I have built and used several pinhole cameras and I am now in the process of building an Ultra wide pinhole camera using a Polaroid MP4 carcass. By ultra wide I mean making the focal length as wide as possible that will just cover the 4x5 film format.
I thought that 25mm would be possible noting that the Zero Image 25B uses a 0.2mm pinhole with an effective aperture of f/138. It is difficult to see for sure but it appears to me that the example image in their gallery covers about 110mm (greater than 4 inches but less than 5 inches).
However, when I plug the numbers (25mm FL and 0.2 Dia.) into the various online calculators I get image diameters from 32.5 mm to 48mm. Obviously not enough. The online calculators seem to infer that a 50mm focal length would be the widest practical giving approximately a 100mm image circle.
Can anyone explain this discrepancy to me? What is the definition of image diameter for pinholes? is there a point where the image continues but is not counted as good enough perhaps?
I am about order the pinhole from Lenox Laser (thought I'd give them a try for comparison purposes) and do not want to end up with the wrong one so your input would be greatly appreciated.
The copal #1 shutter that I am fitting will be mounted in a "lens cone" so that the focal length can be varied in the future but for now I am only interested in as wide as possible with the 4x5 format.
Thanks to all, Terrance