double-concave lenses have negative foccal lengths, diverge 

collimated incident light and form only virtual image which 

are seen through the lenses. They are often used to expand 

light beam or increase focal lengths in existing systems 

and are normally used in combination with other lenses. 

In a single lens application you shoud consider using 

them in prefenrence to a single plano-concave lens 

if the magnification lies in the region.