At this point, we know that aperture is how big the opening in the lens is when you take a picture. We know that big numbers mean a small opening (and therefore less light getting in), and small numbers mean a large opening and more light. f/4 is a large aperture, which means lots of light. f/16 is a small one, and only a little bit of light.
Aperture does way more than control how much light gets into the camera. Those “f-stop things” are one of the most useful tools in photography. Let’s look at why.
I stopped at a park on the way to work this morning so that I could take pictures of flowers to use as examples in this lesson. The flowers weren’t particularly remarkable, but they were useful patient subjects. Here’s one example. You’ll probably look at it and think, “Yeah, a yellow flower”, and see it as fairly messy and uninteresting. That’s because it is messy and uninteresting and an entirely unremarkable picture. Its purpose in life is to serve as a bad example, and it does a rather fine job of it.
Here’s the exact same flower from the exact same angle, but it’s a much more compelling photo. When you look at it, your eye is drawn to the flower, and it isn’t distracted by all the stems and leaves and junk and crap and ick in the background. The only difference between the two images is that in the first one the background is in focus, and in the second it’s soft and blurred.
“Hey, that’s cool! How can I do that?”
It’s easy! All you have to do is pay a little bit of attention to what’s going on, and change your aperture to get the effect you want.
It’s time for me to demystify another technical term– this time, it’s “depth of field”. Depth of field is really just how much of your image is in focus. There’s a lot of complicated stuff underlying it, but the basic premise is really simple.
When you take a picture, it’s almost always true that some things in the image will be close to the lens, and others will be farther away. The way depth of field works is very simple– if your lens is very wide open (i.e. small f-stop numbers) only a narrow range of distances will be in focus. If your lens has a very small aperture (a big f-stop number), then a very large range of distances will be in focus.
Confused? I don’t blame you. Let’s have our six-year-old draw us another picture. Imagine that you’re walking through a meadow. In front of you is a yellow flower. A little bit farther back from the flower is the goofiest-looking tree you ever did see, and in the distance is a snow-capped mountain range.
Clearly, the flower is very close to your lens, the mountains are far away, and the tree is somewhere in the middle. Let’s pretend that you’re going to take a picture of this idyllic scene, and you decide to focus on the tree.
If you keep your lens wide open, the tree will be in focus but the flower and the mountains will be blurry. If you use an aperture that’s in the middle of the range then you can get the tree and the mountains in focus, and if you use a very small aperture then you can get everything in focus. The former might be useful if you wanted to focus on one thing (say a person standing in front of the tree) and the latter is great for landscape photography.
Think of your depth of field as a stripe that is centered at whatever you focused on. Whatever is inside the stripe will be in focus. If your lens is wide open, then the stripe will be very narrow. If it has a very small aperture, then the stripe is very wide.
I’ll close today with two excellent illustrations from the nature photography of Joe Decker. One shows the power of a shallow depth of field, and the other illustrates what a wide depth of field can do. I highly recommend clicking through to see larger images, since the thumbnails don’t do them justice. Oh, and Joe’s flowers are much better looking than mine.
You’re probably still confused by this, and I don’t blame you. In the next lesson, I’ll leave the fluffy flowery examples behind and show you depth of field in a completely different way. If you don’t want to wait, stick your camera in aperture priority mode and take a bunch of pictures at different f-stops. The wonderful thing about digital photography is that you can play around a lot. The feedback is cheap and very fast.
Next lesson: Quick Exercise: Focusing