Choosing Aperture for Coin Photos

Coin Photography – Choosing the Best Aperture

There are eleven pages about photographing coins:

Black Background — White Background — Photographing the Edges — Using Extension Tubes — Choosing the Best ISO Setting — Choosing the Best Aperture (this page) — Choosing the Best Exposure — Editing the Background — Keeping Detail in the Highlights — Processing Your Image — Some Final Tweaks

Two photographic factors affect the best aperture to use for coin photos – depth of field, and diffraction. Here are some strips across coin photos taken with my EOS D60, 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. In fact, the right side of this coin is tilted down slightly because of the irregular nature of the other side, which it is resting on. You'll never get an ancient coin to rest perfectly level, nor of course will it be perfectly flat. Normally, you might tilt the stage a little to compensate for this, but I have not done so here, so as to give a clearer demonstration the effect. So in the photo taken 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 blurring is caused by an optical effect called diffraction, whereby waves going past an edge bend a little. A small aperture has a lot of edge in proportion to the amount of space, so the diffraction effect becomes more important as the size of the aperture decreases. (This is an oversimplified explanation, but there isn't room here for the full one ..)

How much this affects the resulting image depends on the resolving power of the camera's sensor. Some cameras, typically those with bigger sensors, will do better than others. For a 35mm digital camera, somewhere around f11 is probably about the best option for sharp results. If you are using a smaller camera, the optimum might be f8 or less. But please try this test out on your camera – the technology is moving on all the time and you might see a quite different result.

There are eleven pages about photographing coins:

The content of this page was last updated on 27 January 2011