Originally posted by baro-nite IQ can be quite different reversed, depending on the lens. But the M50/1.7 is a good lens either way.
You get more slightly more magnification for a given amount of extension with the lens reversed. In general you can expect lenses to perform best in the normal configuration for 1:1 and less, reversed for greater magnifications. But it's worth experimenting.
My favourite reversing lens for many years was the M 28/3.5. Reversed it gave me about 2.1X magnification because I used a 52>49 step down ring to match a 52mm reversing ring to the 49mm filter thread on the 28/3.5 moving the lens a bit farther from the film.
The distance from the lens mount to the subject with a reversed lens is the registration distance (mount to film/sensor) which is something 45.46 mm (?) with Pentax lenses. The 50mm reversed should give you about 1.1 X. Extension tubes increase the magnification significantly.