a little help with lens

It's the crop factor, ie 1.5x for Pentax.

It's a lens originally designed for full frame, but if you pop it on a crop sensor camera, the effective field of view is narrowed to the equivalent of a 75mm lens on full frame (50 x 1.5 = 75).

Focal length remains 50mm of course, it's just the equivalent field of view that changes because of the smaller sensor.
 
The 50mm relates to 35mm film format,so on a Nikon D3 or D700 for instance with the bigger sensor,it would perform as a 50mm.
But on a cropped sensor as you're 1.7 it performs as a 85mm
on a 1.5 sensor like my D300 it would act like a 75mm.

:thumbs:

If you want to keep to as close as poss to the 50mm for portraites the 35mm is a cracking lens.
 
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