I've been going back and forth here trying to figure out how this all works. I want to take pictures of my fishtank. To get things in the back, I need a focal length of at least 8-9 inches, preferrably up to maybe 12. I have a set of 3 tubes (something like a half cm, 1cm, and 2 or 3 cm thick), and a number of lenses to throw on them. 35mm, 55mm, and 135mm are the prime candidates. How do I figure out what tube combination to use and what lens? I know I can bumble around and trial and error it, but I'd like to have at least some understanding of the theory behind what I'm doing...

also, the 55mm is the fastest lens (f1.4), so I'd like to use that, because fish shooting is tough light-wise to begin with...

oh, yeah, sorry, shooting with a pentax k100d. the 135 is this lens:http://www.pbase.com/knickles/tak135

Originally Posted by tardypizza

I just trial/error it, but this post on another board may help.

Thanks, that's actually really helpful. I can get a general idea of what to do, and then trial and error to fine tune it. I'll hopefully have some shots up soon to show how it goes...