What needs explaining here is what at 'standard lens is - and
that is simple.

It is the lens which has a focal length equivalent to the longest
measurement of the format in use, i.e. the diagonal. It this
follows that a 'standard' lens for 35mm will, by Pythagorus, be
about 42mm, and for 645 will be 75mm.

AP-C cell sizes vary, but assuming 23x15mm this would make a
'standard' lens about 27mm. The ratio of 35mm to AP-C is thus
about 42/27 or 1.5 which is what is generally accepted as 'about'
the magnification factor. Due to fanufacturing processes and the
way the brain 'sees' a picture, 50mm (with a magnification factor
or 50/42 or 1.2) is usually accepted as 'standard' for 35mm.

By the same token a 645 'standard' lens on an AP-C camera will be
75/27 or about 2.8, which in 35mm lens terms would be the
equivalent of about 140mm.

HTH


--
Woody

harrogate three at ntlworld dot com