If a photographer need to focus on object at 120mm?

Question

in order to produce a sharp image he must insert a tube x m so that so the distance available becomes (50+x)mm to (70+x)mm how do i no whats the smallest length of tube i need? and will this tube change the linear magnification?

Answer 1

Trial and error; start with the shortest tube in your bag, and yes, you will get magnification, relative to the longer normal lens.

You may also need extra light as the distance gets shorter.