Choosing ISO setting for Coin Photos

Coin Photography – Choosing the Best ISO Setting

There are eleven pages about photographing coins:

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

ISO setting on a digital camera mimics the behaviour of different films which required more or less light to create an image. The downside was that the faster a film was, the more grainy was the resulting image. Digital cameras have similar characteristics for similar reasons, except of course that you can change ISO setting from one frame to the next. The higher the ISO rating, the shorter the exposure. Less light is used to create the image, and you are more likely to see a grainy result.

With my little Ixus 400, a setting of ISO 400 gives a noticeably poor result. On the right is a pair of photos, one taken at ISO 50, one at ISO 400. You can see that the ISO 400 shot does not look as sharp, and appears to be patchy or grainy. This is particularly noticeable on the lower cheek around the jaw. This image also seems to have phantom colours floating on it.

On the left is an expanded shot, of just the letter "T" from the word "AVGVSTA" from each of the same photos, which shows what is happening at pixel level. You can see a lot of artificial colour has been introduced when the camera has tried to fill in details it really doesn't have. Each of the three primary colour channels captured by the camera is contaminated with "noise" produced internally instead of actual captured light.

I should make it clear that this is an excellent little camera that has given me many great photos. But here, I have pushed it to its limit to demonstrate a point.

How noticeable this colour and grain effect is depends on how good the camera's sensor is. A small cheap camera, or an older one, is more likely to give poor results. However, look at these comparison strips from my EOS D60. These are at the natural magnification of the sensor, and I can see very little difference from one end of the scale to the other.

There is perhaps a little more coarseness in the colour tones of the coin in the ISO 1000 strip, signs that the same effect that the Ixus 400 showed so strongly are just beginning to appear. But it's hard to be sure. I might just be seeing what I expect to see.

Normally I would unhesitatingly recommend using the best quality option, ISO 100. Capture more light, and make a better picture. But having made this test, I think that with a decent camera you could use ISO 200 or even ISO 400 with no problems at all. That might be useful if your setup makes it hard to avoid camera shake on longer exposures – for example, if you live next to a busy road. But please do this test with your own camera first. You can see that there is a lot of variation.

There are eleven pages about photographing coins:

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