As we learned before, when you take a picture a few things happen quickly. First, the aperture closes to block some of the light that’s coming into the camera. Second, the shutter opens and closes. As soon as that happens, the magic gnomes on the camera’s sensor record whatever light they saw when the shutter was open.
I’m sure you’ve taken some pictures that came out way too dark, or maybe even completely black. You’ve probably also taken some that were really light and washed out. We call those dark pictures underexposed, since they didn’t get enough light. The way too light ones are called… can you guess? That’s right, overexposed! Give yourself a shiny gold star.
You’ll see three little bears next to the text in this article. The first one is too bright– he’s overexposed. The second one is too dark– she’s underexposed. Baby bear is at the bottom– he’s juuuuuuusssttt right. IF you go through all of the lessons on this site and do all of the exercises, you’ll know what it takes to get all of your pictures to be just like baby bear.
There are three basic things that go into the equation for how a picture will be exposed. The first is how long the shutter is open– if it’s open for a long time, a lot of light gets in and lands on the sensor. If it’s only open for a very short period of time, a lot less gets in. Because photographers like fancy schmancy terms, we call this shutter speed.
The second thing that controls exposure is how big the aperture is. If it’s closed down so that there’s only a tiny hole, not much light gets in. If it’s wide open, a lot more light comes in and lands on the sensor.
If you can’t make sense of how aperture works, imagine that you’re in a plain room with no lights at all… not even the glow of a computer screen. (Stop shaking… it’s an imaginary room.) All of the light in this room comes from one window that faces out to a bright sunny day. If the room has a really big window, then the room will be bright and sunny. On the other hand, if it has a teeny tiny window, the room will be pretty dark. The aperture on your camera is like the only window the camera has. If the aperture is big, lots of light gets in. If it’s small, only a little bit of light sneaks through.
The third thing that goes into exposure is called ISO, and it’s a measure of how sensitive the sensor is. Remember our sensor gnomes? Think of them as light microphones. If the camera is set to a very high ISO (e.g. 800 or higher), the gnomes will be able to record small amounts of light. At low ISOs (100, 200) the gnomes need a lot of light.
Right about now, you’re probably thinking that you want the gnomes to be as sensitive as possible, but that’s not true. For now, just trust me on this, and I’ll explain later.
Next lesson: Shutter speed, how it works