What is 55mm f1 7 on medium format in full frame?

55mm on medium format gives the view of 43mm on full frame. For the same depth of field, f/1.7 there behaves like about f/1.3 on full frame. Exposure doesn't change: f/1.7 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33) (yours): 55mm at about f/1.7 for the same depth of field
• Full frame: 43mm at about f/1.3 for the same depth of field
• APS-C: 29mm at about f/0.9 for the same depth of field
• APS-C (Canon): 27mm at about f/0.8 for the same depth of field
• Micro Four Thirds: 22mm at about f/0.7 for the same depth of field
• 1-inch: 16mm at about f/0.5 for the same depth of field
• 1/1.3-inch: 12mm at about f/0.4 for the same depth of field
• 1/1.7-inch: 9.4mm at about f/0.3 for the same depth of field
• 1/2.3-inch: 7.8mm at about f/0.2 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.