43mm on 24X36 are "normal", but also 50mm and 55mm...
Pentax say Medium Format and 55mm lens, so i guess it´s like
43mm on 24X36(that´s the diagonal of 24X36mm).
So 43 X 1,33 = 57mm(close to 55mm).
645 has 75mm and 24X36 has 50mm normal lens,
so 75 - 50 = 25 and half of 25 are 12,5 then 50mm + 12,5 =
62,5mm.
That is between 645 and 24X36, but if it is 43mm lens for 24X36..
75 - 43 = 32 and half of 32 are 16 then 43 + 16 =
59mm. Or diagonal for 645 are 70mm and for 24X36 43mm.
Then we have 70 - 43 = 27 and half of 27 are 13,5,
and 43 + 13,5 are
56,5mm.
There it is, almost a 55mm normal lens.
And the diagonal are in the middle of 24X36 and 645(~43X56mm).