Pentax K-S2 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-S2 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.
K-S2 also has a sensor based image stabilization system which means that all of these lenses will be stabilized when used on the K-S2.
Since Pentax K-S2 has a APS-C sensor, it has a focal length multiplier of 1.5x 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-S2.
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Pentax K-S2 Lenses with Image Stabilization
Pentax K-S2 Lenses with Weather Sealings