Pentax K-500 has a Pentax KAF2 lens mount and currently there are 163 native lenses available for this mount.
The latest released lenses that are compatible with the K-500 are the Pentax FA 50mm F1.4 Classic, the Pentax FA 50mm F1.4 and the Pentax 16-50mm F2.8 ED PLM AW.
K-500 also has a sensor based image stabilization system which means that all of these lenses will be stabilized when used on the K-500.
Since Pentax K-500 has a APS-C sensor, it has a focal length multiplier of 1.5× so you have to multiply lenses original focal length with this multiplier to find the Full Frame equivalent focal length of the lens when mounted on K-500.
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Pentax K-500 Lenses with Image Stabilization
Pentax K-500 Lenses with Weather Sealings