How aperture affects depth of field

- In part one of this course, you should have learned a few things about aperture. For example, you should know that apertures are measured in F numbers, which have this odd progression, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16. You should know that each one of these numbers represents the size of the circular opening, that is the aperture in your lens. You should also know that, from left to right, each one of these numbers represents an aperture that is half the size as the previous number. In other words, each one of these apertures represents a one stop difference in exposure. The tricky bit here is that larger numbers correspond to a smaller opening, so f/16 is half as big as f/11 or one stop smaller. In the first part of this course, you used aperture to compensate for shutter speed changes, so, for example, if your camera chooses a 30th of a second as a shutter speed, a speed that might be to slow for handheld shooting, you know that you can increase the shutter speed by one stop to a 60th of a second. But that's a change that will darken your exposure by one stop, so to compensate for that you can open your aperture by one stop to let in the same amount of light that your shutter speed change eliminated. Those equal and opposite changes give you the same level of overall brightness in your final exposure, but with the advantage of a faster shutter speed in that case. However, changing the size of your aperture does more than simply change the brightness of your exposure. As shutter speed changes affect brightness and motion stopping power, aperture changes affect brightness and depth of field. As your aperture gets smaller, depth of field increases. For example, I'm going to look in this camera over here. Notice the background, we're on a tighter shot here and the background is about as sharp as what you've been seeing in this camera over here. Right now we're affecting a change in that other camera, so that when I look back to it depth of field is much more shallow than it was before, and what I mean by that is the background back there is a little bit softer. The change that we affected is that we opened up the aperture of this camera. Before we had a very small aperture which gave us deep depth of field, now we have a larger aperture which is getting us a much more shallow depth of field. Again, one is not necessarily better than the other in any sort objective sense, but they do have very different fields. Let's take a look at some stills side by side. So in the left one you can see, I've got my smaller aperture, which is giving me deeper depth of field. In the right hand image, I've got a wider aperture which is giving me more shallow depth of field. You can argue that the shallower depth of field kind of isolates foreground more, by making the background less distracting. Here's a landscape shot with very deep depth of field. Shooting the scene with shallow depth of field doesn't really work in this case. We want to see all of the details in the shot, so going shallow doesn't work as well, in my opinion. But again, depth of field is subjective and very often your subject matter will drive your depth of field choices. So now you have another factor to work with in your exposure balancing act. Perhaps you're shooting in a dark room and you're meter is reporting very slow shutter speeds, you can, of course, increase the ISO, but you can only take that so far before your image becomes too noisy, so you can open up your aperture. This will allow you to use faster shutter speed, but it will also reduce the depth of field in your shot. Depending on what you're shooting that may or may not be a problem. If you want to shoot in low light, have every deep depth of field, and freeze motion, that may not be possible. In that situation some cameras will do better than others because some cameras can have their ISOs pushed higher before they produce a lot of noise. But still there, there will be situations where, for reasons of practicality and physics, you simply can not get the balance that you want. Aperture then, can be summed up as follows, an aperture as a mechanical circular opening, usually contained in your cameras lens. It can be made bigger and smaller, and as you make it smaller, the depth of field in your scene increases. Smaller apertures are represented by larger numbers. You just need to memorize all of that, won't take long, and soon the particulars of aperture will be second nature. However, these aren't the only things that you need to know to take control of depth of field.