Day 2 of lessons week is about lens aperture and depth of field. They are intimately related, which is why they go together. This is one subject that I know gives a lot of people grief, but that I understood intuitively the first time I heard about it. Hopefully, I can write that intuition directly into your brain.
Your lens is like your eyeball. For this subject, all that’s really important is your iris and pupil, or the front of your lens. The iris is the colorful muscle that controls the size of your pupil, and makes your eyes beautiful. In the camera lens, there is a set of overlapping leaves that functions like the iris, making the aperture of your camera lens larger or smaller. The size of the aperture, or hole through which light passes, is expressed as a fraction, showing its relation to the length of the lens (absolutely none of this is important to you when you’re using the camera, but if I’m going to talk about it, I’m going deep).
Let’s get this math out of the way, so I can show you some photos. You see these numbers with every photo you take, 2.8, 4, 5.6, 8, 11, 16, 22, and a bunch in between. No, that’s not some bizarre photographic Fibonacci sequence. It’s telling you how widely or narrowly your aperture is open. When you see 2.8, your aperture is 1/2.8 as open as your lens is long (expressed as f/2.8, or (focal length) divided by (aperture)). So if you have a 50mm lens, the fraction is 50/2.8, making a 17.85mm hole in the lens (go to your calculator and work it out – I find that helps me). If you’re shooting at f/8, the opening is 50/8 or 6.25mm. And if you’re at f/16, it’s 50/16, or 3.125mm.
If you’ve been paying attention, you’ve noticed that as the aperture number (also called an f/stop) gets larger, the hole in your lens is getting smaller. You get a lot more light pouring through your lens at f/2.8 than at f/16. In a quick aside, this is why your shutter speed is so much faster at f/2.8 than at f/16. Your sensor doesn’t need to be exposed to the light for as long in order to fill up with light. I’ll probably do another bit on shutter speed in a couple days, so this will be explained in more detail then.
What I really want to get to in this post is aperture as it affects depth of field. Light beams do a very cool thing…they focus themselves in the right conditions. When your lens is wide open (f/2.8), the light’s bouncing all over the place, but as you close down the aperture (f/16), the light gets more and more focused. What this means for your photography is that when you’re shooting at f/2.8, your shutter speed is blazing fast, but you have to be focused exactly. When you’re shooting at f/16, your shutter speeds are significantly slower, but nearly everything in the frame is in focus, and this is depth of field. It’s how deep your effective focus is.
Often when shooting landscapes, I’ll set my lens at its smallest aperture and set the focus for about 20 feet (6 meters) away. This will put everything in the scene in focus. Lenses used to be manufactured with a scale that would show you this in action, so you could set your aperture, turn the focusing ring so that infinity was at one extreme, and you could see how close the nearest object would have to be to also be in focus. I use this frequently for close scenes, where I want subjects that are 3 and 10 feet away in focus, but I don’t want things further away in focus. So I’ll set the furthest extreme to 10 feet (3 meters) and use the scale to determine which aperture I need. Wikipedia has a great article (with tons of math and charts) about aperture at http://en.wikipedia.org/wiki/F-number.
So here’s what it means for you as a photographic artist:
Do you see how all but a few of the bottles are hazy blobs that look like you forgot to put your glasses on? And of the ones that are in focus, only parts of them are in focus? This is because I shot the photo wide open. It has a shallow depth of field, meaning you better really want only one little bit in focus. Click on the image to get a larger version of it, if you need.
At f/8, you can see that the detail of everything is a little bit sharper, but still, there’s a lot that’s out of focus. With the first shot, none of the hand-written price tags were legible, but at this aperture, you can get enough detail to read a couple of them.
And then with this one, at f/22, nearly everything is in focus. You can read price tags and descriptions from front to back, and each bottle is distinct.
Now, go out and don’t be afraid of the Av setting on your shooting mode dial. It’s the only one I use, when I’m not in fully manual mode. For me, the aperture is always more important than the shutter speed, and if the shutter speed is too slow, I adjust with the aperture, that way I know without thinking about it how much of the scene is in focus.
If you have questions, feel free to ask them. I’ll be happy to answer.
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