ISO, Shutter Speed, & Amount of Light

[MTVision]

In those illustrations and the tutorial, phoenix, I didn't see anything about the sharpness of the subject in focus, just the sharpness of the foreground and background. In the tutorial, under Controlling Depth of Field, I didn't see a change in the sharpness of the subject in focus, even though the aperture is changing.

I don't know if I made this as clear as I should have. I am photographing a coin lying flat on a piece of paper, directly above it, from 12 inches away. I don't want anything in the foreground or background. Sorry if that was not clear.

Lenses have a sweet spot where they are sharpest. Kit lens f/8-f/11. They don't perform their best, optically, when shot wide open or even completely closed down. You could probably find this information by looking up some reviews of the lens.

 

[analog.universe]

Sharpness at various apertures is a result of the physics of lens design. Essentially, an ideal lens that focuses all colors perfectly and uniformly all the time is outside the scope of current optical knowledge and manufacturing. So instead, different designs attack the different optical aberrations in different ways, keeping straight lines straight, focusing red and blue together instead of allowing them to drift as they would naturally, countering corner vignetting, etc.. All of these aberrations are minimized as you stop the lens down more and more, and different lenses pull off different levels of decency at different maximum apertures, depending on design.

On the other side is diffraction. As a ray of light passes very close to a solid object, it bends slightly. This effect is noticeable in a lens as the light passes the blades of the aperture, they bend slightly and as a result defocus slightly. When the aperture is large, the percentage of light rays close to the aperture blades relative to the total light is not significant. As the aperture is stopped down, the "free area" in the center becomes smaller, and a higher percentage of the light is subject to diffraction effects. Again, diffraction limiting is a consideration in lens design, so different lenses will exhibit these effects at different minimum apertures.

When you balance these two factors against each other, a lens's sharpest region is usually in the middle somewhere, f/5.6 to f/11 or so. You can look up lab tests for different lenses and find out how sharp they actually are at different apertures, all graphed out nicely. Some of the better lens designs will be sharp enough to outresolve the camera sensor from 2.8 all the way through 11, but you certainly pay for it.

 

[KmH]

A simple lens is thicker in the middle than it is at the edges. Consequently, light from the edges of the lens focus at a slightly different distance than the light from the middle of the lens. Hence focus, is less sharp overall (optical aberration) when the lens is used wide open.

By stopping the lens down to a smaller opening (f/8 or f/11), only the thicker, middle part of the lens is being used, so everything focuses to the same point, and is sharper.

Photographic lenses have extra optics that try to correct for the thin edge, thicker middle focus sharpness aberrations. Consumer grade kit lenses don't do as good a job of that as the much more expensive pro grade lenses.

Lens Genealogy « Canon Rumors