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View Full Version : An autoexposure pinhole Polaroid.



ImageMaker
12-11-2005, 10:38 PM
After determining, a few months ago, that the reason I couldn't get a decent image (at anything up to $1.75 per frame) from my $3 Polaroid Automatic 210 was because the front struts were bent, leaving the lens board around 3 mm too close to the film plane for any given focus setting, I also concluded that there wasn't any reasonably obvious way to repair it -- and when I got a (much higher quality) Model 350 from another forum member, I decided to convert the 210 to pinhole. The bent struts wouldn't matter once precise focus was no longer required.

Here's the basic camera, closed (photos taken after conversion): Attached files http://f295.f295.org/uploads/pict0217small_802.jpg (http://f295.f295.org/uploads/pict0217small_802.jpg)

ImageMaker
12-11-2005, 10:38 PM
And here's the basic camera, open.

Attached files http://f295.f295.org/uploads/pict0218small_5701.jpg (http://f295.f295.org/uploads/pict0218small_5701.jpg)

ImageMaker
12-11-2005, 10:39 PM
There were three ways to approach the conversion. The "traditional" way to convert a Polaroid has been to hack off everything forward of the film body -- struts, bellows, lens board, etc. -- or just take a saw to the plastic if it's a non-folding plastic body type. I saw two other possibilities. One, to remove everything from the lens board and install the pinhole with a manually operated shutter (either a flap or a leaf shutter from a dead lens camera), or two, to use the original shutter built into the Polaroid (for which I'd previously spent some time and effort repairing the corroded battery connectors).

The final decision was made by my inability to remove the battery holder where I'd glued it to one of the camera's body panels: I was going to use the original shutter. Attached files http://f295.f295.org/uploads/pict0219asmall_6829.jpg (http://f295.f295.org/uploads/pict0219asmall_6829.jpg)

ImageMaker
12-11-2005, 10:42 PM
I'd found an article some time ago on repairing certain common problems in the more sophisticated automatic shutters of the fully adjustable Polaroids -- the ones that supported four film speeds and had a "Scene Selector" were very versatile, and had good lenses, so are worth some effort to repair. This shutter turned out to be considerably simpler than those, with far less electronics and a much simpler metering system. That's good -- the simpler the electronics, the easier they are to fool.

Here's the shutter section, closed up -- the front and rear panels are held together by four small straight-blade screws, one in each corner of the back side of the lens board. You can see the pinhole down inside where the lens would go, and to the right the metering lens. The bezel of the original lens controls the exposure compensation (Lighten/Darken), so I needed to remove the lens without removing or damaging that bezel and its working lever; the bezel is retained by the metal front cover, which is held in place by contact adhesive (which avoided the bezel area to allow the bezel to rotate). I wound up scoring the front element with a small scredriver (plastic lens, one meniscus element front and one rear) and then breaking it out with a hammer and cold chisel, supporting the edge of the bezel on a wood board to keep from damaging the metal cover plate. That left me with the exposure adjustment intact, but no "glass" in the bezel.

The rear element came out easily, by just removing a spring retaining ring like the ones found in many simple cameras. Attached files http://f295.f295.org/uploads/pict0220asmall_901.jpg (http://f295.f295.org/uploads/pict0220asmall_901.jpg)

ImageMaker
12-11-2005, 11:12 PM
Inside the front cover, there's a two-piece board; one side has the electronics that time the shutter, while the other has the mechanics that actually move the shutter across the opening.

In the image below, arrow #1 points to a piece of matt board used to block the film selector at the 3000 position -- the opening in the sliding plate for the larger aperture used with slower color film wasn't a full round, so I couldn't mount a pinhole on that side. In addition, I didn't particularly want to deal with the color shifts that reciprocity failure introduces in Type 669 or Type 690 films, so I decided to dedicate the camera for Type 667, ISO 3000 black and white print film. This is also the least expensive pack film for Polaroid, best pricing about $19 for a box that contains two ten-exposure packs (the corresponding square format film, Type 87, is a dollar or two less than that).

Arrow #2 points to the metering cell, a cadmium sulfide photoresistor which is part of a simple timing circuit that holds the shutter open until the meter senses that the film has received enough light, then allows the shutter to close (though the shutter will close prematurely if the shutter release is let up). This circuit and the battery power that operates it (3 volts from a pair of AA size cells, after my battery holder modification) are critical; if the shutter timing isn't operating correctly (for instance, with a dead battery or corroded contacts or wires), the shutter will sound as if it's operated normally, but will not hold open for even the minimum 1/200 that this shutter can time; instead, the shutter (which operates somewhat like a focal plane or Minox shutter) will run with the trailing plate pushing the leading plate, and so travel at normal speed, and make a more or less normal sound, but never expose the film.

My modification to the metering system was to mask the cell so it would receive less light than in the original setup, by the same ratio as my intended f/320 pinhole would give the film less light than the original f/42 aperture for high speed film. From f/42 to f/320 is about six stops, so I had to reduce the sensing area of the cell by a factor of 2^6, or 1/64 of its original size. I did this by, first, dismounting a small piece of metal mesh that would have covered the cell when the camera was set for the lower speed color film, and installing it permanently over the cell window, and then masking that with a piece of brass shim drilled to 1/16" (about 1.5 mm), which left three holes in the mesh uncovered; equivalent would have been a single hole about 0.75 mm masking the 6 mm face of the CdS cell.

Arrow #3 points to the pinhole, secured to the front of the smaller aperture with my standard mounting, black masking tape. The hole is (of course) blackened, in this case on both sides to avoid reflections if I ever want to use filters on this camera.

Once the modifications were complete, I resoldered the white power wire, which had broken at the solder joint from handling, tested the shutter to ensure it was operating and metering more or less correctly, then closed up the lens board housing. Dry testing showed the shutter firing at speeds that seemed correct for the light. Testing with a sheet of 3x4 format film (J&C Classic 200, which I pushed mercilessly to what I hoped was EI 2000) showed no fogging and what appeared to be a reasonably sharp image.

Then I just had to wait for the arrival of some Type 667 film.

Now that I've shot the first pack of film, I'll probably rework the mask on the meter sensor; I'd like to reduce the light that passes the mask by about 50%, which would mean about a 0.5 mm hole; doing so will cause the shutter to stay open twice as long in any given light, and allow normal exposure with the lighten/darken control set to the "norrmal" position, giving two stops lighter and one stop darker adjustment from there. This will compensate for the well know tendency (probably due to capacitor aging) of Polaroid shutters in this family to underexpose as they age (and the newest in this family are about 30 years old).

Exposure adjustment aside, however, I'm very pleased with the outcome of this project -- the images, when I hold the camera steady, are sharp and crisp, with little flare, and exposure seems correct in "daylight" conditions. The exposure circuit in this camera is capable of timing exposures up to 10 seconds, which would require about 1.5 stops of additional exposure with Type 667 to correct for reciprocity failure -- easily dialed in on the control -- but I believe the shutter actually compensates for this (the curve is similar, though not quite identical, for all Polaroid peel-apart pack films); I've had good exposures in the 3-4 second range with my 350, which would be a very visible half stop underexposed if reciprocity compensation were not applied. Attached files http://f295.f295.org/uploads/pict0221asmall_7086.jpg (http://f295.f295.org/uploads/pict0221asmall_7086.jpg)

staft
12-11-2005, 11:25 PM
so this is the beast producing the great new shots you've been posting! you know how to document a process. my poloroid is a later non-bellows angle-mirror type, so i think i will probably just plunge in. for now, i wait to test today's autographic mod. more excuses to waste time, film, and money...

ImageMaker
12-11-2005, 11:30 PM
There are a couple operational issues to pinholing with a camera in this family. The less expensive models (those with plastic bodies) don't have tripod sockets, though it's no big deal to add one; for this camera, I'll probably glue a board with blind nut to the back film door, for use with the tripod head tipped over. Potentially a bigger problem, these cameras also don't accept standard cable releases, even with an adapter; instead, Polaroid sold cable releases (and self timers) that friction fit over the shutter release button. I plan to try to pick up one of each, but if I can't, I might try my hand at machining one on my lathe; all I need is a collar that will fit snugly over the bezel around the shutter release, and an inner button that will either depress the release directly, or accept a standard cable release. The tricky part of that is that cable releases have a steeply tapered thread, which is hard to machine on the inside of a hole.

For pinholing, it might be sufficient to use a small piece of wood, to be depressed by a strong rubber band wrapped around the camera body. Or it might turn out that the movement from releasing by hand isn't significant (though I doubt that will be the case).

ImageMaker
12-11-2005, 11:45 PM
Aaah, staft, you can modify the metering the same way -- just install a mask behind the opening that reduces the aperture by the correct proportion. The shutters in some of those cameras are a little harder to time, since they combine aperture and shutter (the SX-70 was the first camera to do that, back in 1972). However, you can remove the front panel on most of them with film in the camera, without fogging the film (there's a second step to removeing the shutter and lens assembly, which would fog the film, except on the SLR models which protect the film except during actual exposure), so you can adjust your mask to give the exposure you want -- smaller hole/denser mask filter gives more exposure.

For that matter, I could have done my modification as easily with a few layers of ND filter material, had I had some and a decent way to mount it. Most of the 600 series and SX-70 family cameras (except the first generation SX-70) could automatically expose up to two minutes, though few if any included a tripod mount and only a few have any provision for even a proprietary remote release.

murrayatuptowngallery
12-12-2005, 05:10 AM
Hmmm, time to go dig in the basement, see if I still have any unaltered Pola.

ImageMaker
12-12-2005, 01:54 PM
FWIW, in the way of filtering for the meter sensor for camera modifications, I've just discovered that a piece of unexposed, developed and fixed Tri-X 35 mm film from my trimmed scraps is almost exactly two stops of neutral density; three layers of this film would provide the needed filtering for the sensor without dealing with the metal masks I used. Do test your own film with a hand-held light meter, since this measure is strongly dependent on the amount of fog in the film (which varies with age and storage conditions) -- but B&W 35 mm film, with its gray base, can make a usable ND filter for non-imaging applications like this.

Daryl
12-12-2005, 07:09 PM
OOHRAH!! WAY TO GO!!


WELL DONE!!






SEMPER FI !!

Carry on.

luscher
12-12-2005, 09:47 PM
i have just lost my winter break project - i recently picked up one of these for $4 at St. Vinnies. well, i guess i'll simply follow in the footsteps of an expert on this one.

thank you IM, for the great instructions !

ImageMaker
12-13-2005, 01:59 AM
What model do you have, Luscher? The plastic lens versions should all be pretty close to this one, but the glass lens versions have significantly different and more complex inner workings -- especially the ones with Scene Selector, which have an aperture plate with a total of eight different apertures and filters or masks for the meter to match each with its corresponding film speed.

luscher
12-13-2005, 10:40 AM
i'm not sure if there were variations within a model, but it seems to be the same as the one you have - unless you were thinking i might have a different model. (note angelic Photoshop 'glow' around the viewfinder)

since you brought it up, is there a simple non-destructive test to determine the material used in a lens? this never occurred to me before, but given your cyclopedic knowledge of all things photographic, you seem the ideal person to ask !
Attached files http://f295.f295.org/uploads/13__automatic_210_4567.jpg (http://f295.f295.org/uploads/13__automatic_210_4567.jpg)

ImageMaker
12-13-2005, 03:45 PM
There are no significant variations within models; during this period Polaroid was bringing out a whole new line every 2-3 years, marketing (fairly expensive) cameras as periodically replaced consumer items instead of either cheap commodities or finely made tools to last generations as earlier camera makers had always done. Rather than modify a model in mid-run, they brought out a new model (still did so throughout the SX-70 and 600 eras, for that matter, and even recently replaced the i-Zone Pocket with the i-Zone 200, including claiming the film -- same speed, same format, same manual ejection method -- is not interchangeable between the two). However, each generation had a number of different models with different features, from the 100-250-350-450 with four film speed settings, Scene Selector switches, high quality rangefinders, parallax compensating viewfinder frames, and excellent glass lenses, through various less featureful and less expensive models to the 150-200-300-400 models with plastic lenses, 2 film speeds, and no rangefinder (not even the simple stadimeter type found in the 210, which still corrected parallax) to aid in focusing, instead using "icon" focusing with 1 face, a single person, group of people, tree, and mountains for various focus zones.

Your 210 will be identical to mine except for potential variation in how much it underexposes from aging of the timing components in the shutter, assuming the shutter works at all. I'd suggest you start by applying 3 volts (careful of polarity) to the battery terminals and testing to be sure the shutter actually opens (fire the shutter with the back open, and watch for light through the lens). If that works, you can just buy a 532 battery direct from Polaroid or from Radio Shack's "Direct2You" program, and it'll probably last 2-3 years; otherwise, you'll need to eliminate corroded wire in order to get power to the shutter. Probably also worth cleaning up the rollers inside the back to be sure they aren't so corroded you'll never be able to get film to run through them.

Once you have the shutter operating, you can start the pinhole conversion in earnest (no sense putting the effort into preserving the auto exposure if the shutter can't be put back in operation). And if you can't get the shutter to work, you can always convert it the way chrism did his, or pull all the works and front plate out of the shutter housing and install a leaf shutter in the hole in the rear plate, with a pinhole in that shutter.