I’ve been an amateurish photo guy for a while. I understand how the lens focuses light onto the film plane (or mirror) and how the focus ring moves the lens elements apart slightly to change the focus field. Here’s what I don’t get:
I don’t quite understand how the aperture controls the brightness of the image. It makes the “hole” that the light goes through bigger and smaller, so I would imagine it would chop off the outside of the image at high f-numbers. Indeed, when I press the aperture-preview lever on my camera, I see a darker image, but the picture is decidedly unchopped.
Also, why does the aperture affect the focus field? The higher the f-number, the deeper the field. And why is the field so deep on wide-angle lenses, and so shallow with telephoto lenses?
From what little I know of photography, a picture is made by light exposing the film. The smaller the aperture, the less light that actually makes it through, thus the film is exposed less and appears darker. The rest I’m not sure about.
This is almost impossible to explain without a figure, so I’ll refer to this page and the figure about halfway down, entitled “Image Formation by a Converging Lens.”
The Object in that figure really represents a single point being imaged (it’s arbitrary - each point in the focal plane has a corresponding point on the film). Notice how the light reflected (or emitted) from that point go out in all directions (the blue lines), and some of them strike the lens. All of that light that passes through the lens will be focused onto a single point on film plane - the Image Location in the figure.
It’s common (and sometimes convenient) to think of what’s going on by only thinking of that single line that passes directly through the center of the lens. That kind of misses the point, though - even the stuff that hits the very edges of the lens gets focused to the same point. That’s what makes the lens useful.
Thinking about this picture, you can probably see that the bigger the lens, the greater the fraction of the reflected/emitted light from the Object point will strike the lens, and thus be focused upon the film plane.
Now imagine a mechanical iris sitting right behind that lens. Wide open, we still use the whole lens. Now close the iris to half its original diameter. In so doing, you block 3/4 of the light that hits the lens front from passing through and getting focused on the film (remember, this is 2-dimensional so area goes as r[sup]2[/sup]).
Hopefully, this sort of shows why opening and closing the aperture does not start cropping the image. It merely cuts down on the amount of light from each point in the image that actually reaches the film plane.
If the aperture was closed to a tiny pinhole, you still wouldn’t get any cropping of the image. Rays would pass only through the exact center of the lens, but you’d still get that tiny cone from every Object Point making it to the film plane.
As I read over that, I’m not sure it will really help or not. I’ll just toss it out there and see what questions it answers and which ones it obfuscates.
Actually, it just reduces the blur. So the focus is “good enough” over a wider range of distance.
Why does it reduce blur? Well, imagine a parallel beam - say, light from a star - being focused by a lens onto an image plane. If you put your film right on this image plane you will get a perfectly sharp image, regardless of what the aperture is. But what if the film was slightly off the image plane? It will produce a blurred point - i.e. a disk. Then you close the iris. What happens? The cone of light originating at the lens and converging on the image plane gets thinner. So the disk, the blurred point, also gets smaller. What was previously an unacceptably blurry image can become an acceptably sharp image.
You have to use these vague qualifiers because focus is not a simple yes-no quality. The books may have “standard” depth of field formulas, but that depends on how sharp you want the photo to be.
I’m a little more fuzzy on this (pardon the pun :)), but I have a vague understanding. Taking a look at that same figure that I was referring to before, imagine that the lens is slightly out of focus. I think the point will be clearer if you imagine that the film plane is slightly ahead of or behind that Image Location dot (same Object Point and lens locations).
All those rays diverging from the Object Point won’t converge perfectly on the film plane, but will hit it in kind of a blur. By looking at the rays that they have drawn, you can see that the outer edges of that blurry spot are the light rays that have passed through the outer portion of the lens. By reducing the aperture, then, you’re eliminating the “worst offenders” from the incoming rays hitting the film plane. All things being equal, then, the out-of-focus object will look a bit less blurred with a smaller aperture.
If we turn that around, we can say that, for a smaller aperture, an object must be farther out of the object plane in order to appear equally smudged on the film. Effectively, this is saying that a smaller aperture yields a larger depth of field: you can get away with being farther off of perfect focus without it being too bad.
That’s pretty hand-wavy, but with luck it will get the point across.
