What is 400mm f5 6 on canon aps c in full frame?

400mm on APS-C (Canon) gives the view of 640mm on full frame. For the same depth of field, f/5.6 there behaves like about f/9 on full frame. Exposure doesn't change: f/5.6 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 810mm at about f/11.3 for the same depth of field
• Full frame: 640mm at about f/9 for the same depth of field
• APS-C: 427mm at about f/6 for the same depth of field
• APS-C (Canon) (yours): 400mm at about f/5.6 for the same depth of field
• Micro Four Thirds: 320mm at about f/4.5 for the same depth of field
• 1-inch: 237mm at about f/3.3 for the same depth of field
• 1/1.3-inch: 178mm at about f/2.5 for the same depth of field
• 1/1.7-inch: 139mm at about f/1.9 for the same depth of field
• 1/2.3-inch: 114mm at about f/1.6 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.