The switch that locks it to F/22 is for use on any modern SLR or DSLR: the camera bodies control the aperture. If you were to use it on something older (manual focus, usually), you'd have to unlock it and turn the ring to control aperture directly. For use on anything after the 1980s, jsut leave it locked and use it like you would anything else.
The 20mm f.2.8 is quite nice. I've been told to look for a used f/3.5 example instead, as it's sharper and, for IR use (my intention), better.
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