In a flurry of productive activity, I finished a pretty good looking (IMHO) extension to mount on the front of the adapter plate and a half-assed bezel bezel plate to hide the screw holes and give a "finished" look to the project. I'll add a couple of photos of the 3D printed prototype.
I've been very pleased with my pinhole camera designs and I expected the same with this project, even though I am using an entirely different kind of film. I loaded the Instax film back with a new pack of film and toted the assembly to work yesterday. In the true spirit of hubris and irony, my first photos are somewhat underwhelming. There are ten sheets of film in an Instax pack, I have only shot 5 so far.
In the interest of your understanding, allow me to 'splain a few things about (pinhole) photography before I get on with the analysis of my fail.
To make a photograph, I don't care how, you have 4 intrinsic variables that govern the exposure:
1 - Available Light (exposure value - EV)
2 - Film/sensor speed (ISO or ASA number)
3 - Aperture (F-stop)
4 - Shutter speed (hours:minutes:seconds)
#1 is what it is, but you can add light sources/reflection, and you can shade your subject.
If you're shooting film, #2 is what it is, but digital can vary to suit the situation.
#3 is a function of the size of the opening that allows light to the film/sensor. Fixed in pinhole cameras, adjustable as F-stops in other cameras.
#4 is how long the film/sensor is exposed to light reflected from the subject.
All exposure variables are adjusted in "Stops" or a doubling/halving of values. Sometimes fractional stops are used, but let's keep this simple. For a conventional lensed camera, for a given amount of available light, shooting film of a fixed speed, there are an infinite set of complimentary apertures and shutter speeds that permit the same amount of light to impinge on the film/sensor. Think about it - halve the size of the aperture, and double the exposure time, and the same amount of light hits the film/sensor. And vice verse. This is known as "reciprocity"
Except that this is an ideal model of behavior. In the real world, when film is exposed to light for a "long time" (which varies and is film-specific), its sensitivity decreases. Which means that for long exposures, a photographer must add some amount of additional exposure time to overcome this effect. Every film is different in this regard and some films are well documented, and others... well not so much. To make things even more cumbersome, "Reciprocity Failure" increases by a unique exponential factor for every film. Some are well-behaved and others need vastly more, even hours, added to a given exposure time.
With a pinhole camera, the aperture is very tiny and fixed. The only exposure variable controlled with the camera is the relatively long shutter speed. Depending on the film, some time will necessarily be added to the shutter exposure time to compensate. Instax instant film (unlike other Fuji films) is poorly documented as regards this behavior.
Instax is ISO 800, which means that it is 3 STOPS faster than ISO 100 - remember that each doubling/halving is a stop and 800 is 3 doublings of 100 (or 2^3). This makes for potentially very fast pinhole exposure times, so there is a neutral density filter ND8 inside the camera that slows the exposure 3 stops or back to ISO 100. This filter is a cheap, small, and unfortunately plastic element that is prone to scratching and fogging. I think I will rework the camera to take a filter that can be removed for low light situations.
The first photos have some notations on them to allow understanding the exposure. The film is effectivley ISO 100 with the ND8 filter, the EV is the "Exposure Value" i.e. EV 10 is twice as bright as EV 9, the ND8 is noted, and the exposure time.
Photography is purely mathematical if you know all the values, but it's hard to see...
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