What is 17 28mm on aps c in full frame?

17-28mm on APS-C gives the view of 26-42mm on full frame.

The same view on every common format:

• Medium format (44 x 33): 32-53mm
• Full frame: 26-42mm
• APS-C (yours): 17-28mm
• APS-C (Canon): 16-26mm
• Micro Four Thirds: 13-21mm
• 1-inch: 9.4-16mm
• 1/1.3-inch: 7.1-12mm
• 1/1.7-inch: 5.5-9.1mm
• 1/2.3-inch: 4.6-7.5mm

Crop factors are by the sensor diagonal, rounded as they're usually quoted; a format with a different shape (4:3 against 3:2) frames a little differently across the width. More in the focal length converter.

Computed from the sensor sizes, not written by a model.