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Pin Hole Imager Project Part 2

Acancarter

I'm retired and living in Northamptonshire, so plenty of time for photography.
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Pin Hole Imager Project Part 2

17 Jan 2021 2:01PM   Views : 292 Unique : 232

Quite a few views on part 1, so here is part 2. Thanks for taking the time to look at this - I hope you found it interesting. At least it will have confirmed that there are crazier people about than you! Please leave any comments you might have. Message me if you have any questions.

There are some commercial pin hole ‘ lenses’ available, such as the pinhole pro shown below. This is small, portable and, for a pin hole, fast, so it can capture video so probably good fun! The down side is resolution. I haven’t evaluated one of these, but I expect it could only resolve 200-300 pixels across a diagonal, well below my target of 1000pixels. You could try making something like this with an old body cap and maybe an extension ring.
331023_1610891425.jpg

How to increase the resolution? What is the best diameter of the pinhole? For a large pinhole, effectively you just get the shadow of the pinhole on the sensor, it is just geometry. As you reduce the diameter of the pinhole, the image spot gets smaller and smaller, until you reach the ‘diffraction limit’ then it rapidly gets larger again, as well as dimmer. This was all worked out long ago, by none other than Mr Petzval of retro lens ‘swirly bokeh’ fame! What this means is that you need to increase the size of the sensor, as well as keeping the pinhole near the diffraction limit to increase resolution. Film pin hole cameras achieve this by having a large bit of film; the larger, the better. The largest pin hole camera ever was made in an aircraft hanger, with a massive film! Not very portable… My way round this was to have an intermediate imaging screen, where the film might have been in a conventional pin hole camera, and then to photograph the screen with the camera. Sounds simple! See the diagram below:
331023_1610891464.jpg


One disadvantage will be the light level, as much of the light will be scattered in all directions by the screen, rather than most being absorbed by the film. The effective speed drop depends on the aperture of the re-imaging lens, but it is likely to be several stops. The low light sensitivity of the electronic sensor compared to the film makes up for this.

This is my first attempt. Lots of lessons to be learned. Stray light is a killer! This only worked when covered in cloths, blankets, boxes… anything I could find to keep the light out! The cardboard wasn’t completely opaque either and light scattered forward from the screen gave the ‘brown’ cast to the image. The pin hole was made in thin aluminium tape, using a sewing needle.
331023_1610891505.jpg

The screen inside the box was a problem as well. I didn’t have a sheet of ground glass so used thin paper (supported on glass). I think it would be difficult to make ground glass for this, as it would need to scatter the light in all directions (Physicists would call this ‘isotropic scattering’ - the surface of the moon scatters isotropically, which is why it looks like a flat disc when full rather than a sphere!).

The texture was evident with some papers I tried. The flowers below had an interesting ‘impressionistic’ rendering, due to thin airmail paper - this was attractive, but I think an identical effect could be achieved in photoshop.
331023_1610891539.jpg

So this version didn’t meet my target resolution and had all sorts of drawbacks, but was fun and encouraging and didn’t cost anything! In the next part I’ll describe my final version… I say final, depends on any further ideas or suggestions!

Comments


dudler Plus
18 1.9k 1928 England
19 Jan 2021 12:14PM
Ahhhh... A two-stage device - my attempts have worked round a direct image on the sensor: pinhole size is determined by my dexterity (or lack thereof). I realised that if I put extension tubes behind the pinhole I could alter the focal length (the first attempt used a broken lens, so records weird EXIF data: subsequently, I've used a pinhole on a body cap.

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