Quote:could I just replace a broken DCX lenses with PCX of the same FL.
Yes and no, depending...
A converging lens, DCX or PCX, will focus a collimated beam to the focal point of the lens at its focal distance. Collimated light travels parallel to the focal axis. The light rays from an object at an infinite distance from the objective lens are considered to be traveling to the objective lens as a collimated beam of light.
If the next lens is a DCX and is placed at the proper spacing, the converged rays from the objectives focal point should converge again at your DCXs back (rear) focal point, toward the ocular end.
However, if your next lens is a PCX, and depending on how you orient it, your point image coming from the objective focal point could be converted back into a collimated beam, which would be imaged to infinity, and not the focal point of your next lens downstream.
My post about splitting a DCX into two PCXs was a hint. With the orientation of the two PCX halves from the DCX kept unchanged, and when slowly separated, you should have collimated light between the two halves. Now combine that idea to obtain an
effective focal length to replace a DCX lens that you can't order in exactly the focal length you want, and you've got a possible work-around.
Selecting a lens is kinda like selecting a new canister powder. You need to gain an understanding of its particular properties, as compared to others, and experiment a bit to see if its suitable.
Snell's Law and ray diagraming is an important part of understanding the particular properties of a lens.