Hey all, time for another unscientific test... actually, it's quite scientific, just not as rigorous as other tests (make observation, form hypothesis, test hypothesis, observe results, rinse & repeat).
Anyway, I've had a sneaking suspicion that the 12-50 kit lens has some minor light-loss that does not correspond to the aperture. Meaning, I can get the right depth of field for a given aperture, but my exposure will be just slightly under exposed. It's not a huge deal, and I'd guess it to be less than 1/3 stop. Also, I suspect it's a little worse at the wide end.
Here are some samples, all in full manual. Also, lighting conditions were pretty equal for each focal-length comparison. Natural lighting in my office, not a cloud in the sky, each comp shot was less than a minute difference.
The following two images are with the 17mm and the 12-50 set to 17mm
E-M5 @ ISO 1600, 1/40s shutter, f/5.6 for both lenses (LN jpeg).
1) The 17mm prime
2) The 12-50 @ 17mm
Next, the Oly 45 and the 12-50 set to 45mm.
E-M5 @ ISO 1600, 1/20s shutter, f/8 for both lenses.
3) The 45mm
4) The 12-50 @ 45mm
As you can see, the difference is slight, and more pronounced at 17mm. If you want full-res, let me know.