This is an update that builds on the previous discussion of exposure and reciprocity failure.  I don't claim to have written The Book on the subject, but referring to the previous log may help you understand the following. 

-----

When last we left our hero, he had cleverly tucked a cheap neutral density filter inside the workings of the Pinstax in an effort to sloooow down exposures to a workable shutter speed. The film is ISO 800, which is 3 STOPS faster than ISO 100 ( or 3 doublings -8 times faster - to correct a typo in the last project log). A ND8 filter brings the speed of ISO film down to an effective ISO 100. Why would I want to do this (no one asked. ever.)?

I'll tell you: In most of my camera designs, I use a shutter that consists of a blade that rotates out of the way to open the pinhole to light and expose the film.  I like my design, it's simple, and it lends itself well to 3D printing.  Other cameras use adapted shutters from existing cameras, bits of opaque tape, floppy disk shutters (really), or caps that variously snap, thread or clip over the pinhole. Every shutter has its advantages/disadvantages: cost, complexity, scarcity, remotability® (I just coined that word - the ability to remotely operate). 

I prefer an exposure time in excess of four seconds. Primarily, this is owing to the slight amount of camera movement induced in opening and closing the shutter. Even on a tripod. Even with the steadiest of decaffeinated hands. Moving the shutter open and 

1... 2... 3...

closed shakes the camera just a wee bit which blurs an already-soft photograph. Any longer, and the motion becomes insignificant in the context of the longer duration, in the same way that a person strolling through the scene of a minutes-long exposure will be invisible. 

Keep in mind, one of my goals for pinhole photos with my cameras has been proof-of-concept, and I want zero camera motion during exposures to demonstrate that.  There are all kinds of artistic goals that would allow for some - or much - camera motion during a pinhole exposure, but for me, not so much, most of the time. I am deeply, profoundly satisfied with many of the long exposure pinhole shots I make and I think that has become an element of my style. 

Which brings me to the subject of ISO or film speed. I like ISO 50 or 100 for pinhole because (except for bright days) my shutter can be open for at least 4 or 5 seconds, and I have taken to using ND filters on bright days to allow me to slow the exposure to minimize the effects of camera motion. I could design a remotely operated shutter that uses a cable release (and I have), which is a standard trick of pinhole photogs everywhere. But design is about both form and function, and I prefer the form of my clever and simple blade shutter. 

So, owing to my vanity and a fondness for a simple but motion-prone shutter design, I endeavored to put a tiny (28mm) ND filter inside the Pinstax to bring the exposure response for ISO 800 Instax film back into my comfort zone.  The small filters are readily available through Amazon. They're plastic, rather than optical glass, so they're inexpensive. But inexpensive also means cheap, and they scratch with the lightest kiss of wind-borne dust or pollen.  A pinhole camera, as you may know, has a near-infinite depth of field. Which is photo-speek for "everything is in focus".  Including the tiny mote or scratch on the ND filter cleverly tucked inside the pinhole camera (where it is devilishly impossible to clean). 

So there's that.

Also, Reciprocity Failure (RF)- The unique property of non-linear response to lengthened (or shortened, sometimes) exposures, outside the design parameters of the film. For Fuji Neopan Acros 100 B&W film, this means a fairly trivial amount of time is added to a calculated exposure to compensate for this effect. Fuji Velvia 50, on the other hand (a delicious color slide film), requires substantially more time be added. Turns out that Instax instant film RF is both undocumented (in ATYPICAL fashion for my beloved Fuji film guys), and catastrophically poorly behaved in this regard.  

In my initial tests, I am finding that WITHOUT the ND filter, I don't get good exposures until more than +10 stops. That's 10 doublings, folks. In relative terms, if the metered exposure for a given scene is 1 second (distressingly fast to me), I don't see passable (forget beautiful) results until around 14 MINUTES. Adding the ND8 filter would push that to 2 HOURS (along with all the undesirable baggage of cheap dirty plastic filters). 

So, What's next for the Pinstax? Currently, it is sitting in the windowsill, perched on top of a rugged wooden pinhole camera I built years ago. It looks cool.  Aesthetically, it pleases my eye and design sense. Too bad it's crap.  

All my pinhole cameras have been designed to allow for disassembly and tinkering by the folks who 3D print, build, and/or use them.  The Pinstax is no exception.  I know that people shoot Pinhole with this crazy film called Instax - I've seen the photos. So, it may come down to using a larger aperture (pinhole), faster shutter speeds, or some other trick I haven't deduced. It certainly means that I have to come out of the Schlaboratory and talk to the people who are making it work. 

Clearly, using a new and different film, but expecting results consistent with my previous efforts has been a pinhole schooling. In shooting 35mm and 120 film I have a bias as to how I design cameras, and, as always, you start with the film and design the camera for the medium.  

Stay tuned!