Pinhole camera out of a Mamiya RB67?

[littlebird]

Hello! I saw that there's a pinhole attachment to the RB back but I wonder if I can make a pinhole out of it with putting a piece of cardboard with a pinhole (0,2mm size) where the lens goes. Would it work? Would it give a circle big enough to cover the negative? And what f-number would this give me? I know that the diameter is 0,2mm but approximately how far is the hole from the film? Thanks for the help!

 

[DWThomas]

Hello! I saw that there's a pinhole attachment to the RB back but I wonder if I can make a pinhole out of it with putting a piece of cardboard with a pinhole (0,2mm size) where the lens goes. Would it work? Would it give a circle big enough to cover the negative? And what f-number would this give me? I know that the diameter is 0,2mm but approximately how far is the hole from the film? Thanks for the help!

Not familiar with the RB67, but for my Bronica SQ-A I took the lens off, did whatever to raise the mirror, and measured to the film plane from the lens mount front surface ("register distance") with a small steel scale. Some old school vernier calipers have a strip that comes out of the bottom end that can measure depth also.

I would expect coverage to not be a problem as the pinhole is much smaller diameter than any normal lens and mounted right at the body front should likely have a clear line of sight. Assuming you can get a piece of cardboard attached without a light leak I'd expect it to work. (There's one way to find out! )

I made my own pinhole setup for the Bronica by starting with a body cap normally used to close up the body with no lens attached. At the time it was done I took advantage of access to some metal working tools I no longer have, but it was a fun project. The 'oh yeah' considerations are things like defeating/bypassing interlocks dependent on sensing film moving to allow winding to the next frame and that sort of thing.

 

[Donald Qualls]

A pinhole mounted in a body cap is one of the classic ways to make a pinhole camera out of an SLR. I haven't done this with my RB67, but I've got a pinhole body cap for M42 and have used it a few times.

Coverage isn't a problem with pinholes -- as long as you don't have something physically blocking the light behind the pinhole, the coverage runs to 160 degrees or more (I've got a cylindrical pinhole camera that covers the film up to the back surface of the pinhole mount).

You'd calculate the focal ratio by dividing (average) distance to the film by the diameter of the hole.

With a body cap on an RB67, that distance would be the same as the flange-to-film for RB67 lenses, 112 mm, so your 0.2 mm hole in a body cap would give approximately f/224.

 

[juan]

Search online. There are people making pinhole attachments for RB67. Essentially a lens cap with a pinhole.

 

[DWThomas]

Additional comments:

The kluge I linked to could not use the normal camera shutter because it's in the lenses, there's only the mirror and a safety flap in the camera. So the camera has to be fooled into thinking it's ready to take a picture and the mirror has to be up. That's OK because one can't see diddly thru an SLR viewfinder with an f/250 pinhole in place!

Thinking more, I hope you meant putting some sort of thin metal plate with the pinhole over a larger hole in the cardboard. The ideal pinhole plate is infinitesimally thin; e.g., a knife edge. Many folks use a piece of aluminum from a soda can, or brass shim stock found in auto supply or hobby shops, something around .002 to .005 inch thickness. If one drills or punches through a substantially thicker plate it starts to behave like a tunnel, increasing the off-axis light fall-off and can vignette.

 

[DWThomas]

With a body cap on an RB67, that distance would be the same as the flange-to-film for RB67 lenses, 112 mm, so your 0.2 mm hole in a body cap would give approximately f/224.

Mmm -- more like f/560? Pinhole Designer thinks it's 560 and also the optimum for that distance is about twice that diameter, circa f/250. (Those numbers vary quite a bit depending on the magic constant used in the calculation, but 0.2 is pretty small for that scale.)

 

[littlebird]

Yes I also calculated something like f/450 which is not that ideal as my light metre can only work with f/90. The lens cap version sound nice, maybe I'll order one.

 

[DWThomas]

my light metre can only work with f/90

Don't know what you may have available, but there are metering apps for smart phones that specifically target pinhole work. I have Pinhole Assist on an iPhone that goes from f/1 to f/1024. It also takes light readings and can log a thumbnail of the shot along with exposure data and do a few other tricks.

 

[Donald Qualls]

more like f/560?

Yep, you're right. I multiplied by 2 instead of dividing by 1/5 (= multiplying by 5). Oddly, that would probably have given okay exposures; it's only four stops overexposed, and reciprocity effects would eat most of that.

But yes, 0.2 mm is about right for 30 mm projection distance, for 112 mm you probably want more like 0.7 -- which is about f/160.

If you get f/560 and can only meter to f/90, just meter at f/64 and increase exposure by 6 stops (multiply by 2.8 per stop when you go over 1 second, that will automatically compensate for reciprocity with most cubic grain films).