What ISO Is and What Noise Costs You
Okay. New module. Shutter's in your hands, aperture's in your hands, so now we finish the triangle with ISO.
I put this one last on purpose. Most classes teach ISO first because it's the easiest number to understand, low number bright light, high number dark room, done. But I think teaching it first is backwards, because people learn "just bump the ISO" as their fix for everything and never really deal with light. I'd rather you understand shutter and aperture as your first tools and treat ISO as the one you reach for last. That's a real opinion of mine and I'll probably repeat it more than once tonight.
What ISO actually is
ISO is how sensitive your camera's sensor is to the light hitting it.
Low ISO, like 100 or 200, means the sensor needs more light to make a good exposure, but it does it cleanly. High ISO, like 3200 or 6400, means the sensor will make a bright image out of very little light, but it does it by amplifying the signal, and amplifying the signal amplifies the noise along with it.
Here's the thing that took me a long time to really get. ISO doesn't add light to the scene. It's not a flashlight. It's the camera turning up the volume on whatever light is already there. Turn up the volume on a quiet room and you don't just hear the conversation better, you also hear the hum of the fridge and the static in the wires. That static is noise. Same idea.
What noise actually looks like
Noise shows up as speckling, mostly in the shadows and the dark areas of your photo first. Push the ISO high enough and it creeps into everything, and colors start looking a little blotchy and wrong too, not just grainy.
A little noise isn't the end of the world. I'll be honest with you, some of my favorite hiking shots have visible grain in them and I don't mind it, it almost looks like film. But there's a point past which the noise eats real detail, and once detail's gone you can't get it back in editing. I don't do much editing myself, I can crop and nudge exposure and that's about it, so I try to get it right in camera rather than count on fixing it later.
The tradeoff, laid out plain
- Low ISO (100-400): clean image, needs good light or a slower shutter or wider aperture to compensate
- Medium ISO (800-1600): usable in most indoor and evening light, some noise starts showing in shadows
- High ISO (3200 and up): lets you shoot in genuinely dark conditions, but noise gets obvious, especially on smaller sensor cameras
Your camera has a "native" ISO range and then an extended range that pushes higher. I'd stay out of the extended range except when you have literally no other option. It's there for emergencies, not for regular use.
A story about learning this the hard way
I tried to shoot the Milky Way from up Alpine one night, years ago now, and I sat out there for three hours in the cold and came home with nothing but noise. I had the ISO cranked up because it was dark and that felt like the obvious move, but I didn't understand yet what I was trading away to get there. Every frame looked like static with a few stars poking through. I was cold and irritated and a little embarrassed, honestly, because I'd told my wife I'd be back in an hour.
I read about ISO the whole next morning until I actually understood the tradeoff, not just the number. That's usually how I learn things, slow and irritated first, then thorough.
How to practice this at home tonight
- Put your camera in manual mode, same as we've been doing.
- Pick one still subject indoors, a lamp, a bowl of fruit, whatever's sitting on your counter.
- Set your shutter speed and aperture and leave them alone.
- Shoot the same subject at ISO 100, then 800, then 3200, then whatever your camera's max is.
- Load them up on a bigger screen if you can, not just the camera's LCD, and zoom into the shadows on each one.
You'll see it plainly. The noise isn't subtle once you know where to look.
One caution here, worth saying plainly: don't judge noise off the little LCD screen on the back of your camera. It's small, it's bright, and it lies to you about how clean or noisy a shot really is. You need to see it bigger.
Before next time
Do the ISO ladder test above on one subject in your house, and bring your worst noisy frame to next class, the ugliest one. We're going to look at exactly where the noise shows up first and why, and it helps to have a real bad example in front of us instead of just talking about it.