Microplanar 4.5/40mm hi-res lens
Microplanar is a special lens with flat image field purposed for micro photography. This series has very good chromatic correction, large field of view, relatively high speed. It can be used with an eyepiece or without it.
LOMO Microplanar series is similar to Zeiss Luminar, though they are not of so high quality. But price is lower as well, so you have a choice between quality and price.