According to Camera Wiki, the Pentax MZ-50/ZX-50 uses KAF2 mount lenses. It likely also works with older (manual focus) K-mount lenses, though beware the Ricoh variant, which has a pin that can get stuck on the screw drive of some Pentax bodies.

There are significant trade-offs in functionality when using old lenses with digital cameras. With old Pentax-branded lenses, it can be worthwhile. However, you have a third-party lens that would likely lose autofocus when adapted. Unless you specifically enjoy using adapted lenses, this is likely not the best way for you to move into digital.

To use your old lenses, your options include:

• Using a mirrorless camera with adapter. However, you would lose autofocus and electronic aperture. See Can I use lens brand X on interchangeable lens camera brand Y?

• Use a Pentax camera that uses a modern variant of the K mount. There are full-frame DSLR (K-1), APS-C DSLR (K-3 and others), and APS-C mirrorless (K-01) cameras. New models are released regularly, so check to make sure you are looking at the latest ones.

  • The lenses you have likely require a screwdrive to operate autofocus. Check the specific camera models you're interested in if autofocus matters to you. The following image shows what to look for:

    

  • According to Mark Roberts: Pentax Lens/Camera Compatibility, there are some other issues you should be aware of (emphasis added):

    Some third-party lenses (Vivitar and others) from the manual focus era have an issue that causes problems with later Pentax cameras: A flange which is meant to protect the diaphragm actuation lever but which is much bigger than is really necessary. If your particular lens has this "feature" it will prevent the lens from being mounted on many Pentax cameras, including all the digital SLR's.

    Older versions of the Sigma 24-70 f/3.5-5.6 autofocus lens are incompatible with Pentax Digital SLR's — They won't autofocus or stop down the lens for shooting and have several other issues. Newer versions of this lens are labeled "ASP HF" and should be OK.

See also:

• DP Review: The long, difficult road to Pentax full-frame
• Pentax K-Mount Lens Series Explained: The differences between various Pentax lens series