Using an extension tube or additional bellows draw beyond the infinity position is the same. In both cases you’re placing the lens farther from the film than usual.
In close-up photography we sometimes wonder, “At what point do we need to increase the exposure for the additional bellows draw?” That’s best determined by the exposure error we’re willing to tolerate. But how can we quickly determine the error so that we can decide?
On the top of page 32 of the 1975 edition of the Kodak Professional Photoguide:
“When the exposure for a close-up photograph is determined with a hand-held meter, or if exposure is found by using a guide or dial, and if the close focusing is accomplished by extending the lens, then additional exposure is required when the camera is closer than eight times the focal length of the lens.”
When the lens-to-subject distance is eight times the focal length of the lens, the required compensation is 0.39 stops. I presume that the author of the article selected an integer multiple of the focal length at which the exposure error is 1/3 stop or slightly greater as the minimum error that triggers compensation.
Here’s a procedure that works for cameras that don’t have a through-the-lens light meter:
The following table gives the exposure error and a corresponding estimate of the lens-to-subject distance in multiples of its focal length. After measuring the lens-to-subject distance and consulting the table, you can choose at what point compensation is warranted, and know by how much to compensate.
A lens of focal length f requires the following combinations of exposure increase and lens-to-subject distance given in multiples of the focal length (reckoned from the optical center of the lens):
0.1 stop, 29.4f
0.2 stop, 14.9f
1/3 stop, 9.2f
1/2 stop, 6.3f
2/3 stop, 4.8f
1 stop, 3.4f
4/3 stop, 2.7f
1.5 stop, 2.5f
5/3 stop, 2.3f
2 stops, 2f (1:1 image/object ratio)
A quick check with a tape measure and looking at the table can tell you if compensation is warranted and by how much.
