Several people have given the answer, but let me give it again in words.
You divide the distance from the subject to the lens by the focal length. Then subtract one. Finally, take the reciprocal to find the magnification.
Alternately, you can do the following, which is a bit easier. After focusing on the subject, find the total bellows extension, and divide by the focal length. Then subtract one. That is the magnification. In practice, this is easier to use since you don't need to measure the distance to the subject. You make your measurements along the rail. However, if you wanted to know the magnification for a given subject distance and focal length without actually setting up the camera, you would use the other method. The point about not having to buy the lens before deciding if it will work is well taken.
Of course, all distances have to be in the same units. To convert, remember that one inch equals 25.4 mm, and that there are 12 inches in a foot.
Let me remark that to use these rules, all you need is some way to measure the distances and simple arithmetic. It is also helpful to have a simple calculator to do the division. If you can't remember the rules, jot them down somewhere and carry it with you. Many of us use some log book to record apertures, speeds, and other information about camera setup, anyway, and that would be a good place to keep such information. There is no abstruse, difficult mathematics. I can't imagine that anyone who is incapable of doing the arithmetic can successfully take pictures with a large format camera. In fairness, however, I should add that for telephoto lenses and some wide angle lenses, the true extension is signficantly different from the distance between the standards. One way around that difficulty, is to measure the extension beyond the infinity setting instead and to add that to the focal length. That will give you the true bellows extension. (You can also do it using the rear flange focal distance, which is usually included as part of the specifications of the lens.)
Of course, the other methods for determining magnification may be more convenient for some people. It is really a matter of taste and what you would rather not think about. For me, it is easier to measure and remember how to do the arithmetic than to set a scale in the scene, fiddle with something on the gg, and then remember to remove the target from the scene. Others may not want to remember how to do it with arithmetic. What I don't understand is the insistence that one method is better or worse than another. It may be easier for you, but that doesn't mean it is easier for the next guy.
Bookmarks