If you’ve looked closely at your camera, you’ve probably noticed a strange circle/line symbol somewhere on it (you can see it above). It’s generally near the viewfinder but could be anywhere on the side.

What’s most unusual about this symbol is that, unlike most of the other weird things all over your camera, it’s not paired with a button. It just sits there on its own. So what does it mean?

The circle/line symbol marks the exact placement of the sensor (or film plane) inside the camera. It’s one of the many throwbacks to old film cameras.

Now, just because it’s a throwback doesn’t mean it’s not useful. Knowing the exact sensor location lets you accurately measure the distance between it and the subject. Measuring from the front of the lens is fine if your subject is a dozen feet away, but if they’re close, an inch or two makes a big difference.

In particular, macro photographers still use the focal plane mark to measure the distance to the subject so they can manually focus accurately and product photographers use it to dial in their lighting set up.