How does the aperture diaphragm [or a human iris for that matter] change the amount of light that passes through without changing how much of the image gets projected onto the focal plane?
To put it another way…When I stop down the lens on my camera why isn’t the result a small circular crop of the original image in the center of the frame?
This seems like something basic but I’ve never figured it out, and it’s bothering me that I don’t know how it works. I tried google but, apparently, I don’t know enough about the field to come up with the right search terms.
First of all imagine that the light emanates from an object in all directions, all travelling in a straight line so they spread out as they get farther from the object. It is then easy to imagine that different rays of light from this object (and all other objects in the field of view) will strike the lens over it’s entire surface.
It is the job of the lens to change the direction of all these spreading-out rays (by refraction) so that they all come together again at a point as they strike the film/retina/ccd, forming an image. That is what it means to bring the object into focus.
When you have that picture clear in your mind, consider what would happen if you cover all but the center of the lens, i.e. reduce the aperture. Light from all parts of the subject will still be hitting the still-open part of the lens, still forming an image, but when the rays are converged the image will be darker because some of the image-forming rays have been blocked off.
OK, in the real world things are a little more complicated. There is always some light-falloff farther away from the center of the image (known as vignetting) and this gets more pronounced at smaller apertures but the principle holds.
But here’s another version that might be easier to picture.
It’s the lens’s job to shoot light rays from each point on its entire back surface to every point on the image plane - every point on your retina, or on the film, or whatever. Note this is not one ray from each point on the lens to only one corresponding point on the film. Each point on the lens sends a ray to every single point in the image.
The aperture alters that job by starting to turn off all the rays coming from the outermost points on the lens. But each point on the film is still getting rays from all the remaining points on the lens. The image on the film gradually gets dimmer as you close the aperture, because there are fewer points on the back of the lens contributing. But the remaining points on the back of the lens are still throwing rays at the entire film.
Hey, here’s a third version. The aperture’s job is to make the lens act like they made it from a smaller diameter blank of glass in the first place. The image a lens makes doesn’t look like a picture of the lens, right? So the edge of the lens doesn’t decide where the edge of the image is. Well, neither does the edge of an aperture if you build one into the lens.
Vignetting (if you mean loss of film coverage) decreases as you stop down. Using a view camera and movments, if I use a lot of rise or tilt, I may be on the “ragged edge” of coverage. As I stop down, I reduce light cutoff because the projected cone of light from the lens is not being cut off by the collar and flange. Stopping down also helps to “conceal” a lack of exact focus due to a deeper depth of field, and any flaws that may be in the lens itself (you are operating in the “sweet spot”). You also have to worry about light falloff if you use a short (wide angle) lense. It sits closer to the film, and so the cone of light has less room/distance to spread out and cover. If you shoot 35mm or medium format it all becomes pretty much a nonissue, the manufacturer has taken care of it and the lens/body relationship is fixed. It helps to think of your lens as a projector. It projects an image onto the film. Look up Leslie Strobel’s book, or John Sexton and Q. T. Luong on the web. Luong runs a website devoted to this stuff.
Let’s say I’m taking a picture of a person. Light is reflected off of every part of the person, in many directions. Let’s consider a single point on the person, such as the tip of the left ear. Some of the light from that point will hit the top of the lens, some will hit the middle of the lens, some will hit the left edge of the lens, et cetera. But no matter where on the lens the light from the tip of the left ear hits, it’ll all hit the film at the same spot, and that’s the spot on the film that’ll be the image of the tip of the left ear. Now, suppose that I decrease the aperature of the lens. Some of that light that went through the edges of the lens doesn’t get through, now, but of the light that does get through, all of the light from the left ear still hits the same spot. So my image will be fainter (because it’s got less light), but the whole image will still be there.
At least, this is the case if the image is focused. If it’s not focused, then light from the left ear hitting different parts of the lens will not quite all reach the same spot on the film, so the left ear on the picture will appear smeared out. The rays which are going through the edges of the lens will be the ones which are smeared out the worst, so if you decrease the aperature, an out-of-focus image won’t be as bad. This is why decreasing the aperature on a camera increases your depth of field, or allows a wider range of objects to be in good enough focus.
If we take this to the extreme, we can stop down the aperature until we have only a pinhole. With a pinhole, the depth of field becomes infinite, so everything will be almost in focus. We don’t even need a lens any more at this point. Of course, the drawback to this is that you’re only getting a tiny fraction of your original light, so your image will be extremely faint.