What is 800mm f6 3 on aps c in full frame?

800mm on APS-C gives the view of 1200mm on full frame. For the same depth of field, f/6.3 there behaves like about f/9.4 on full frame. Exposure doesn't change: f/6.3 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 1519mm at about f/12 for the same depth of field
• Full frame: 1200mm at about f/9.4 for the same depth of field
• APS-C (yours): 800mm at about f/6.3 for the same depth of field
• APS-C (Canon): 750mm at about f/5.9 for the same depth of field
• Micro Four Thirds: 600mm at about f/4.7 for the same depth of field
• 1-inch: 444mm at about f/3.5 for the same depth of field
• 1/1.3-inch: 333mm at about f/2.6 for the same depth of field
• 1/1.7-inch: 261mm at about f/2.1 for the same depth of field
• 1/2.3-inch: 214mm at about f/1.7 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.