What is 45mm f2 8 on medium format in full frame?

45mm on medium format gives the view of 36mm on full frame. For the same depth of field, f/2.8 there behaves like about f/2.2 on full frame. Exposure doesn't change: f/2.8 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33) (yours): 45mm at about f/2.8 for the same depth of field
• Full frame: 36mm at about f/2.2 for the same depth of field
• APS-C: 24mm at about f/1.5 for the same depth of field
• APS-C (Canon): 22mm at about f/1.4 for the same depth of field
• Micro Four Thirds: 18mm at about f/1.1 for the same depth of field
• 1-inch: 13mm at about f/0.8 for the same depth of field
• 1/1.3-inch: 9.9mm at about f/0.6 for the same depth of field
• 1/1.7-inch: 7.7mm at about f/0.5 for the same depth of field
• 1/2.3-inch: 6.3mm 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.