In your Hasselblad body, behind the mirror, there are a pair of doors/light baffles that open when the shutter fires. The firing sequence on a Hassy is as follows:
-depress shutter button
-lens stops down to working aperture
-mirror pops up
Winding the film advance then triggers the mirror to return, and re-cocks the shutter, opening the lens to maximum aperture for viewing.
If those baffles are not operating properly, they can fail to get out of the way of the light, leading to partially exposed frames. This is what I'm suggesting you check first, by dry-firing the camera without a film back on, to observe their actuation. I don't remember if the problem with them occurs with short or long exposures - I THINK the problem is with exposures greater than 1 second because they don't stay open, but I could have it ass-backwards (it's been 15 years since I had the problem, on a body I no longer own). The suggestion to check the lens aperture diaphragm was a secondary thought, because you're shooting with the lens almost to minimum aperture anyway.
Are you calculating reciprocity with these exposures? Although color negative film requires much less exposure compensation than black-and-white, it still requires one stop additional exposure for times over 1 second. Depending on your film, you may have to add two stops past 30 seconds, or some other time - be sure to check the films' data sheet.
Thanks for the info, although I'm familiar with the mechanics of the camera. It didn't click immediately what you meant by 'baffles' as I know them only as the auxilary shutter - might be a British thing.
I think only a couple of exposures exceeded 1 second with this roll, but no longer than 3 or 4 and
I do use a reciprocity chart yes. Even if I hadn't of accounted for reciprocity, it would only have
been be a couple of stops difference and there would still be some showings of an image
because of the massive dynamic range of Portra 400. Might have been a different story with
It seems the problem has been narrowed down to an intermittent shutter error in colder weather.
I will observe the problem in similar conditions and make a test roll, perhaps uploading the results.
How cold was it where you were shooting? If you're having a temperature-related shutter issue, then you need to get your lens in for a CLA - those should be good down to 0 degrees Farenheit or below.
Around 10 degrees C.
The body broke in cold around -6 in December.