Talk:Depth of field
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In photojournalism we went over "circle of confusion" again--and after hearing for the 3rd or 4th time, I think I finally understand it. It has something to do with the diffraction of light as it passes through a lens, causing blur b/c of overlapping on the negative. I think. Could someone who knows, not suspects, go through this again, maybe providing an image to help illustrate it? Thanks, Koyaanis Qatsi
The circle of confusion is due to refraction, not diffraction. Generally speaking, if you have a point source of light in front of an (ideal) lense, then refraction and the particular shape of the lense causes light rays from the source to meet in a single point behind the lense. If that point happens to lie on the film, the point will be in perfect focus on the photo. If the point lies before (or behind) the film, then the rays haven't completely met yet (or are already diverging again) when intercepted by the film. The precise circle on the photo results from the fact that the light rays, before hitting the lense, went through a diaphragm of fixed aperture; without the aperture, the bright region on the film would be much bigger, since a lot more light rays from the source would contribute. AxelBoldt, Saturday, May 25, 2002
Refraction, right. I had the right idea but wrote the wrong thing. What I don't understand, though, is why having a wider aperture results in shallower depth of field and a smaller aperture results in greater depth of field. I keep thinking I understand it, and deciding I don't. Which is why I thought a diagram and an explanation would be nice. Koyaanis Qatsi, Wednesday, May 29, 2002
Making the aperture smaller makes the circle of confusion smaller. Imagine the ideal case: a tiny tiny aperture, letting only a "single" light ray through. That light ray would be refracted at the lense, but would stay a light ray, and the circle of confusion would be a single point. Now a little larger aperture will let several rays through, these diverge a bit, the lense brings them together again, but the film intercepts them "too early", and you see a small circle. The larger the aperture, the more light directions get through, the lense tries to bring them together again, but they hit the film too early and you get a larger the circle of confusion.
Now, all the distances for which the circle of confusion is small enough will be more ore less in focus on the film. If the aperture is smaller, a larger range of distances will qualify. AxelBoldt, Wednesday, May 29, 2002
Is it worth commenting on the depth of field of a pinhole camera? David Martland 07:28, 10 Dec 2003 (UTC)
Sure