Originally posted by bluestringer Adding 50mm of tubes to a normal 50mm lens gives you 1:1, correct? So if you add 50mm of tubes to a 50mm macro lens that already does say 1:2, does that still only give you 1:1 ?
Close, but not quite. I think it is more a case of the subject distance vs image distance and how focus can be achieved rather that purely how much tube you put in place. Optical magnification is given by the fraction (Image Distance)/(Subject Distance). So if your subject is 50mm in front of the optical center of a standard 50mm lens, you will have 1:1 magnification. The problem is that a 50mm lens is meant to focus infinite rays at 50mm behind the optical center. As the subject get closer to the lens, more bending must be provided to get the rays to focus on the sensor or the lens elements must move forward away from the sensor thus inherently changing the magnification. For example, the 18-55mm has a minimum focus distance of about 23cm, so at 55mm it should only have a theoretical magnification of about 0.23x while the actual quoted magnification in the database is 0.34x. In reality the main elements are moving forward thus moving the optical center of the lens away from the sensor thus increasing the Image Distance in the fraction.
Tubes change the minimum focusing distance (Subject Distance) by largely increasing the (Image Distance). So adding the tubes increases the magnification by changing both top and bottom of the fraction. The resulting focus range however is restricted greatly by the tubes and quickly becomes a very narrow band between minimum and maximum focusing distance but very close to the lens front.
So all in a bit more complicated that you initial thoughts.
Adding tubes to a macro certainly increases the magnification, just not sure of if there are any definitive ratios. Have used about 35mm of tubes on DFA-50 f2.8 macro for some technical macro photos at work as part of a debris ingress wear investigation, and the magnification was amazing. When blown up on a computer monitor the images are like looking through a low magnification high quality microscope.
I sure others with great insights than I will also chip in.