Those are linear magnification scales.
For example at 39cm you are at 19x with the 50mm lens.
Knowing your magnification is important for choosing an enlarging lens. When you are outside the manufacturer's magnification specs for a lens, you are likely to have difficulty with blurry corners.
For example if you have the popular Rodenstock Rodagon 50mm f2.8 lens, you will be exceeding the manufacturer's recommended guidelines to use it at 19x. So you can be hitting your head against the wall trying to get all 4 corners and the center sharp at that magnification.
Last edited by ic-racer; 02-21-2012 at 08:55 PM. Click to view previous post history.