Not Enough Light
It’s dusk. The window is as big as it gets. The curtains can’t stay open any longer, or things start to blur. The room is still too dark.
Turn Up the Volume
ISO doesn’t let in more light. It turns up the light you already caught, like a volume knob. Same window, same light in.
Each click doubles the brightness, and adds a little more grain. Turn it up too far and the whole room goes white.
The Catch
Crank up a quiet radio station and you hear the song, and the static too. High ISO works the same way: the picture gets brighter, and so does the noise.
The Details
Want the full story? Keep reading.
How Much Light Do We Need?
ISO is probably the most misunderstood part of the Exposure Triangle. People often hear that ISO makes the camera “more sensitive to light.” That’s a useful shortcut when you’re first learning, but it’s not really what is happening in a digital camera.
For our purposes, let’s keep it simple. Think of ISO as how much light we need to make the photograph. A lower ISO requires more light. A higher ISO allows us to work with less light. And, just like everything else in photography… Nothing is free.
Start With ISO 100
When I pick up my camera, I generally start at ISO 100. Why? Because, all other things being equal, a low ISO generally gives me the best image quality my camera can produce. That means less noise and, depending on the camera, the most dynamic range available. So if I have plenty of light, I’ll take advantage of it. But sometimes I don’t have enough light. And that’s where ISO becomes useful.
Think About the Light We Need
Let’s go back to our box. We’re trying to fill the box with the right amount of light. At ISO 100, we need a certain amount of light. Now change the ISO to 200. We need half as much light for the same exposure. Change it to ISO 400. Now we need half as much light again.
So: ISO 100 → ISO 200, half as much light required. ISO 200 → ISO 400, half as much light required. ISO 400 → ISO 800, half as much light required. Each time we double the ISO, we reduce the amount of light needed for an equivalent exposure by half. That’s the basic relationship I want you to understand.
Why Not Just Use ISO 6400 All the Time?
Because… Nothing is free. Higher ISO allows you to work with less light. That’s great. But increasing ISO generally comes with increased image noise and can reduce image quality and dynamic range. You may see more grain or noise in areas of the photograph. Fine details may not look as clean.
The overall image may not have the same quality as a photograph made at a lower ISO. How much this matters depends on the camera, the lighting, the exposure and what you’re doing with the photograph. Modern cameras can do some pretty amazing things at high ISO. But that doesn’t mean there is no trade-off.
So Why Increase ISO?
Because sometimes you need to. Imagine you’re photographing a football game at night. You want a fast shutter speed because you need to freeze the action. Let’s say you want 1/1000 second. That’s a very short amount of time. You’re already letting in very little light. Maybe you’ve opened your aperture as far as you reasonably can. But the photograph is still too dark.
What can you do? Increase the ISO. You’re making another trade. You’re giving up some potential image quality in exchange for the ability to use the shutter speed and aperture you need. And sometimes that’s absolutely the right choice. A sharp photograph with a little noise is much more useful than a beautifully clean photograph with a completely blurry subject.
ISO Gives You Another Choice
This is why I don’t think of ISO as simply a setting that you should keep as low as possible. It’s a tool. Maybe I’m photographing a landscape in bright daylight. I can probably use ISO 100. Great. Maybe I’m photographing the same landscape after sunset. Now there isn’t nearly as much light.
I might need a slower shutter speed. If I’m using a tripod, that’s often fine. But what if I’m handholding? Now I have a problem. Maybe I need a faster shutter speed to keep the photograph sharp. I could open the aperture. If that’s not enough, I can increase the ISO. Again… Nothing is free. I’m making a choice.
ISO and the Other Two Parts of the Triangle
This is where the three pieces start working together. Let’s say I’m photographing water. I want a slow shutter speed because I want the water to blur. I also want a smaller aperture because I want a lot of Depth of Field. So I might choose f/16 and 1/4 second.
But now I look at the exposure. Maybe there’s not enough light. I have a few choices. I could open the aperture, but that reduces Depth of Field. I could slow the shutter down, but that changes the way the water moves. Or I could increase the ISO, which gives me more flexibility, but I may introduce more noise. Which one should I choose? There isn’t a universal answer. It depends on what matters most for the photograph. That’s the Exposure Triangle.
This Is Where “Nothing Is Free” Really Matters
Every setting solves one problem while potentially creating another. Aperture can give you more or less Depth of Field. Shutter speed can give you more or less motion. ISO can give you more flexibility in low light, but potentially at the cost of image quality. So when you change ISO, don’t just think: “I need more light.” Think: “What am I willing to trade?” That’s a much more useful question.
How I Think About ISO
My starting point is usually simple: ISO 100. Then I look at the photograph I’m trying to make. If I have enough light to get the aperture and shutter speed I want, I’ll stay there. If I don’t, I have to make a decision. Maybe the aperture needs to change. Maybe the shutter speed needs to change. Maybe ISO needs to change. I don’t automatically reach for ISO. I look at the photograph and decide which trade-off makes the most sense.
ISO Is Not a Measure of Brightness
This is an important distinction. Changing ISO does not physically make more light enter your camera. The aperture controls the size of the opening. The shutter controls how long that opening is available. ISO changes how the camera processes the signal produced by the light that was captured.
So if you are standing in a dark room and change ISO from 100 to 3200, you haven’t magically created more light. You’ve changed how much signal is needed to produce the resulting exposure. That is why ISO is different from aperture and shutter speed.
Try This
Find a scene where you can keep your camera and composition roughly the same. Put the camera in manual mode. Start at ISO 100. Choose an aperture and shutter speed that give you a reasonable exposure. Take a photograph.
Now change only the ISO. Try ISO 200, ISO 400, ISO 800, ISO 1600, ISO 3200. Keep the aperture and shutter speed the same. Look at what happens to the exposure. Then zoom in and look closely at areas of the photograph that are dark or contain fine detail.
Can you see the difference in noise? How much does it matter? At what point do you start noticing it? Your camera may behave differently from mine. That’s okay. The goal isn’t to find the “best” ISO. The goal is to understand what ISO does.
The Question I Want You to Ask
Don’t memorize: “Always use ISO 100.” That’s not the lesson. Instead, ask: “What ISO gives me the flexibility I need while maintaining the image quality I want?” Sometimes that’s ISO 100. Sometimes it’s ISO 400. Sometimes it’s ISO 3200. Sometimes it’s much higher. There is no magic number. There is only the number that makes sense for the photograph you’re trying to make.
Learn what ISO gives you. Understand what it costs. Then make the choice.
Beta Help me make this better. Tell me what you think →