Can you make a more focused beam by using convex and concave lens?

[physics-james]

When you put a convex and then concave lens in front of a light source, the light will be parallel but narrower than when it came in such as in a laser beam expander/compressor. Using a pen laser and a convex and concave lens, is it possible to focus the beam by putting a convex then a concave lens in front of it using this method? Taking this to the extreme, would it allow you to create a laser from a normal flashlight with enough lens?

 

[Andy Resnick]

When you put a convex and then concave lens in front of a light source, the light will be parallel but narrower than when it came in such as in a laser beam expander/compressor.

This is a common misconception. It's only true of the initial beam is collimated, which is a limiting case for a zero-size source. 'Beam concentrators', in the sense you are using, is conceptually identical to Archimedes' 'death ray' and both violate of the second law of thermodynamics.

https://apps.dtic.mil/dtic/tr/fulltext/u2/281847.pdf

The optical system you describe is the optical prescription for a telephoto lens
https://www.pencilofrays.com/lens-design-forms/#telephoto

Or, when reversed, a retrofocus (wide angle) lens:
https://www.pencilofrays.com/lens-design-forms/#retro