What is 21mm f1 5 on canon aps c in full frame?

21mm on APS-C (Canon) gives the view of 34mm on full frame. For the same depth of field, f/1.5 there behaves like about f/2.4 on full frame. Exposure doesn't change: f/1.5 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 43mm at about f/3 for the same depth of field
• Full frame: 34mm at about f/2.4 for the same depth of field
• APS-C: 22mm at about f/1.6 for the same depth of field
• APS-C (Canon) (yours): 21mm at about f/1.5 for the same depth of field
• Micro Four Thirds: 17mm at about f/1.2 for the same depth of field
• 1-inch: 12mm at about f/0.9 for the same depth of field
• 1/1.3-inch: 9.3mm at about f/0.7 for the same depth of field
• 1/1.7-inch: 7.3mm at about f/0.5 for the same depth of field
• 1/2.3-inch: 6mm at about f/0.4 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.