Here’s why is there a London Underground logo on top of your camera.

It’s a circle with a line through it, and you find it on many SLR and mirrorless bodies. But what is it? It’s actually surprisingly useful…

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, that little logo is called a Focal Plane Indicator.

Blink and you’ll miss it, but you can find the focal plane indicator on most camera bodies, if you start looking for it.

The Focal Plane Indicator is a line with a little circle on it, usually located on top of the pentaprism on the top plate of the outside of the camera body. The line is aligned perfectly with the focal plane of the camera.

On a film SLR, the focal plane indicator shows where the film is located on the inside the camera. On a digital camera, it indicates the location of your imaging chip.

For the vast majority of photography, you don’t need to know the distance to your subject to the exact millimeter — but there are exceptions. For example, this might come in handy in macro photography, 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.

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