I have a Hasselblad 501C/M and would like to do some close-up photos of ....well, whatever looks good close-up. I (think) that I understand the concept of extension tubes, however I don't understand the overall technical use of the different sizes; 8mm, 16mm, etc., and what to expect when used with different focal length lenses, such as additional exposure. I have a 50mm, 80mm, and 180mm lenses. Can anyone give some advice?