What's the circle with a line through on top of my camera?

The little circle with a slash through it? It's not just there for decoration...

Quite a few of you liked my "Let's get geeky about ISO" article, so in the same vein, I have a new mini-mystery (and solution) for you guys... If you grab the SLR camera closest to you, you'll see that it probably has a little circle with a line on it on top of the camera. Have you ever stopped to think what it means?

It's a funny one; and one that even many professional photographers is unable to answer. Ladies and gentlemen... It's the...

Focal plane indicator

The Focal Plane Indicator is a line on the outside of the camera, that is aligned perfectly with the focal plane of the camera - on a film SLR, this is where the film runs inside the camera. On a digital camera, it's the front of your imaging chip.

This indicator can be used if you ever have the need to manually measure the distance to your subject precisely. This might come in handy in macro photography, for example, or if you want to know precisely what your minimum focussing distance is on a lens. Where it reads "0.4m - ∞" on the side of your lens, for example, the "0.4m" refers to the distance from the focal plane to the item you are trying to photograph - and not the distance from the front of the lens to the item, as many people believe.

So there you have it, a delicious morsel of photography trivia to start your month of September. What's your favourite photography fact that many people don't know? Leave a comment!