Originally posted by johnha I'm curious about the adapter - which one are you using? The manual focus Mamiya 645 lenses have an A/M aperture switch (similar to the P6x7) to stop down the aperture for DoF etc, is there a reason why you can't focus wide open then stop the aperture down with the switch to use Av and the camera's metering?
Medium format lenses don't resolve the same IQ as the best 35mm format lenses, but there are no 'consumer' grade medium format lenses, apart from the odd exception, they're all good, very good or excellent. As stated you're only using the centre of the lens, on film I'm pretty sure they're capable of good or better results. The biggest drawbacks are the additional size/weight, larger filters and the lack of automation.
The adapter I use is from Photodiox. It has no electrical connections, so everything is manual. However, since the Pentax ME only has 1 manual speed (1/100) that makes the rig essentially aperture priority. I've been using my Sekonic Dual Spot F (Model 778) which is a bit of overkill - but again, it's the one I have. (One advantage is that I can shoot film at iso greater than 800 - a limitation on the ME.)
I set the iso on the Sekonic and fix the shutter speed at 1/125. On B&W film (see above) the difference in shutter speed doesn't seem to matter. I'm trying some Ektar 100 now and I'll let you know how that turns out.
The size/weight isn't too bad, with a 45, 55, or 80mm lens, it's pretty easy to carry around. I imagine it's similar to attaching a big lens to a micro 4/3 camera in terms of weight and size.
Again, this isn't the rig I'd recommend to anyone starting out but it allows me to have some fun with existing equipment, while using an old camera to good effect. So, if you have some decent MF lenses and want to expand your shooting repertoire, you might give this approach a whirl. All you will be out is the cost of the adapter (and maybe some film if you really muck it up.)