"blacked out" 645 viewfinder masks and the effect on Hasselblad PME prisms

To effectively compose 645 images in the field with my Hasselblad (500C/M body + PME 51 meter prism + A16 back) I want to use a

completely opaque (black) viewfinder mask (I'll either make one or paint out a Hassy-made one) to define a nice crisp 645 viewing

rectangle. BUT, of course, this will affect the reading given by my PME 51 meter prism (or any older PME) which meters 50% from a

26mm centered circle and 50% from the remaining area on the older focusing screens, BUT when using the Acute-Matte D screen (what I

have) that centered circle changes to different oval patterns determined by the focal length of the lens. (All that comes from Ernst Wildi's

"Hasselblad Manual" - did I even get that right?) That's the murky situation my research has presented to me so far. So, before I start

doing a lot of (possibly fruitless) tests to determine a system of meter compensations, does anyone have any practical insights into this?

The PME51 measure exposure from the ground glas, and the pattern of sensitivity varies with the focal length. However it is always smaller than the ground glass and does not have to be recalibrated. If you blacken the glass to mask a 645 field of view, the meter will attempt to read from the blackened area. You would need to calibrate the meter differently for each lens.

 

If you cannot discipline yourself to compose within a thin outline, I suggest you get an hand-held meter.

Eric, try drawing a thin rectangle w/ a sharpened grease pencil. Don't black out it's edges. You should be able to frame your shots just fine (ala Leica RF frame lines) and your metering should be unaffected.

I make overlays for an Hasselblad finder using laser print transparencies, cut them to fit the ground glass inside the metal frame and hold them in place with Scotch Magic Tape. The lines are sharp and much finer and cleaner than you can get with a drafting pen.

Too bad hasselblad ground glasses doesn't have composing gird, this would avoid the need for a mask.

 

The meter is metering on the ground glass, I can see no way the focal lenght of the lenses could affect it's oval metering area on it.

 

With such fresnel-based ground-glass, if present, the metering issue is present should you have a black mask or not.

 

Anyway, if you can't go with only pencil's lines, try a black mask, I don't think your metering will be much affected, if else, you'll have to

correct 1/4 nearly 1/3 IL, wich is close to nothing if you always set your camera to exactly what the meter says.

 

For me, with a center weighted metering system, metering interpretation must be systematic, so adding 1/3 IL correcion if needed to

your interpretation won't be too hard to do.

Thanks for your responses. I think I'm going to try making a laser print overlay, and then stop fussing and start shooting.

For the benefit of future readers of this thread: (according to Ernst Wildi's Sixth Edition of "The Hasselblad Manual") : All

the PMEs through PME5 have their metering pattern affected by the lens' focal length. The book provides some guide

diagrams for this. PME51 (the last version, produced in 1994-95 only) has a consistent meter pattern (supposedly!) ..an

oval which sits just below the horizontal 'crosshair' line on many Hassy screens and extends just above the top vertical

line.

The metering pattern being affected by focal length is due to the interplay between focal lengths of the lens (size and position of the exit pupil) and the Fresnel lens, the optics of the thing that holds the metering cell (often a prism finder), and the lens that is placed directly in front of the metering cell. It is not something that only Hasselblads display. ;-)<br>It however usually does not lead to great variation and can usually be ignored. I think the reason they don't mention it for the later finders (the reason why other manufacturers do not mention this at all) is that - though meant to be helpful advice - it is easily seen as pointing out a shortcoming of these finders, i.e. bad marketing.<br><br>Jérôme,<br><br>There of course are focussing screens with grid available from Hasselblad too. They're just a bit more expensive than the clear plastic masks you can lay on top of them, whether self made or those from Hasselblad. ;-)

Q.G.> Did the old Hasselblad 1600f/1000f ground glasses have Fresnel lens? If not, are they less affected by this issue?

 

This should be true for any SLR having a Fresnel lens, and the metering device placed upon the prism?

 

Sorry, as every Salyut-Kiev 88 has a composing grid, any how the focussing help should be, or not, I was assuming

Hasselblads did.

Yes, the 1600 F/1000 F focussing screens have a Fresnel lens too.<br>And yes, it should be true for all systems using an optical device between lens and metering cell. To what degree depends on what there is exactly.