35mm equivalent, not 28... Seems the X1D has a 50Mpx sensor measuring 33mm x 44mm. That's similar in dimensions to my Leica S2 sensor (30x45mm). On the S2, the Leica 45mm lens is equivalent to 35mm while the 35mm lens is equivalent to 28mm. I have the Leica 35 but for 45mm I use a Pentax-67 lens adapted. The Hassy could be fun, Helen, and that's a bunch of megapickles... 🙂
Edit: FWIW, the conversion factor is 0.8x, so multiply the medium-format lens by 0.8 to get the equivalent on the smaller camera. For example, a 100mm lens on the Hassy or S2 is equivalent in angle of view to a 80mm on the 35mm camera.