Originally posted by RiceHigh But that lens is not really that wide. Its just equivalent to something like 37.5mm for 135 film.
And f/2.8 is not really very fast. I think its standard for the MF.
The 645 33-55/4.5 and the 35mm have both a filter diametre of 82mm. Unless Pentax have some trick up its sleeve a 25mm with that speed (2.8) would have a truly large front element. It is also rather pointless as F:4.5 will be satisfactory for most MF users in this focal lenght range.
You got your math wrong. A 25/2.8 for the full 645 format is like a 15/2.8 for an FF K-mount camera. Take a look at the A 15/3.5 to get some idea.....
If I'm not completely mistaken a 25mm has never been made for MF (except possibly a fish-eye lens). I believe the Pentax FA645 33-55/4.5 is the widest wide angle ever for MF.