I have just bought the Pentax 17-70mm f4 to replace my Sigma 18-50mm f2.8/4.5. Of course the first thing I did was to carry out a comparison shoot. Lo and behold the
the frames taken with my new lens were under exposed by quite a margin at the wide end. I was using my K30 on aperture priority set at 200asa and the exif data showed:-

Sigma(18mm) f8 @ 1/125
Pentax(17mm) f8 @ 1/250

As the focal lengths increased so the difference in exposure decreased. At 50mm the exif showed

Sigma f8 @ 1/60
Pentax f8 @ 1/80

I am not particularly concerned (indeed of the 50mm exposures the Pentax was the most pleasing). I just wonder if any one can hazard a guess at why?

I've been a Pentax user since 1987 AND PRESENTLY OWN A k30 AND A p50.