around ISO 640 or 800, so a photo shot above that switch can come out cleaner than the same scene shot below it and brightened later.
So give the photograph the light it needs at the time, with whatever ISO that takes. Shooting dark to protect a number protects the wrong thing.
Base ISO, and the one time low really is better
Every sensor has a base ISO, the setting where the amplifier is working the least. On most cameras that is 100, on some it is 64 or 200. It is where the file holds the most tone, the best shadow detail and the cleanest colour. It is also the reason a tripod is worth carrying: with the camera on a tripod and nothing in the scene moving, you can stay at base ISO and let the shutter stay open as long as it likes.
Ignore the extra settings at each end of the range, the ones written L and H, or ISO 50 and ISO 102400. They are not real steps of the amplifier. They are the nearest real setting with a brightness tweak applied on top. At the bottom they cost you highlight detail, and at the top they give you nothing.
Set it once and stop thinking about it
Auto ISO has a bad reputation it stopped deserving about ten years ago. Used properly it is not the camera guessing. It is the camera doing sums you would otherwise do badly, in a hurry, while your subject walks off. Set three things and it becomes the most useful control you have.
- A minimum shutter speed. This 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 would actually be happy to print. On most modern cameras that is 6400 or 12800, which is higher than you think.
- A starting point of 100, so it drops back down as soon as the light allows.
Then put the camera in aperture priority, choose how much you want in focus, and let it protect your shutter speed by spending ISO. It will spend as little as it can get away with, which is what you would do, only faster.
How high can you go?
There is no one answer. It depends on the camera, on how bright the scene was, and on how big the photo will be seen. But there is a rough shape to it, and it is more generous than most people expect.
| 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 goes with every row. If the choice is between a grainy photograph and no photograph, it is not a choice.
Grain is not a fault
Half a century of photographs people love were shot on film grainier than anything in that table. Grain reads as texture, as atmosphere, as proof that the light was low and the moment was real. What people actually dislike is not grain. It is the smeared, waxy, blotchy look of a photo that was shot too dark and rescued afterwards, which is a different thing and is avoidable by getting the light right at the time.
Noise-reduction software has also got very good. The current crop rebuilds detail instead of smearing it away. Use it gently, and late, once the brightness and colour are settled, and stop before skin starts to look like plastic. A photograph with a little grain left in it still looks like a photograph.
The triangle, closed
That is all three. Aperture decides how much of the scene is sharp. Shutter speed decides whether movement freezes or smears. ISO decides how clean the file is. All three move in stops, all three trade against each other, and every photograph is one arrangement of the three, chosen either by you or by the camera.
The order that works is nearly always the same. Decide what the photograph is about and set that dial first. Set the second one to whatever your subject cannot do without, which is usually a shutter speed fast enough to hold it still. Then let ISO pick up whatever is left, 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.
- Noise hides in the shadows, so a bright scene takes a high ISO much better.
- Raising it in the camera beats brightening it afterwards, 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 blurry one, every time.