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Comment by jonplackett | Hacker News Reader

How come this wasn’t completely over-exposed after so long?

The way this technique works is not a standard film photographic exposure, the paper responds very differently in this process to normal processes. In effect what you have is the Sun burning a line onto the paper. This actually means that you don’t need to develop or fix the paper afterwards either. However, the final print does not last very long - the paper is still light sensitive when you take it out of the can so you need to scan it rapidly, in a dark room.

Here's a quote from the Solarcan (a premade product that uses the same principle) instructions:

> Inside the Solarcan is a single sheet of light sensitive photographic paper. Once exposed to light a chemical reaction happens to the emulsion and it begins to change colour. Traditionally photographic paper captures a latent image that is not visible to the naked eye and requires chemical processing, however with a Solarcan the light from the Sun is so powerful that as it passes across the paper it darkens visibly and no chemicals will be required to ‘develop’ the image.

https://solarcan.co.uk/wp-content/uploads/2019/06/solarcan-t...

Maybe if you make the pinhole the right size and match the ISO of the paper? I can barely get digital photography exposures right lol.

Yeah maybe it was just a really tiny hole!

It’s strange though because you usually need to figure out how long the exposure would be and then use the right size hole / paper. But this sounded like it was accidentally left for all this time.

I wonder how much post-processing they had to do?

From the instruction manual [1] for the Solarcan (a premade product that operates on the same principle), it appears pretty much all you do is scan the image and then invert it. Take a look at the images in the PDF - they're pretty cool.

[1] https://solarcan.co.uk/wp-content/uploads/2019/06/solarcan-t...

There's a few different actions. First, since it isn't developed the paper effectively acts extremely slowly. Whereas normal photographic paper would be around 6 or up to 20 ISO, doing it this way is almost immeasurably slow. Getting similar results without a pinhole and just something layed on top (photogenic drawing), it would take several hours in direct sun to get a decent image. The next contributing factor is that some papers "print out" easier than others. The hard ones will take hours in direct sunlight, while the easier ones might take "only" 30 minutes or so. The final factor is that pinhole is a very slow and low contrast method of exposure. Silver halide paper suffers from "repricocity failure" where basically during a very long exposure, the material becomes massively less sensitive. This is why film is so hard for astrophotography. A 100 ISO film over several hours can become something more like 1 ISO. Either way, the end result is that the paper gets very slow and gains a very wide exposure latitude, nearly impossible to over expose.

You can push/pull photographic chemistry a couple stops... and for photo paper I think you can push/pull a bit more than regular C41 color film.

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