"Curtis P. Hedman" wrote:
>
> I recently tried taking some Christmas lights photos with my 4T, using ASA
> 800 film. While taking the shots (in AUTO mode), it seemed like the
> exposures were too short, and when the film came back, most of the frames
> were severely underexposed - except for a few which I shot using the "B"
> setting and timed myself external to the camera. I then started fiddling
> with the camera, firing off some shots with lens cap in place (film loaded,
> of course). I found that, at the ASA800 setting, the exposures were running
> 8-10 seconds at f/2.... shifting to ASA400 resulted in 3 minute exposures!
> Thinking the contacts might be dirty I ran the selector back and forth a few
> time, and then seemed to get the long exposures at a number of ASA settings.
> When I repeated the experiment a day or so later, it was back to short
> exposures at ASA800, long at ASA400. Seems that the camera doesn't like the
> 800 setting! Anything below 400 seems to work OK. Anyone have any
> suggestions? Could there be a bad spot on the encoder or whatever, or
> persistent dirty contacts? Are there any other experiments I can try to
> narrow things done a bit? I did do a 'dark current' check - at ASA 3200 and
> f/2 (but lens capped), the meter read 1/4 second, which the list archives
> suggests is about right. I did find that even at ASAS25, the longest
> exposure the camera will deliver is 3 minutes, not the 4 minutes everyone
> seems to claim. Do I need to worry about light entering through the eyepiece
> during auto exposure, or does the mirror completely block off any stray
> light during exposure?
> Lots of questions! Hope someone has some suggestions..... Thanx in
> advance.
You might be experiencing what is called "reciprocity failure".
Off-the-shelf-film assumes that exposures will normally be in the range
from 1/1000 to 1/10 second. Exposures outside of this range frequently
require additional exposure. For example, quoting from book 1 of the
Ansel Adams' Basic Techniques of Photography book which in turn cites
data for Kodak Plus-X, indicated exposures of 1, 10, and 100 seconds
require lens aperture adjustments of +1, +2, and +3 stops [or substitute
2, 50, and 1200 seconds] respectively. The reason for these exposure
adjustments is that the chemical composition of the film's various
layers react differently to exceptionally short or long exposures than
they would had the layers been exposed to light in the "normal" or
expected range of time.
John Hudson
Vancouver, BC
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