What is 21mm f2 4 on aps c in full frame?

21mm on APS-C gives the view of 32mm on full frame. For the same depth of field, f/2.4 there behaves like about f/3.6 on full frame. Exposure doesn't change: f/2.4 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 40mm at about f/4.6 for the same depth of field
• Full frame: 32mm at about f/3.6 for the same depth of field
• APS-C (yours): 21mm at about f/2.4 for the same depth of field
• APS-C (Canon): 20mm at about f/2.2 for the same depth of field
• Micro Four Thirds: 16mm at about f/1.8 for the same depth of field
• 1-inch: 12mm at about f/1.3 for the same depth of field
• 1/1.3-inch: 8.8mm at about f/1 for the same depth of field
• 1/1.7-inch: 6.8mm at about f/0.8 for the same depth of field
• 1/2.3-inch: 5.6mm at about f/0.6 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.