Rain and film

[Richard Littlewood]

Taking a shot not long ago, late in the day, low light, I had an exposure time of 5 mins. During the exposure it started to rain, not heavily, but a light wind driven rain, and after about 4 mins the camera filter was so wet that the drops of water started to run. I took one look at the saturated filter and ended the exposure, thinking that particular shot was a write off.
Odd thing is, that even though the sheet is slightly underexposed, I can't see a trace of the filter being drenched. The image is sharp and blob free all over. I have other shots that have single water spots that stand out like a sore thumb, and others taken in fine mist that have a mottled look, so the lack of evidence of a water spotted filter totally baffled me.
Then I got to think that maybe during a long exposure what happens is the film is much more responsive to initial exposure to light, I mean much more, than it is towards the end of the exposure time, and in a 5 min time, 80% exposure is complete within 1 min, with the remainder of the time just adding density. Hence the invisible rain.
Is this a known reciprocity thing? Am I just being dumb?!

[dazedgonebye]

Sounds like you've got it figured out.

That last minute contributed very little to the exposure due to reciprocity failure.

[Andrew O'Neill]

That last minute contributed very little to the exposure due to reciprocity failure.

I would have to agree with this. When I look at my reciprocity exposure compensation curves that I made a while back, the curve starts starts off as a straight line and begins curving upward very sharply at about 5 minutes. By about 15 minutes it's almost vertical. I wonder if at some point, the curve will bend back on itself and reverse of tones will occur? I should mention that the curves are for HP5 and FP4.
I've had rain drops on filters which ruined short exposures and the same for long exposures but with no apparent damage to the image.

[Ron Marshall]

... after about 4 mins the camera filter was so wet that the drops of water started to run.

Perhaps the drops had been running through most of the exposure, so that individual drops were not stationary and/or stable long enough to leave a distinct image?

Interesting!

[sun of sand]

Rons is my idea as well. Water itself is not the issue as you can shoot underwater and be fine ..it's stationary blobs that leave defined edges
I'd doubt its as sharp as otherwise would have been but i dunno

I think film is more responsive to light towards the end than beginning
Thats a misnomer maybe
maybe latent image "fades" as time goes on
but then

Anyway
when using flash i've read that it's the later flashes that will show up brightest on the negative

If after 1 minute you were only off by a stop or two then it wouldn't make too much difference in the end? underexposed a bit is all
You set off a bomb at 45 minutes into the exposure your negative will be ruined? overexposed
As in film is always equally responsive to light it's the long length of time for dim light to reach correct density that is the issue
2 minute exposure -calculated- on polaroid is much different than a 10 minute exposure on polaroid

[sun of sand]

"At low light levels, i.e. few photons per unit time, photons impinge upon each grain relatively infrequently; if the four photons required arrive over a long enough interval, the partial change due to the first one or two are not stable enough to survive before enough photons arrive to make a permanent latent image center."

at certain lengths to get correct density you're just chasing your tail
EV-3 with no light anywhere you're not going to get an acceptable exposure ever?