The 645D's sensor is larger than 135 (35mm) film, true, but does that make it truly "medium format"? I don't care. What is important for me to know is that the sensor is 44x33mm in size. So let's see how this compares to other film/sensor sizes.

Relative frame sizes between 645 film, the Pentax 645D, 35mm film, and the Fujifilm X system.

Dropping down from 645 film to 645 digital is straightforward – the 69.7mm film diagonal is matched to a 55mm of digital with a 1.27x multiplier. E.g. the standard SMC Pentax-A 70mm F2.8 lens, used on the 645D, has the same field of view as an 88.7mm lens would on film.

Pentax 645 film to digital field-of-view equivalences

645 Focal Length 135 Equivalent FOV
35mm 44mm
45mm 57mm
55mm 70mm
70mm 89mm
120mm 152mm
150mm 190mm
200mm 253mm
300mm 380mm

Converting from 135 film focal lengths

You can see in my chart above that the Pentax 645D sensor is not just bigger than than a "full frame" of 135 film, but it's also a slightly different shape. The change from a 4:3 aspect ratio in 645 world to the 3:2 ratio of 135 (and APS-C) means finding an equivalent field of view is less straightforward. The difference between 4:3 and 3:2 isn't as big a difference as when we compared with 1:1 squares, however, so we can just throw together a chart and put an asterisk on it.

The difference between a 4:3 and 3:2 frame size with the same diagonal

*Field of view equivalences will not be exact, due to the change in aspect ratios. The 4:3 image will have slightly less width and slightly more height than the 3:2 image, even with equivalent focal lengths.

Now, we're ready for the chart. The 135 diagonal is 0.787x smaller than the 645 diagonal, and the reciprocal is the 645 diagonal is 1.27x longer.

Pentax 645D to 135 film field-of-view equivalences*

645D Focal Length 135 Equivalent FOV
35mm 28mm
45mm 35mm
55mm 43mm
70mm 55mm
120mm 94mm
150mm 118mm
200mm 157mm
300mm 236mm

I didn't realize until seeing the numbers spelled out this way that the 645D system ends up with two different "normal" lenses: the 70mm "long" normal, and the 55mm "wide" normal.

In any case, happy shooting! And now here's some photos I've taken recently with the 645D, learning how to use it best, from visualization to presentation.