The camera is the MZ-5n, which is on loan from a local list member, and the lens is a Pentax short tele 1.8 aperture. When using the lens on the A setting it will sometimes show the aperture in the viewfinder as 1.7 ;-)) I didn't check to see if that's the case in aperture priority mode as well.
In a Pentax booklet, they mention that the A135/1.8 will be shown as a f/1.7 lens. I understand the camera cannot show 1.8.
If this the lens you tried, it must be front heavy on the MZ-5n although the grip would help.
Andre
