Here's another interesting comparison to go with the "choice
of exposure" thread
. These are strips across a coin, each one taken at a different aperture, from f2.8 to f45, as shown on the image. The focus was set once at the start, and was not adjusted between these shots.
Depth of field
is very important. At f2.8, there is very little tolerance. You can see that here, either the coin or the camera is not positioned perfectly level. You'll never get an ancient coin to be perfectly level, nor
of course perfectly flat. So in this case, at f2.8, the entire right side
of the photo is blurred.
Reducing the aperture improves depth of field
, until at f8 and f16, the result is pretty good
. This also has the advantage of directing the light only through the part
of the lens which is most likely to be very accurately made, and which should have the least optical or chromatic aberration.
Reducing the aperture further blurs the whole of the coin. At f45, the smallest aperture my camera can do, this is very noticeable. This effect is casued by an optical effect called diffraction.
For a 35mm digital camera, f16 is about the best option for sharp
results. If you are using a smaller camera, the optimum might be f8 or even f4.