What is 90mm on aps c in full frame?

90mm on APS-C gives the view of 135mm on full frame.

The same view on every common format:

• Medium format (44 x 33): 171mm
• Full frame: 135mm
• APS-C (yours): 90mm
• APS-C (Canon): 84mm
• Micro Four Thirds: 68mm
• 1-inch: 50mm
• 1/1.3-inch: 38mm
• 1/1.7-inch: 29mm
• 1/2.3-inch: 24mm

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.