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As you may recall from high school biology class, there are two types of light-sensitive cells in your eye: rods and cones. Codes are the color-sensitive cells and most of the cones in your eye are gathered into a very small area at the back of your eye, called the fovea. The fovea is responsible for the focused part of your field of view. Now it is not immediately obvious, but the only part of your field of view-- that is, the only part of all of the stuff that you can see-- the only part that is in focus is an area roughly the size of the tip of your thumb when held at arm's length.
That is this little bit right here. Now if you don't believe that, give this a try. Get a piece of paper that has some text on it, tear a page out of a magazine or just use a book or something. Hold it at arm's length and put your thumb in the middle of it. Now, focus your eyes on your thumb, and with your peripheral vision try to read the text that's around your thumb. You should find that you can't, that it is completely out of focus. Take your thumb away, and what was underneath your thumb is in focus. Since only a small part of your field of view is in focus, you subconsciously move your eyes around to sample different areas of your field of view, and your brain assembles this into a big image that gives you a sense of an overall impression of focus.
But when it comes time to closely examine something, like reading text on a page, then you actually look at that thing, and you focus your eyes. In other words, you choose which part of your field of view that you want to focus on. Now your camera's autofocus mechanism is very similar in that you must choose where you want it to focus. Choose the wrong place, and your subject may be soft or blurry. Now earlier we discussed the importance of half-pressing the shutter, but this is such an important topic I've decided to nag you about it some more, before we launch into a discussion how to use the focus points that appear in your viewfinder.
So if you look here, you will see that we built a simple little scene here, some nice old, antique cameras. And my digital SLR is pointed at that scene. Now what this big thing is here, this monitor, we have taken a video feed out of my camera, and so what you are seeing here on the screen is what I would see in my viewfinder. This is going to allow you to see exactly what I am seeing when I am shooting. So I am going to do what I am supposed to do and press the shutter button halfway down. When I do, my camera calculates, auto focus, it meters the light in my scene, it calculates a white balance. When it's done it, it beeps, and it flashes a light in the viewfinder, and most importantly, there, it has done it.
You can see--oops, wrong finger-- you can see right down here it has calculated a shutter speed of a third of a second, an aperture of about 4. Now it has also turned on a bunch of lights in my viewfinder. Your viewfinder may not look exactly like this. What each one of these squares is is a different point that the camera can choose to auto focus, and you can see these ones that are lit up in red are where it has chosen to focus. They are all sitting on top of this antique slide projector, and so I know that the camera is choosing to focus on my subject. That's great! When you stop to think about it, you'll realize that auto focus is a very difficult thing to pull off.
In any given scene, like this one, there might be a lot of things that could be the subject of the image. The auto focus system has to try figure out what thing in the scene is supposed to be the subject, and then it has to drive the lens to focus on that thing. This is why individual camera vendors tout their specific autofocus mechanisms: it's a really hard thing to engineer. Now in this scene we got a pretty simple situation, because our subject, the slide projector, is in the very center of the frame, so all of the focus lights that lit up were right there in the center as they were supposed to be. Let's look at what happens if we go to a more complex scene.
Okay, check out our new scene here. We've placed three antique cameras in the front of a scene, and we have got the old slide project in back. Now the important thing to remember about autofocus is that when you half- press that shutter button to focus, your auto focus system does not pick a particular object to focus on. In the last example, it didn't say oh, there is a slide projector. I'll focus on that. It picks a depth to focus at. So watch what happens when I half-press my shutter button to focus. It's lit up a focus point on this camera and this camera and this camera and the front of the table.
In other words, it's lit up points on any thing that's at the distance that it has chosen to focus on. Let's look at this a little closer here. This camera, this camera, this camera, and the front of the table all sit on this plane right here, and that's the distance that the camera has chosen to focus at, so all of these things will be in focus. That thing back there is behind that plane of focus, so it is not going to be in focus. One of the most important things to remember about your autofocus system is that it chooses to focus at a particular distance and lights up the point on the object that is at that distance.
Therefore, when you press the button, you need to be really careful to check that your subject has a focus point lit up on it. If this was lit up, I would have bad focus. These are lit up on what I want to actually be in focus in the image, so I am in good shape. Something else to know is that there will be times when your autofocus system picks the wrong thing. On a lot of cameras, you can work around that by simply pressing the shutter button again--see how it has picked some different points here. I can press again, and it gives me different sets. So that allows me to quickly try, or quickly get access to some different focus points.
These are the basics of autofocus, but your camera probably has many more auto focus features in there, some other auto focus techniques that are good to know about, and you can learn about all of those in the "Foundations of Photography: Lenses" course.
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