If your lens is what I am thinking it is, and I don't have one, but I do have another Focal brand 135mm lens. The Focal brand was sold by K-Mart in the 70s and 80s. K-Mart used to have quite an extensive photo department with even chemistry and paper in some stores but that's long gone. Anyway, the Focal lenses were re-badged versions of other lenses so I wouldn't be surprised to find it was the same as a Vivitar. They work OK but aren't stellar performers. But, then again, they didn't cost like stellar performers either. It certainly should be usable but the Pentax should be a much better lens. Depending on how you use it, the quality may not make much difference to you. My Focal 135mm works quite well, but I would agree that it is a bit soft - contrast is lower due to poor control of flare by the less expensive optics.
