What is 15mm f4 5 on aps c in full frame?

15mm on APS-C gives the view of 22mm on full frame. For the same depth of field, f/4.5 there behaves like about f/6.8 on full frame. Exposure doesn't change: f/4.5 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 28mm at about f/8.5 for the same depth of field
• Full frame: 22mm at about f/6.8 for the same depth of field
• APS-C (yours): 15mm at about f/4.5 for the same depth of field
• APS-C (Canon): 14mm at about f/4.2 for the same depth of field
• Micro Four Thirds: 11mm at about f/3.4 for the same depth of field
• 1-inch: 8.3mm at about f/2.5 for the same depth of field
• 1/1.3-inch: 6.2mm at about f/1.9 for the same depth of field
• 1/1.7-inch: 4.9mm at about f/1.5 for the same depth of field
• 1/2.3-inch: 4mm at about f/1.2 for the same depth of field

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.