Re: Finding camera leaks

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Wed, 30 Oct 1996 15:02:54 -0600
>From: "P3D Gregory J. Wageman" <gjw@xxxxxxxxxx>
>Subject: Re: Finding camera leaks

>Greg E. writes:
...
>>  And could your eyes see that small fraction of light leaking
>>out of your camera, coming from a small bulb (even a 2W
>>halogen)?

>I don't know for sure either, but I believe a dark-adapted eye
>in a dark room is an extremely sensitive light detection instrument.

>I remember reading somewhere that the sensitivity of the eye to
>light is logarithmic, something like 12 orders of magnitude, and
>*increases* at lower light levels.

>How long would you have to expose that ASA100 film at f1.8 in order
>to take an acceptably-exposed picture of, say, a forest in moonlight
>which your eye is quite capable of seeing in "real time"?   Tens
>of seconds?  Minutes?  What does this say about the effective "speed"
>of the dark-adapted eye?

I agree - the eye can trigger fairly reliably on just a handful of photons.
At low light levels, film suffers extreme reciprocity failure - thus the
need for exposures of many minutes to capture views that can be seen by the
unaided eye. I've processed 100-speed color negative film in lighting
conditions where I could actually see a little bit, but the light did not
affect the film to any perceptible degree.

For the best results, the eyes must be dark-adapted - techniques for this are
described in Astronomy, Sky & Telescope, etc.

For light leaks too small to be detected by eye, the answer is "don't store
your camera in direct sunlight for a week between shots". :-)

John R


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