Originally posted by Tremaine ...Sorry for the lengthy post and bragging but 23 bucks for two lenses isn't to bad right?
Shhh... RioRico pounces on bragging like this like a panther and starts talking about 645 folders and something called enlarger lenses.
There are two types of lenses that are not quite compatible with Pentax DSLRs and your 28mm may have both incompatibilities. Just from the serial number, it sounds like a Vivitar lens made for them by Cosina. The lens may not mount because it has an extended light shield that runs into parts of the camera mount. It's common on Vivitars. Compare the lens to your kit lens. Between the aperture lever and rear lens element is the shield I'm talking about. On the kit lens it only covers the range of the lever. On your $23 special, it might extend about 1/3 around the lens. The quick fix is to remove just that flat black piece, usually 3 screws holding it in. The more complete fix is to file or cut it to resemble the kit lens, paint the shiny parts black and reinstall.
The second incompatibility is called the Ricoh pin. It was designed to work with Ricoh program cameras. Your lens has the pin because it says PK-A/R. The pin can fall into the slot for the auto-focus drive pin on the camera and prevent you from removing the lens. If it's rounded, it is not likely to get stuck, but it will feel different when it's installed. The third photo here shows a rounded Ricoh pin:
https://www.pentaxforums.com/forums/pentax-lens-articles/87589-photos-k-m42-m...ns.htmlhttp://
If your lens is a Vivitar, the pin is probably OK. It can also be easily removed or ground down with a Dremel tool.