Pentax K20D 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 K20D are the Carl Zeiss Makro-Planar T2/50, the Samsung D-Xenon 16-45mm F4 and the Samsung D-Xenon 18-55mm F3.5-5.6.
K20D also has a sensor based image stabilization system which means that all of these lenses will be stabilized when used on the K20D.
Since Pentax K20D 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 K20D.
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Pentax K20D Lenses with Image Stabilization
Pentax K20D Lenses with Weather Sealings