One of the reasons I bought a Pentax DSLR is because Pentax is the only company currently offering full compatibility with all the lenses they ever made.

The older, screw-mount lenses need a 15$ adapter to be used with their bayonet mount. Each lens retains the capabilities it had when first sold. A manual focus lens will not suddenly autofocus.

Pentax active autofocus system

When manually focusing a lens, the autofocus system on Pentax DSLRs is still active and will tell you when accurate focus is achieved. In other words, your hand is the focus motor but the camera still confirms focus.

At the press of a button, the metering system of the camera is also active even when using a manual lens.

Pentax users can use the millions of legacy lenses without issues, although using a manual lens is slower than using a fast autofocus lens. But those older lenses can be purchased very cheaply and are often high-quality optics with great build quality. Some old lenses are actual legends, still avidly sought after by users willing to sacrifice autofocus in exchange for high quality.

I personally own four manual lenses, two of which are still very well regarded today, the other two being comparable to entry-level lenses.