Hasselblad 500cm Flange Focal Plane - Polaroid Back

[byte]

Hi,

 

Does anyone know what the flange focal distance changes to when attaching a Polaroid back to a Hasselblad 500cm. I uderstand the 500cm has a FFD of 74.9mm. But the polaroid back has an additional piece of glass in between the camera body and focal plane.

 

I'm trying a little hack of the camera, but cant figurine this dimension out.

[ed_farmer]

I just double checked a few videos and other sources. I don't see any additional glass in the polaroid back for the 500cm? The film to flange distance should be exactly the same.

 

That's the whole idea . . .

[orsetto]

The glass plate in Polaroid backs for the Hasselblad was a Polaroid-specific "hack" - you shouldn't need to concern yourself with it when customizing a 'blad for any standard negative or transparency films.

 

At various points in its history, Hasselblad has gotten tangled in its own too-clever-by-half camera body design. One of the most difficult issues was accommodating the body's dark slide "feeler finger" to the presence of a Polaroid film pack: the film pack collided with the feeler, preventing the camera from firing. Hasselblad's eventual solution to this was moving the physical film plane of the Polaroid back out a couple millimeters from the nominal film plane of the roll film backs, thus clearing the feeler finger. Doing this required the glass plate, which shifts the film plane point of focus backwards to the new Polaroid film surface location. The glass plate introduces some aberrations, but this was considered acceptable for the expected use of Polaroid proofs in that era (to roughly check lighting , exposure, composition before committing to roll film: tiny 6cm x 6cm Polaroids were rarely the final end goal of a Hasselblad shoot).

 

Unless your own hack involves utilizing a Polaroid back, or an Instax or Graflok modification that might also foul on the dark slide feeler, its unlikely you'd need to worry about incorporating the glass plate. If you do, make sure any Polaroid back you obtain for modification includes the glass plate: they tend to fall out after several decades of dried-out adhesive, so I've seen a number of backs on eBay that don't have them. Sellers are rarely aware there ever was a glass plate: be sure to ask for confirmation if it isn't obviously there in listing photos.

Edited December 20, 2019 by orsetto

[orsetto]

And FWIW, the glass plate isn't even crucial for use with Polaroid: it was one of those ultra-nitpicky Hasselblad inventions. Strictly speaking, Polaroid resolution was rarely good enough to reveal the slight image plane discrepancy, which would only really show anyway at wide apertures with perfect SLR focus on the screen. NPC and other third party Polaroid backs for Hasselblad did not bother with the glass, and pros used them in droves because they were much cheaper than Hasselblad's own. Besides adding some slight optical aberrations, another drawback of the 'blad version glass plate is it got easily scrtached, dirty or cracked: you're more likley to notice physical defects imparted by the now-ancient glass than miniscule focal plane discrepancies without the glass.

 

Also FWIW, Hasselblad's own explanation for the glass has varied over the years. Their last mention of it on the Hasselblad Bron website claims its required due to re-arranged attachment hooks. On my own Hasselblad Model 100 back, I can't really see what they're getting at with that statement, but whatever.

[tom_chow]

... Their last mention of it on the Hasselblad Bron website claims its required due to re-arranged attachment hooks. On my own Hasselblad Model 100 back, I can't really see what they're getting at with that statement, but whatever.

Polaroid film is bigger, and thus extends below (or above) the film gate, and will thus overlap the film back hooks on the bottom of the camera, or the latch hooks at the top, both of which extend past the film plane.

[orsetto]

Thanks, thats a much clearer description of the hooks issue than Hasselblad's own. Between the hooks and dark slide interlock, the Polaroid film plane needed to be set back a bit.

[byte]

Thanks all - The hooks at the top and bottom of the body are definitely want interfere with polaroid film sheets. So I understand why Hasselblad included the extra glass to move the focal plane back on their version of the polaroid back. I just cant find anything online that indicates how much further back the focal plane moves. 1 or 2mm is a Grand Canyon esc gap when focal plane is concerned, surely?

[orsetto]

Just dug out my Hasselblad Model 100 Polaroid back, to see if there was any way to measure the glass thickness without destroying the back. As it happens, one corner of the glass plate is beveled, leaving a tiny gap where I could insert a toothpick and mark the depth. While this is a very very rough measurement against an ordinary ruler, not a precision caliper, I repeatably got approx 3mm thickness of the glass itself (from film surface to body face). The complete focal plane adjustment is probably a hair more or less than this measurement, since my measuring ability is crude and the glass is held in position by a paper-thin metal frame on the camera side which may add fractionally to the film plane offset. If you're extremely serious about whatever hack you're planning, it may be best to simply buy a beat up Hasselblad polaback for measuring purposes (Model 80 is much cheaper usually, since the film for it has been long discontinued). Remove the glass completely, then you can measure it (as well as the framing depth) accurately.

 

[q.g._de_bakker]

The focal plane shift, though dependent on, is not equal to the thickness of the glass. It depends on the refractive index of the glass, and the simplified but very usable formula to calculate the shift (for paraxial rays) is:

 

Shift = Thickness of glass * (1 - (1 / refractive index of glass used))

 

Using typical 'household glass', the shift is approx. 1/3rd of the thickness of the glass. So a 3 mm thick glass plate will shift the focal plane by 1 mm.

 

A flat plate will introduce a considerable field curvature. So nothing very good will come of using a plate.

It was already mentioned that this did not matter much for Polaroid use, since these tiny thingies were only used to show art directors that the photographer did indeed know what he or she was doing. The photographer knew how to use a viewfinder and exposure meter, and did not need a tiny, fuzzy picture, that had terrible contrast and never looked like the final picture.

[tom_chow]

Ah, the days of Analog Chimping!

 

Then there was the NPC polaroid back that used a fiber-optic plate, which did not need to extend into the body, and worked with the 200/2000 series bodies - they could move the focal plan as far back as necessary, making it easy to clear all sorts of mechanicals. (Note that later Pol100 back worked with 200/2000 bodies, by using a thinner & higher index of refraction glass plate)

 

I think we only ever use a polaroid back to confirm lighting - keep in mind that flash equipment back then was more finicky & failure prone, a cooked transformer or cap would drop the power without you knowing, a poor contact would drop out a flash, no TTL etc... And if you had an art director - then you had to produce something they could look at before giving the thumbs-up.