The Shoebox constant radius pincam

More pinhole fun, this time with a shoebox.

The plan

It seems to be the way of things with me that ideas spring to mind in the oddest of places at the oddest of times. Sometimes they are gone forever before I have the opportunity to make a note, or they reappear in search of recognition at another odd time. Occasionally I get lucky and the idea stays with me long enough for consideration, perhaps acceptance and ultimately is put into practice.

So it was a couple of weeks ago on a crowded commuter train home at the end of a busy day, that I had the idea of creating a constant radius curved image plane pinhole camera out of a shoebox. A geometric puzzle to keep my mind awake amidst the noise of the train and the jostling of my fellow passengers.

The mental conundrum was too much to deal with but my mind wandered to thoughts of how I would record such an image. Perhaps a strip of art paper with SE1 emulsion brushed on? That would be cool – but it would be a negative image and how would I develop what would likely be a quite long strip of paper? Would I be able to make a positive contact print from it, perhaps on to another strip of SE1 coated paper?

Problems to be surmounted already and the idea still just at the stage of bouncing around inside my head. But how about direct positive paper? Would I be able to tape together enough of the 5″x4″ sheets I use in film holders to fit the image plane? It’s paper that I’m well accustomed to and as individual sheets it would be simple to develop. If nothing else it would be a good way of testing the camera design before moving on to ideas for other, more tricky media.

As I alighted the train for the short walk home my mind was buzzing. I had a plan. I had a project. I was going to create some pinhole art!

Construction

Sturdy shoeboxes are too good to throw away. They make useful storage boxes but better still, can be adapted to make pinhole cameras. It happens that I had two or three going spare. The box I chose has approximate internal dimensions of 33cm wide, 18cm deep and 13cm high.

Once made light-tight, the box could be fitted with a curved support on a radius of 16.5cm with a pinhole set centrally in the long side. Assuming full 180º coverage, the image plane would extend to 518cm wide by 12.5cm high and because it follows the radius of the curve, it would be free of both distortion and light fall-off along its entire length. The optimum pinhole diameter for a 165mm projection distance is roughly 0.6mm.

To ensure that the pincam would be light tight, the box and its lid were lined with thin black card. A strip of sticky-backed black felt was attached inside the lid to seal against the tops of the sides of the box. The card lining the sides of the box was extended about 1cm above the rim and folded inwards to form an additional light baffle.

The pinhole was made by pushing a dressmaking pin through a 3cm square of ArtEmboss mat black aluminium foil and positioned with black PVC electrical tape in the centre of the front panel of the box. More black PVC tape positioned over the pinhole aperture served as a shutter.

To form an image-plane support I cut a strip of card 518mm long by 130mm high, the length being calculated as the semi-circle of radius 165mm. A couple of spare pieces of foamcore served as a filler to support the back of the strip within the box.

Under safelight in the darkroom, I joined together five sheets of Harman Direct Positive paper with masking tape to make a strip 500mm long by 125mm high. This fitted neatly inside the box, supported by the image-plane card. With the top of the box secured by a couple of lengths of string I was almost ready to make my first test image.

Tripod support

I know from experience that although a viewpoint might be found, it is often unworkable because there is no way to support for the pincam in position! To get around this I like to make some provision for attaching my pincams to a tripod.

For the Shoebox pincam I took an offcut of plastic rectangular conduit cut to a couple of centimetres longer than the pincam is wide. I drilled a 7/32″ hole in the centre of the base and screwed the 1/4″ thread of a tripod quick-release plate into the hole. The metal thread of the mount effectively acted as a die to cut a thread in the softer plastic. To avoid stripping the new thread by over-tightening, I then fixed a 1/4″ threaded nut to the inside of the conduit, with the help of a big dollop (technical term!) of hot glue.

When the top of the conduit is slid back in place, the support is quite rigid and can be carefully mounted on a tripod. The pincam and support are held together by the string which also keeps the lid of the box in place. It may not be rock-solid but in pinhole terms it does the job perfectly! Being tripod mounted opens up a whole range of viewpoint options, levelling and height adjustment.

First test

I took the Shoebox pincam to the banks of the River Almond on a bright Saturday morning. Shaded by the trees, my hand-held Sekonic L-308S meter and the iPhone Pinhole Assist app agreed on an EV(100) of 9.8, giving an exposure time of 53 minutes for direct positive paper rated at ISO 3. During exposure the light changed as the sky cleared and the sun lit the trees on the far bank of the river, so I reduced the exposure time given to 45 minutes.

Back in the darkroom I opened the shoebox to find that the semi-circulat image-plane support had slumped a bit towards one end of the box. That would need to be fixed.

With the masking tape holding the five sheets together removed carefully so as not to tear the back of the paper, I proceeded to develop each sheet individually, paying particular attention to time (3 minutes each) and temperature (19ºC) to ensure consistency from one to the next as all five would make up the single final image.

Exposure was good, perhaps a little over exposed for my taste. There was distinct vignetting at the end that had slumped while at the other end the print was lighter with lower contrast. Overall I was extremely happy with the result and after making an adjustment to the image-plane support, loaded up the pincam for a second test to be made next day.

Adjustments

20170924_048

To resolve the problem of the image-plane support moving, I strengthened it with four pieces of flower arranging wire taped to the back, and pushed two drawing pins through the centre into the foamcore filler at the back. My expectation for the second test would be to find out whather the vignetting noticed in the first was due to movement of the paper or actual cut-off of the field of view.

Second test

For the second test I joined a couple of friends for a walk across the tidal causeway to Cramond Island, with an idea to make a pinhole exposure of the war-time gun emplacements and concrete huts. It was another bright day, this time with steady soft light from a light grey backlit sky.

Again, my Sekonic and iPhone agreed on the light, metering EV(100) of 11.8. To allow for yesterday’s perception of slight over-exposure I rated the paper at ISO 4 (1/3 Stop faster) and gave the metered 12 minute exposure.

Back in the darkroom again, the fix made to the image-plane support had done its job and I processed the paper as before. Two things were immediately obvious: Clear vignetting at either end of the image suggesting an effective field of view amounting to about 145º rather than the full 180º semi-circle I had hoped for; and rather disappointingly an inexplicably overexposed image!

The puzzle

The over exposure of the second image was quite unexpected. I rechecked all of my measurements and calculations and finding them all correct I proceeded to remove the pinhole to accurately check its diameter.

I mounted the pinhole alongside a steel milimeter rule on the bed of my scanner and scanned at maximum resolution. Viewed at full size on screen I was able to measure the pinhole and by reference to the magnification of the known milimeter rule, calculate the diameter as 0.612mm, exactly as intended and giving an aperture of f/269.

With all the variables checked and found correct, I was left with only one conclusion to explain the over exposure -human error – mine! On the first test the sky had been obscured by the trees while the second was under an open sky. I can only conclude that on the second test I have pointed my meters at the wrong part of the scene and failed to account for the brighter conditions.

More care needed next time methinks! And I do intend that there will be a next time as soon as possible. I want something worthy of a frame on my wall.

EDIT:

There is a follow up to this in my next blog post. In it I come to the correct conclusion and find a resolution!

Intrepid pinhole projections

Pinhole experimentation with the Intrepid Camera.

My Intrepid 5×4 Field Camera is one of the original Kickstarter models. It came with a lensboard fitted with an 0.5mm pinhole, the optimum diameter for a pinhole camera with the pinhole set 140mm from the image plane, giving an aperture of f/280. (I refuse to refer to this distance as the ‘focal’ distance, there being no lens to focus, and instead refer to it as the pinhole ‘projection distance’ or ‘PD’ for short.)

I have used the camera with its lensboard pinhole on a few occasions, always setting the front standard at 140mm or so. However the camera’s bellows adjustment gives the flexibility to alter the PD to cover a range from about 60mm all the way out to almost 300mm. With a little spare time on my hands I decided to investigate the field of view obtainable at different PDs and to see whether any image degradation occurred due to using the fixed 0.5mm diameter pinhole at ‘non-optimum’ PDs.

I set up The Intrepid on a tripod with a selection of my grandchildren’s old toys, an old rabbit hut and an even older garden shed as subject matter. My plan was to make exposures on Ilford MGIV RC Satin paper rated at ISO 6, setting the bellows for 60mm, 100mm, 140mm, 180mm, 220mm and 260mm PDs and using the Pinhole Assist iPhone App for calculation of exposure times. In the event, I ran out of sufficient daylight to complete the series and made do with four exposures.

Here they are:

20170916_00120170916_00220170916_00320170916_004

The developed paper negatives have been scanned, inverted, flipped and been adjusted for black and white points in Affinity Photo.

Pinhole exposures will never be sharp due to diffraction but I am unable to detect any noticeable image degradation over this range of PDs although only the third image is exposed at the ‘optimum’ PD. This points to the accuracy and cleanliness of the pinhole supplied with the Intrepid. Exposure times are all satisfactorily consistent, given that changing light conditions during longer exposures have an uncontrollable effect on the outcome, so my long-standing faith in Pinhole Assist as a great tool for pinholers is well justified.

There have been times when pinholing that I’ve wished I could narrow the field of view. I think I’ve found a solution!