Okay, as requested I am posting samples from the two lenses at f/2.8, f8, and f16. Unfortunately they are not at 50mm as requested.
I already returned the 17-50mm, so I cannot take any new pictures. For the tests I conducted when I still had both lenses, I chose 35mm, thinking that I wanted to avoid the extreme focal lengths of either lens. Shooting at 50mm would be at the very extreme end of the 17-50mm and somewhere in the comfortable mid-range of the 28-75mm. I don't know whether this would have any bearing on the tests, but some lenses are known to perform worse at their extremes.
Both lenses have 35mm markings, which I tried to use, but looking at the EXIF data shows that the 28-75mm was at 38mm, while the 17-50mm was at 34.4mm. I don't think this should effect the results.
Lighting was provided by two Paul C. Buff White Lightning Ultra600 monolights with softboxes. The lights were positioned to the left and right of the still life, and their position and output settings remained constant the entire time. They were triggered using a hotshoe-mounted radio transmitter/trigger set.
The shoot was conducted in a basement with no windows and no natural light.
The camera was mounted stationary on a tripod, and ISO was set at 200 throughout the shoot and white exposure was set for flash lighting. I was careful to not move the camera when changing lenses. I used a wireless remote and 3 second mirror lockup for each shot.
In order to maintain proper exposure as I changed aperture settings, I would turn the shutter speed down by one click every time I closed the aperture a click. So, for example, at f/2.8 shutter speed was 1/13, and at f/3.2 it was 1/10. Shutter speed settings were identical between lenses for each aperture setting.
Please note that the focus point ended up being slightly different between the two sets of images, so these images should not be used to compare sharpness of the two lenses.
Here is a picture of the testing setup:
And here is a link to a directory where you can download the original RAW files:
https://oncourse.iu.edu/access/content/user/ealucas/Filemanager_Public_Files...mm_vs_28-75mm/
And in case you prefer, here are direct links to each of the files:
https://oncourse.iu.edu/access/content/user/ealucas/Filemanager_Public_Files...-50mm_f2_8.PEF https://oncourse.iu.edu/access/content/user/ealucas/Filemanager_Public_Files...-75mm_f2_8.PEF https://oncourse.iu.edu/access/content/user/ealucas/Filemanager_Public_Files...17-50mm_f8.PEF https://oncourse.iu.edu/access/content/user/ealucas/Filemanager_Public_Files...28-75mm_f8.PEF https://oncourse.iu.edu/access/content/user/ealucas/Filemanager_Public_Files...7-50mm_f16.PEF https://oncourse.iu.edu/access/content/user/ealucas/Filemanager_Public_Files...8-75mm_f16.PEF