woodsliv Loc: Tehachapi,CA

have a SMC Pentax-M 1:2 50mm lens, would this be a good reverse mounted lens on a D5100 for macro?

oldtigger Loc: Roanoke Virginia-USA

yes

Nikonian72 Loc: Chico CA

Wide-angle lenses work best reversed, but a 50-mm f/2 could be used reverse-stacked on a longer focal length lens. Read more here:
FAQ: Differences between Reverse-Mounted and Stacked-Lenses for Macro-Photography?
http://www.uglyhedgehog.com/t-90623-1.html

Screamin Scott Loc: Marshfield Wi, Baltimore Md, now Dallas Ga

Wide angle lenses will give you greater magnification reversed, but will also put you right on top of your subject, meaning it will be hard to properly illuminate it. If that subject happens to be a skittish insect, you won't be able to get close enough before it moves away. Douglass's suggestion of reverse mounting a 50mm onto a longer focal length lens is actually better as then you would still retain metering... The other problem with reverse mounting lenses is that the rear element of the reversed lens is exposed & easily damaged (having to get so close to get the subject in focus means a higher probability of getting too close & striking objects). Damage to a rear lens element is much worse than damage to a front element since it is closer to the sensor. I shoot Nikon & they used to make an item called a BR 3. What it is, is an adapter that attaches to the exposed rear lens mount & allows one to screw a "protective" filter onto it, thus giving the reversed lens some "protection". I'm not sure whether anyone else made such an adapter but it is likely...