[alt-photo] Re: Ultra Long Exposure Reciprocity Failure?

[Greg Schmitz]

I've always run simple tests using a step wedge and a light source I can 
control / or a grey scale and a camera.  It's simple to make estimates 
for long exposures when they are compared to shorter exposures.  Without 
looking at my notes, my recollection is that for black and white there 
may be a decrease in contrast and developing time may need to be 
adjusted as well as exposure.

FWIW when you get into really long exposures you've got lots of wiggle 
room.  A 50% error with a 400 second exposure is 200 seconds.

--greg


On 1/19/12 5:07 PM, Francesco Fragomeni wrote:
> Hi all,
>
> I've been trying to figure something out with this for a while now and my
> guess is that the only real answer is testing but I'm hoping that someone
> here might be able to offer some mathematical insight that I haven't
> thought of or seen elsewhere. I am interested in making some ultra long
> exposures and I am looking for a way to approach calculating the rate of
> change in reciprocity failure as the exposures get longer. I understand
> that reciprocity failure is not linear but logically there should be a way
> to mathematically work out the rate of change of the failure over the
> increase in exposure. I'm probably wrong about that otherwise someone would
> probably have solved for it by now but alas I'll still put the question out
> there.