I need to reduce the beam diameter of a laser I am using. I plan to do this by first passing the beam through a positive spherical lens with focal length f1 and then a negative spherical lens with focal length f2 and f2 < f1. By placing the lenses so that the two focal points overlap I will have a new beam diameter this is reduced by the factor f2/f1. I know what I want f2/f1 to be but I can accomplish this with different combinations of f1 and f2. For example if I want f2/f1 = 0.5 then I could use f1,f2 = 200mm and 100 mm or 400 mm and 200 mm.... So is there some other criteria I can use to decide what f1 and f2 should be. I know space is a consideration. I only have so much physical space so f1 and f2 can not be extremely large. But is there anything else to consider that will affect the beam quality?