One of the benefits of being a long term Pentax User, at least in my own personal photography, is that there is a magic wonderland of lenses out there that can still be used today on the current DSLR bodies. I have chosen a selection of lenses that I have used, or still use all the time, on my full frame Pentax K-1.
We start off in the 1960s, and, with a simple Pentax Adapter K, even screw thread-era lenses can still be used. They are of course manual focus and they need to be stopped down manually. Manual exposure and Av modes work on the K-1, and they are a viable set of lenses to use if we wish to. They are also very nice lenses, and I keep my Super-Multi-Coated Takumar 135mm f/3.5 simply because it's too nice to sell for the pittance that it would fetch.
Once we get to the Pentax K mount (1975-current) then no adapters are needed. The original K mount is manual in that there are no electronic contacts, so we have manual focus and still need to stop down manually to the working aperture. However, there are some very fine lenses available and one of them is the Vivitar 55mm f/2.8 Auto Macro, a highly regarded macro lens that I do sometimes use.
The first major step forwards in zoom lenses came with the Olympus 70-150mm f/4, and, soon afterwards, the SMC Pentax-M 75-150mm f/4. This latter lens is a great performer on the K-1 and I have actually ended up with two of them. One is the lens I reviewed for EPZ, see their Vintage Reviews section, and the other was going to be thrown aside after a friend of mine died. I can't bear to let either go because they are both in beautiful, pristine condition. Particularly Olympus and Pentax lenses of this era are just items of beauty in their own right.
Moving into AF lenses, there is one lens that pentax has stuck with over the years, and that is the Fish-eye zoom. The current APS-C lens is a 10-17mm, but the original full frame lens is the SMC Pentax-F Fish-Eye 17-28mm f/3.5-4.5. Cosmetically this has more in common with the FA series lenses than the F series and it looks like a half way stage in that progression. However, it's a cracking good lens, going from full frame Fish-eye at 17mm to a much more rectilinear wide angle at 28mm. Lines are still not totally straight at 28mm, but it makes for a very useful lens. It is in current full time use.
Also in current full time use is the equally useful SMC Pentax-FA 20-35mm f/4 AL lens, an excellent, very compact wide angle zoom. It is always with me on architectural shoots.
Finally, a compact telephoto zoom. The SMC Pentax-FA J 75-300mm f/4.5-5.8 AL is the first of the J series lenses that accompanied the last film camera, the Pentax *ist and the first DSLR, the Pentax *istD. There is no aperture ring, for the first time with Pentax, this being controlled by the camera bodies. Older bodies would not be able to do this, but they could still use Program and Tv modes to work well with these lenses. This lens is not expensive, and at one point recently SRS Microsystems in Watford found a batch of new ones that they sold for les that £120. This was a mega-baragain and I snapped one up. It performs well, not as sharp as more expensive optics but brilliant for portraits and flower studies. It has a very nice character and sublime bokeh.
Sometimes I see articles discussing using older lenses and they do seem to be pre-occupied with mirrorless systems. However, they are missing a trick by not recognising that the Pentax DSLRs are about as compatible with older and varied types of lenses as they could possibly be wished to be. Don't tell anyone or prices might start to creep up.......