What is 85mm f1 8 on aps c in full frame?

85mm on APS-C gives the view of 128mm on full frame. For the same depth of field, f/1.8 there behaves like about f/2.7 on full frame. Exposure doesn't change: f/1.8 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 161mm at about f/3.4 for the same depth of field
• Full frame: 128mm at about f/2.7 for the same depth of field
• APS-C (yours): 85mm at about f/1.8 for the same depth of field
• APS-C (Canon): 80mm at about f/1.7 for the same depth of field
• Micro Four Thirds: 64mm at about f/1.4 for the same depth of field
• 1-inch: 47mm at about f/1 for the same depth of field
• 1/1.3-inch: 35mm at about f/0.8 for the same depth of field
• 1/1.7-inch: 28mm at about f/0.6 for the same depth of field
• 1/2.3-inch: 23mm at about f/0.5 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.