Saturday, 5 February 2011

Project: Foth 6x9 Pinhole Part 2

Well, that didn't take long. yesterday I said that the foth conversion would be simple. It turned out easier than I thought. The camera is now an f250 pinhole with a 75mm focal length and an a Prontor-S shutter. Here’s how it went.

The original Foth dial-set shutter worked on the T and B settings I needed. But as it it only worked by first running the self timer, I decided to swap it out for a much later Prontor-S. It was just lying around waiting for a project. (This shutter also has a fault but not one that will cause me any problems: The blades briefly open as you move the cocking lever. But in a pinhole camera, an extra 1/100 second exposure won’t make the slightest difference) It has no T setting but the Prontor has B as well as 1 and 1/2 second exposures, which will be useful on sunny days. The speeds from 1/5 to 1/300 won’t be used!! Installing the shutter just a matter of unscrewing the original and installing the new one in its place.

The Prontor was from a 1950’s Agfa Isolette 6x6 folder so was equipped with the usual three element Apotar 85mm f4.5 lens.  This had to come out to make way for the pinhole. Two elements unscrew from the front and one from the back. I’d already made a 0.3mm pinhole from an old beer can, and that’s goes in front of the shutter, held in place by the retaining rings that originally held the rear element. Easy.

Lens stop
The bellows make it easy to set the focal length to whatever you want. Hey, a zoom pinhole! Pretty cool, but exposure calculations would get complicated. So I need to set a stop on the bellows rails to get consistent results. I like my pinholes with a focal length of about half the frame diagonal. That gives a nice wide angle of view, similar to a 24mm lens on a 35mm camera. So on 6x9, I’d aim for around 50mm. On this old folder, with a view that wide, the lens door would get into the picture every time. 75mm is about as short as I can go, a compromise but without sawing the end off the door, what else could I do? Still wide angle, like a 30mm lens on a 35mm camera.  I taped piece of black card on the rail as a stop. At 75mm my 0.3mm pinhole works out at f250. Sunny f16 exposure works out at 1/2 second with iso400 film. Glad I fitted the Prontor!

Black tape
That’s pretty much it except for some black tape. There’s a bit of corrosion on the film gate, so I taped over that to prevent scratched film. Some more tape covers some missing paint on the pressure plate and made a little cover for the red window, yep, with more black tape. To prevent light leaks, after I load film, I’ll tape up the the body joints.

I’ve a roll of HP5+ ready to go and as soon the weather improves, we’ll see what the Foth 6x9 pinhole will do.


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