[davidmanze]

 As we are talking precision focal equivalents, (probably the most tedious process known to man), just for kicks and giggles I decided to resurrect my brain and verify the crop factor figures.....using the Pythagoras's theory (for the diagonal dimension).

 

  The crop factor for the D500 (23.5 X 15.7mm) as 1.53067 so 1.53 is near enough.

 

  For the 80D  (22.3 X 14.9mm)* I have it at 1.6156 so 1.615 will do!

 

 

  *Dimensions from Canon's official site for the 80D. ...

 

  http://www.canon.co.uk/for_home/product_finder/cameras/digital_slr/eos-80d/specification.aspx 

 

 

   So, 18 X 1.53067= 27.55206/  1.6156 = 17.05376 mm!