Telescope optical path length vs focal length

[balaonspace]

Hi

I am trying to design a three mirror off axis telescope. The free parameters are the offset between the apertures and distance between the surfaces.

My question is for a system with 'n' elements/surfaces (n=3 in my case) is there a relation between optical path length and focal length of the system.

I don't want to go too descriptive about this problem. If it might help I can upload a pic in some webpage.

bala

 

[cucumber1]

the distance between well yes`. if you have one eye on a lens and the other open. Eye with more light will give more distance but less focus. some what a distortion ? or a dispacement of light'. i would like to see it

 

[balaonspace]

http://img828.imageshack.us/img828/5718/phyfor.png

In this image 'c' (which i interpret as the optical path distance) and 'f' the focal length can be constrained independently.
For example when i design a system with high focal length with optical path, and optimise the same in zemax, the system focal length reduces to optical focal length for a design with smallest 'rms' radius.
I cannot quiet grasp the difference between the two quantities here.
Thank you for reading.
I am not convinced that i explained my problem clearly. let me know if i need to clarify any part.