I've had a monster lurking in my darkroom for some time now. It's a huge lens made for World War II reconnaissance work; A 36" (900mm ) f/6.3 Dallmeyer, originally fitted to a special camera mounted in a Spitfire for photographing targets, battlefields etc. It was made to cover 5x5in or 7x7in negs on roll film (in 150 foot lengths!) but the coverage is quite a bit more than that. The problem is that at over 20kg it's ridiculously heavy and difficult to mount on a conventional camera.
F/6.3 is very fast for such a long lens so there's potential for wet-plate or calotype work. The bokeh is very beautiful and the depth of field is tiny, even at long distances. I've an idea it will suit the scanner-back camera too. I decided it was worth building a simple camera for it.
Quick and Dirty Prototyping
While I suspect this lens has a properly big image circle, mounting it on the Behemoth 20x24" camera would require quite a bit of hacking about and make an already huge and heavy camera twice as unwieldy. I decided a simple sliding-box design would be quite adequate. If I made the back plate a replica of the one on my LSC folding 10x8" I could use its backs- allowing both wet-plate and conventional film holders to be used. I'd also simplify the build process a lot.
To get some rough dimensions for the boxes I knocked up a prototype from cardboard boxes and tracing paper. The good news was that the lens is a true telephoto design, needing only about 21 inches from flange to film to focus on infinity. Quite a reduction from the 36" I was expecting. To accommodate the LSC backs it needed to be just over 12" square at the back.
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| cardboard prototype with lens installed |
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Note the shredded paper focusing screen thanks to my "assistant" Algernon who burst through it while playing with the box.
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The Boxes
I made the boxes from cheap 5.5mm plywood from Homebase. It's pretty awful stuff, imported from China and warped all over the place even if you pick and choose your pieces but it's hard to find anything which is much better, short of full-on certified marine ply which is ruinously expensive. To keep the weight down and to make it as close as possible to an original 1840s Calotype camera as possible I comb-jointed the pieces together. This is time-consuming and fiddly as all the slots need cutting on a router with a specially made jig but the result is very strong and needs no bracing.- Important as the boxes need to be a close fit.
I made a custom lens board so the lens could be easily demounted for transport. The camera box accounts for only about a quarter of the total weight. The boxes are a close sliding fit, with a felt flap on the inner box edge to act as an extra light trap.
Once glued up, stained and varnished, the boxes received their fittings to take the lens board and focus screen/film backs. The base has the same 'keystone' plate fitting as the Behemoth so it will fit on the heavy duty tripod.
Testing
A quick test using RC paper confirmed that the camera's lightproof. I made a simple filter holder from foamboard as grade 00 is usually needed to cope with the subject brightness range.
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| Ilford Multigrade no. 00 filter installed behind the lens. The are 6" square acetate. |
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| Mk 1 Spitfire Camera completed. Chief Assistant Algernon providing scale. He is quite a large cat too. |
Paper is around ISO 1 with the filter giving an exposure time of 8 seconds at f/16 which is typical. Film will need faster shutter speeds, particularly as the aperture only goes down to f/16.
The next stage was to make a drop or "guillotine" shutter. These are very simple: A plate with a slot in it is dropped in front of the lens. The width of the slot controls the exposure.
The next stage was to make a drop or "guillotine" shutter. These are very simple: A plate with a slot in it is dropped in front of the lens. The width of the slot controls the exposure.
Shutter
While a wood and brass shutter would be nice, it was more important to build something which would work quickly, easily and cheaply while ironing out the bugs of the design. I made another QAD prototype from black foamboard and card, starting with the biggest slot (8 inches) I could get out of the card. This will define my slowest shutter speed. Faster speeds are achieved by making smaller slots.
A useful reference is Bill Kumpf's article on largeformatfphotography.info - While quite heavily technical it gives the maths necessary for calculating speeds for different slot widths. I calculated my 8" slot should give a speed of about 1/11 second. Testing it against the excellent Shutter Speed App developed by Lukas Fritz I confirmed a reasonable match: 1/10.3.
From here it was a simple, if tedious job to plot out the slot widths for standard shutter speeds: 1/15, 1/30, 1/60. 1/125 and 1/250 are all possible by reducing the slot width. I've made a variable-width slot with a second piece of card held in place by velcro:
Slo-Mo filming
Another test revealed an unexpected problem: By filming using the iPhone camera's "slo-mo" function. This records at 240 frames per second so it allows the operation of the shutter to be watched at around 1/10 normal speed. Theoretically you can count the frames too as a check of speed but this isn't too easy or accurate. What it did show was a hell of a bounce when the shutter reached its stop:
The shutter plate was bouncing so much it was re-exposing the last part of the image! This was easily cured with bit of foam to act as a buffer.
Images.
I really want to use this with direct positive colour paper prints but the events of this year have made that unfeasible (the colour machine at Farnham was drained down before Easter and there's no access to chemistry for the time being at least. Most of the B&W workshop images I've given away but this one is nice:
