Paul Noble , Nov 16, 2006; 01:27 p.m.
First of all, is the Pentax lens a K-mount or screw-mount lens? There are adapters to allow a Pentax screw-mount (M42) lens to be used on a Nikon camera, with limitations. As far as I know, there are no adapters to allow a Pentax k-mount (bayonet) lens to be so used.
M42 is the formal name of the so-called Pentax or Universal screwmount lenses. It refers to the fact that the thread is 42mm in diameter x 1 mm per thread.
Tamron's Adaptall II is a proprietary system used on some lenses from that manufacturer. You buy an Adaptall lens, which is the same for all cameras, and you buy an adapter that mates that lens (or any Adaptall lens) to your specific camera. If you get a new camera, you simply buy a new adapter and keep the lens. This system will not help you mount a Pentax (screw or k-mount) lens to a Nikon camera.
The problem with adapting Pentax lenses to Nikon cameras is that the Nikon body is thicker than the Pentax body, by a couple of millimeters. This means that, even with an adapter, a Pentax (screw mount) lens can not focus to infinity on a Nikon body. There are adapters that include a lens that modifies the focal length of your pentax lens, so that it will focus to infinity. I strongly suspect that the image quality suffers at least a little when using such an adapter. In this respect, such an adapter is like a teleconverter and we all know that the image suffers in that case.