Should the indicated focus distance on the lens be checked by measuring from the subject to the front element on the lens, or to the film plane?

-- Alan Shapiro (ashapiro@yorku.ca), December 02, 2001

Answers

It should be to the film plane. The front element of many lenses move in and out when you focus. That would make it a nightmare to calibrate distances to the front element!

-- Johnny Motown (johnny.motown+bwworld@att.net), December 03, 2001.

Depending on your camera, look on the top plate. There may be a small circle with a short line running through it (parallel to the rear of the camera body). That is the film plane marking.

-- Jeff Polaski (polaski@acm.org), December 04, 2001.

Well...a simple explanation is that the focal length is the distance from the lens (simple lens) to the image of an object at infinity, which would be at the film plane in the camera. Unfortunately, with complex lenses, the answer is more complex. In complex lenses, the focal length is the distance from the second nodal point (an imaginary point) to the plane in which parellel rays are brought to a focus.

-- Al Fairclough (Lensman@hotmail.com), January 10, 2002.