What is ISO?

The Digital Amplifier

You now know how to control the size of the light (Aperture) and the time of the light (Shutter Speed). But what happens if you are in a pitch-black room, your Aperture is as wide as it goes, and your Shutter Speed is as slow as you can hold it, and the picture is still too dark? This is where you use the final tool: ISO.

ISO Diagram

3. ISO (The Volume Knob)

Unlike Aperture and Shutter Speed, ISO is not a physical, mechanical part of the camera. It is a digital setting. Think of ISO exactly like the volume knob on a car radio.

If you are listening to a very quiet radio station (a very dark scene), you can reach over and turn the volume knob (the ISO) all the way up to 10 so you can hear the music (brighten the picture). However, when you turn a radio up to maximum volume, you don't just hear the music louder—you also hear the ugly, fuzzy static in the background.

  • Base ISO (Often 100, sometimes 64 or 50) (Low Volume): This is the base level of the camera. The picture is perfectly clean, smooth, and beautiful. There is zero static. However, it requires a lot of real light (from the sun or a flash) to expose the picture properly.
  • ISO 800 - 1600 (Medium Volume): The picture is artificially brightened by the camera's computer. It is very useful for shooting indoors or in the shade. You might see a tiny bit of static if you zoom in very closely, but it looks great on a phone or printed small.
  • ISO 12800 (Maximum Volume): If it's pitch black outside and you are trying to photograph a moving animal, you can crank the ISO up to maximum to artificially brighten the picture! But because the volume is so high, the picture will be covered in ugly, colorful digital "noise" (grain). The static is overwhelming.

The Master's Rule of ISO: A master photographer always tries to keep their ISO as low as absolutely possible (their camera's Base ISO). They will always try to add more light by using a wider aperture, a slower shutter speed, or an external flash *before* they touch the ISO knob. ISO is your last resort to save a dark photo.