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Pin Hole Imager Part 3


I'm retired and living in Northamptonshire, so plenty of time for photography.
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Pin Hole Imager Part 3

18 Jan 2021 9:06AM   Views : 427 Unique : 279

Thanks to everyone who dropped by to look at part 2 of this blog. Today I’ll describe the construction and results from my final model, which did meet my resolution target. Unless anyone knows otherwise, I think this might just about be the highest resolution portable pin hole imager ever made. Of course, that doesn’t mean it was that useful!

The problems with the first version were the resolution and scattering from the internal imaging screen. If I had had an ‘ideal ‘ screen - ie one with isotropic scattering, it would have had as good resolution as a conventional pinhole camera with an A4 sheet of film, which would have been awesome. The paper screen seriously messed this up. I thought long and hard about what else I could use, then a ‘light’ came on! I’d previously bought a couple of 600mm square LED ceiling light panels to experiment with (less than £20! What photographic anything can you get for that?...I'll maybe write some nots up on that next week). Take one of these panels to bits and you find two strips of very high power white LEDs, a ‘light guide’ plate that you can use for some great filter effects (my image ‘Into the Void’ was made with a bit of this….. AND a 600 by 600 mm sheet of very good white diffuser…ideal for an imaging screen. Well almost, just a bit thick. The frame from the Light panel was also pushed into service, as the central frame for the imager.

The diagram shows the basic configuration, similar to part 2, but considerably wider, to use the full size of the panel (well why not!). The geometry was chosen to replicate a 20mm focal length on a full frame sensor, using a 20mm f/2.8 lens as the reimaging lens. A faster, longer lens could have been used, giving shorter exposure times, but the 20mm made the imager reasonably compact and with the same dimensions for the front and rear parts.

The sides were all made of cardboard (print mounts) painted black to reduce reflections. The scattering sheet was flat and self supporting. You can see a couple of shots of the beast under construction below, the second with a large aperture on the input, showing a blurred image of the window frame.

The ‘business end’ was built with a sliding part, giving the option of a large (1.5mm) pin hole for live view composition, and a smaller pin hole (0.4mm) for capture. This is slightly larger than the diffraction limit. In practice, I found that the screen limited the resolution, due to its thickness. It isn’t possible to have a spot size smaller that the thickness, so I kept the diameter up to give better brightness. If I ever find a better screen material, I’ll add a smaller pin hole. My expected central resolution is approximately the screen width divided by the screen thickness, around 1200 pixels across, maybe around 1MPixel total. The resolution will fall off towards the edges, due to the angle the light rays meet the screen.

The whole thing was mounted on a wooden frame, with tripod mount and small ball head for the camera. Not exactly gadget bag sized, but small enough to carry in the car or take for a short walk.

These are a few shots taken around the garden and close to home. Exposures were typically 30 seconds at ISO 6400, with the imaging lens wide open. The live view screen was clear to view for composition with the 1.5mm pinhole, so you could see more or less what you were going to get. I set the camera to long exposure noise reduction, so there was a minute to wait before seeing theactual exposure.

I was very pleased with the overall sharpness and rendering. There is always some circular vignetting due to the pinhole thickness, and radial field curvature, both of which give a dreamy look to the images. They also all benefit from the square format, which arises naturally from the method.

The images respond quite well to a sharpen filter removing ‘Gaussian Blur’ as this is exactly what a round pinhole should produce. I think I should be able to improve the noise, as the files from the camera have vastly higher resolution than needed.

The images visually work better if they are not over magnified - panels of three images look good as the eye doesn’t see the limited resolution and concentrates more on the pictorial quality. I think my technology here is a bit ahead of my artistry. I need to find suitable subjects - which should be easier post lockdown. However, I don’t think this will be much use for sports, wildlife, street photography, astro, macro… but if you have any ideas please let me know!

Thanks for reading this!

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dudler Plus
19 2.0k 2018 England
19 Jan 2021 12:16PM
It's not just gaffer tape that's not necessarily opaque! Light leaks are going to be a constant worry with anything involving bright light AND high ISO, I reckon!

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