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• The Curious Case of Instax Reciprocity Failure

  theschlem • 08/17/2014 at 03:18 • 0 comments

  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. 

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  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...

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• Learning Curve

  theschlem • 08/01/2014 at 02:31 • 0 comments

  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 a clear relationship between the exposure data on this small sample of test shots.  

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