Every camera, whether it be film or digital, is, in essence, a light-proof box. To take a picture, you open a hole in the lens, called the shutter, to let light in. The light then hits the film or the digital sensor in the camera and the picture is taken.
As I described last time, there are two ways to control the amount of light that goes into the camera -- you can change the size of the hole that the light goes through (the aperture size) and/or you can change how long the hole is open for (the shutter speed). It's important to get the right amount of light into the camera -- if you have too much light, the picture is overexposed; too little light causes an underexposed picture. But by controlling the two variables described above (aperture size and shutter speed) you can control just how much light gets into the camera -- make the aperture wider or hold the shutter open longer, you get more light. Narrow the aperture or allow less light, you get less light.
You might wonder why there are two ways to control the amount of light. After all, you get the same amount of light whether you use a wide aperture and fast shutter speed or a narrow aperture with a slow shutter speed. So, what difference does it make?
Well, there are some differences, and this post is about one of them. Specifically, we're going to discuss a topic called depth of field. In short, depth of field refers to how much of the picture (in terms of distance from the lens) is in focus. The general rule is this -- if you use a wide aperture, you will have a shallow depth of field. If you narrow the aperture, more of the picture will be in focus. Here's an example:
This picture was taken with a narrow aperture, while holding the shutter open for 10 seconds.
The next picture is pretty much the same shot, except that I widened the aperture as far as the lens would go and kept the shutter open for only 1/6 of a second.
As you can see, when I made the aperture wider, the flowers in the back were thrown out of focus. Whereas in the first picture, the depth of field of the lens reached all the way to the flowers in the back, in the latter picture, the DOF ended right after the sunflower.
So, remember this: when you widen the aperture, more of the picture will be thrown out of focus. When you narrow the aperture, you'll get more in focus (but you'll have to keep the shutter open longer to get more light).
As always, comments, critiques and criticisms are welcomed and appreciated.
Statue of Liberty
Trinity Church, September 11, 2008