Originally posted by mcgregni P-TTL automatic flash exposure mode is not compatible with manual focus lenses, only autofocus. A series types allow the aperture control, but flash exposures can be expected to be inconsistent.
I have to differ on this point. I have several A-series lenses (both Pentax and non-Pentax brand) and they all expose well in all the auto-exposure modes (including with built-in flash) as long as they are on the "A" setting on the aperture ring. This is with all my Pentax DSLR bodies.
If you are not getting good automatic exposure with the aperture ring in the "A" setting, make sure the electrical contacts on the base of the lens and the lens mount of the camera are clean. Wipe both with a dry lint-free cloth.
However, as mentioned, it is possible your aperture ring is not free and snappy, so there is a delayed closing of the aperture. One way to test this is to remove the lens, set the aperture to the smallest aperture (e.g., f16), pull back on the aperture lever with your fingertip to open the aperture then let loose of the aperture lever. The aperture should instantly snap closed. If it's sluggish, that's a problem. Another way to test more critically with the camera is to put the camera in Av mode, with the aperture on the lens in the "A" position, then with the Av dial, set an aperture of wide open, or nearly wide open (say f2), pop up the flash and take an exposure. It should be a fairly good exposure. Now dial a stopped-down aperture (say f16) and take an exposure of the same object at the same distance with the flash popped up, and see if the exposure is good. If it is significantly overexposed, your aperture may be sluggish. However, note that not every lens is perfect, and some over- or under-expose slightly because of aperture nonlinearities, but it should be fairly close if all is working properly. I have found on all my DSLR bodies that I prefer a -0.3 exposure adjustment of my built-in flash to help prevent blowing highlights in high-contrast subjects.
I hope this helps.
-Joe-
-Joe-