Careful lens selection will allow getting as close to 40 megapixels as possible.
The table below captures some options for Pentax 645 lenses and related items.
Equivalent focal lengths vs 35mm DSLRs
The aspect ratio of the Pentax 645D is 4:3, so there is no correct focal length equivalent for a 3:2 DSLR.
The diagonal equivalent is the traditional metric, but is least useful in terms of making images; I think in terms of the horizontal span, but some photographers think in terms of the vertical span. Which is more useful depends on what types of prints you tend to make (e.g., 16 X 20 or 16 X 24). The differences are not large no matter which approach you take.
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