What is an “image plane indicator”?

Question

There is a symbol, a circle with a horizontal line passing through it, on the right side of the top-plate LCD of my Pentax K-5:

This symbol also appears on my Pentax K-r on the left side of the viewfinder assembly, only that it is engraved rather than painted on. The K-5 manual calls this the image plane indicator. What does this symbol mean, and what is it used for?

Asked by DragonLord

Answer

That marks the location of the sensor (or the film plane on a film camera). You won't often have any commerce with it, but it is the "start point" when talking about focus distance. If a lens says on its spec sheet that its closest focus point is, say, 45cm, it means 45cm from that plane. (Because of the viewfinder and prism housing, it's not practical to mark on the camera body where the sensor is actually located).

It's used mostly for macro photography, where the difference between "rough distance" (the distance from the camera to the subject) and actual distance makes a difference when calculating exposure or magnification.

Answered by Stan Rogers

Check more discussion of this question.