How much exposure must I add if I am using 25mm of extension on a 90 mm lens. It is a non metered P6x7 Thanks Chri

-- Chris Hansen (chrishansen@saber.net), April 25, 2002

Answers

The usual rule when adding extension is that adding extension equal to the lens' focal length reduces light by 2 stops and gives a magnification ratio of life size on film, or 1:1. Since you are using a 90mm lens, 90mm of extension reduces light by 2 stops, 45mm of extension reduces by light 1 stop (1:2 mag ratio), and 22.5mm of extension reduces light by 1/2 stop (1:4 mag ratio). So a 25mm extension tube (very close to 22.5mm) will reduce light by about 1/2 stop and give a magnification ratio of 1:4, with the lens' focusing ring set at infinity. If you turn the focusing ring to closest focus, the magnification ratio will increase only slightly, and the light loss will be only slightly more. Just to be safe I would start by adding 1/2 stop exposure to compensate for the 25mm tube and bracket a little, maybe 1/2 stop + & -.

-- Bobby Mahaffey (mahajen@prodigy.net), April 25, 2002.

Chris,

Check the archives. This question came up a while back and someone posted a chart that listed the compensation required for all three tubes individually and combined.

Joe D.

-- Joseph A. Dickerson (jadphoto@aol.com), April 25, 2002.