I need to know if there is a way to put lenses in front of each other to make their focal length shorter. I tried putting them directly in front of on another, but I wasn't sure if there was a better way. I don't know anything about lens physics. Thank you in advance!
If you put the lenses very close together and if the thin lens approximation also holds, then their dioptric strengths (inverse of the focal lengths) are approximately additive. If they're not very close together, then in general the combination will not show behavior that's equivalent to that of any simple lens.