645 intermittent problem :(

[Asahiflex]

The Pentax 645 I bought appears to have a problem

Basically, it appears that in P mode there is a 50% chance that the camera selects the smallest f/stop (f/22 or smaller) during exposure, even though the camera shows a much larger f/stop in the viewfinder. So, at least 50% of the pictures are badly underexposed (near black). If the camera does not select f/22 all by itself, then the shots are perfectly exposed.

The lens does not seem to be a problem, as all lenses I tried behave the same. So it must be on the body side. Any ideas about the source of the problem?

[yurihuta]

My first line of defense is to always clean the contacts on the lens and body (inside the body mount). Some of these cameras are decades old and a little cleaning can be helpful.

[Peter Zack]

I'd agree and go get some proper contact cleaner. Get a magnifying glass and inspect them before and after. A good cleaning can cure a lot. How are the batteries? New?

[Asahiflex]

Thanks, I'll try to clean the contacts and report back my findings!

[D W]

In P mode is your lens set to "A"?

[Asahiflex]

Originally Posted by D W

In P mode is your lens set to "A"?

Yes.

I haven't cleaned the contacts yet, but with old film loaded in the camera I observed that the camera is behaving rather strange.

In P mode, in a dark room, the camera should use the widest f/stop. When I look into the lens and trip the shutter, the shutter speed is for instance 2s. That's OK and after tripping the shutter the lens stays wide open. However, just before the shutter is going to close the diaphragm also closes to f/22,...

[D W]

Besides cleaning all the contacts you need to check to see if the diaphragm levers on the body and lens is sticking during the operation.