ISO: What is it?

We’ve learned that the three things that control the exposure of a digital photo are shutter speed, aperture, and ISO. We know that shutter speed is how long the shutter is open, and that the longer it’s open the more light gets in. We know that aperture is how wide the lens is open, that wider openings let more light in, and that the size of the opening also controls how much of the image is in focus.

So what’s this ISO stuff? Way back when, we learned that there are magic gnomes that live on your camera’s sensor, and that when the shutter opens they record whatever light they see.

But first, a diversion. Imagine you go to a club to see a show– it’s a weird kind of show, but bear with me. There’s someone playing ballads on the guitar and singing along. There’s a guy playing punk rock on the accordion. There’s a trio singing political songs. And then someone gets up and whispers poetry. (Hey, I live in San Francisco. This kind of thing actually happens.)

All of a sudden, you can’t hear the show. The poem whisperer is much quieter than everybody who came before him, and the microphone isn’t sensitive enough to pick up his voice. Eventually someone kicks the sound guy and wakes him up, he turns up the sensitivity on the microphone, and unfortunately you get to hear the end of Ode to a Medium Rare Hamburger With Onions. Oh well.

Now our accordion player comes back for his Sex Pistols encore, and he blasts your eardrums. Whoops! Cover your ears until the sound guy turns the microphone’s sensitivity back down.

If you’d brought your camera you might have gotten a great picture of a room full of people covering their ears while staring at an accordionist, but other than that what does this have to do with photography?

Let’s go back to our imaginary gnomes who sit on your camera’s sensor waiting for light, and then record what they see. They do a great job on a bright sunny day, but what happens when you shoot inside the dark club? All of a sudden there’s not enough light for the gnomes, and you get a very dark image. You can open the aperture, but it only goes so far. You can use a slower shutter speed, but at some point you just get a blur of motion. There’s hope, though. Just like you have to turn up the sensitivity on the microphone to pick up more of a quiet sound, you can turn up the sensitivity on your sensor to pick up more light when the scene is dark. It’s like magnifying the light so that the gnomes can see it better.

That sensitivity is called ISO. Of course, there’s a technical description with lots of math stuff for exactly what it is and how it works, but the truth is that you don’t care. It doesn’t matter. Here’s what you do need to know about ISO– it’s pretty simple:

ISO 100 is generally considered the baseline, and it’s where your camera is set by default. ISO 200 is twice as sensitive as ISO 100. ISO 400 is twice as sensitive as ISO 200, and therefore four times as sensitive as ISO 100. ISO 800 is twice as sensitive as ISO 400, and ISO 1600 is… well, I’m not telling you. You’re smart enough to figure that one out on your own.

In the olden days, you bought film with a particular ISO, and you set your up camera so that it knew what film it had in it. These days, you just tell your camera what ISO that you want it to use, and it factors the ISO into its metering. Most dSLRs have a range of ISO from 100 to 1600, often 100, 200, 400, 800, and 1600. A few will have settings in between. Some will go as low as 50 and as high as 3200, and at least one model will go as high as ISO 6400. I’m pretty sure ISO 6400 will let you take pictures without taking the lens cap off.

See? That wasn’t so hard. Turning up the ISO is basically just magnifying the light that comes into your camera.

Next up: how to use ISO, and what side effects it has.

Next lesson:  ISO: Why You Care


Filed under ISO, Lesson

5 responses to “ISO: What is it?

  1. Marco

    I like you … not in a creepy internet-love-of-my-life-and-loins way, but in a smiling liking you way. You not only know how to explain stuff, as many others can and do too, but you also make it all seem reasonable to the point your handing out eye openers all over the place. Hope that made some kind of sense. If it didn’t I just wanted to give you my compliments and thanks. Thank you very much. I like you…quite a lot.

  2. stopshootingauto

    Thank you!

  3. Frank Kelly

    Yo are the bomb. I have learned more from you in 2 days then I have over the past few months reading the “recommended readings” in the bookstore.
    Thank You for making me take better pictures.

    Today was the first day shooting in Aperture Priority and I was giddy as a little kid. Most of my shots came out be-u-ti-fully and now I have to work on what I did wrong with the action moving shots. I was at a pro football game. All of my posed shots came out great, but anything moving came off slightly blurred. Just a wee bit not sharp.

    I shot in F8 (talk to me) yes I followed your advice……..

    So, for 4 months since I got the camera Nikon D40, I have shot in only auto, they a day or so in “P”, today in “Aper Pr”, can manual be far behind.

    Again, you are the bomb and Thank You .

    • stopshootingauto


      For sports, you generally want shutter priority mode. When things are moving quickly, as in sports, a fast shutter speed is important. Set the camera to shutter priority mode and pick a shutter speed that works, then increase the ISO until you can shoot at a reasonable aperture.

      You might start by looking at what shutter speed the camera was using for the slightly blurry shots, and using something a little bit faster than that.

  4. Taryn

    I too wanted to say Thank you….I’ve learned more in one evening of reading and playing around with my camera and actually UNDERSTAND things I’ve been trying to get for months. This is finally making sense thanks so much for explaining it in english

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s