This topic came up in a different discussion and it seemed like it deserved its own thread so others can benefit. Here goes...

When you read the specifications for a lens, one of the specs is its minimum focus distance. It tells you how closely you can get to your subject and still focus. Unfortunately, the specs don't provide much more information than that and this can create confusion. Is the focus distance measured from the front of the lens or the optical center of the lens? Answer: Neither.

Most camera and lens makers measure the focus distance from the focal plane inside the camera as illustrated below (I prepared these illustrations for a software manual that I've been writing). In other words, they measure it from the surface of the image sensor if its a digital camera or the surface of the film if its a film camera.

Some cameras like the E-1 mark the location of the focal plane on the outside of the camera body for the convenience of photographers. It is shown below (the circle with a line through it: ). I haven't seen this label on many consumer cameras and I'm not sure if any of the other 4/3rds cameras have this label. Even on the E-1, Olympus doesn't explain it anywhere in their manual so most owners who don't know what it means have no idea why the symbol is there.

You can estimate the location of the optical center of a lens by measuring forward from the focal plane of the camera by the focal length as shown below. In other words, the optical center of a 50mm lens will be about 50mm in front of the camera focal plane.

This is only a close approximation of the optical center because of the complexities of modern lenses. But there's more: The optical center of many lenses changes. Obviously, it changes with the focal length setting of a zoom lens. But what is not obvious is that it can also change with different focus settings---even with a prime lens---because many modern lenses shift the focal length when you focus at distances closer than infinity. In fact, if you read the fine print from many lens specs you'll see that the focal length rating is almost always referenced to a focus distance of infinity.

For most of us, knowing exactly how the focus distance is measured and where the lens optical center (also called its "null point") is unimportant. But two photo specialties that need this information are: macro photographers and panoramic photographers.