Reciprocity using multiple exposure technique?

[Eric Jones]

Hello,

I was wondering if you decide to use the multiple exposure technique to build up a long exposure do you need to take into account the reciprocity of your film? I'm leaning towards, no I do not need to account for it since I'm hitting the film with "bursts" of light but naturally all of your expert opinions would be appreciated.

Thanks

 

[noseoil]

Decidedly not an expert, but reciprocity is a factor in long exposaures, multiple or otherwise. I'm sure others can explain this better. It is sort of like the film toe vs. shadow density relationship. There is a point at which a small amount of light does not affect the film at all, but once the toe is reached and passed, things begin to change.

A similar effect with multiple exposures and reciprocity exists. Figure out what the cumulative exposure actually is by adding up exposure times, once you have reached the limits of your "normal" shot, add for reciprocity to make sure there is the correct exposure. You may want to try bracketing to see at what point this actually occurs with your film's curve / developer / camera / meter / etc. just to see when and how it affects the image.

 

[Eric Jones]

I was planning on shooting some tests tomorrow night..so I'll naturally try all of the variables, take notes and report back.

Thanks

 

[galyons]

Eric,
What you are describing is actually Reciprocity Failure, from the failure of the film to follow the reciprocity law. Low light intensity fails to activate the same amount of latent image in the silver crystals in the same period of time as would occur at "normal" levels of light. One can not avoid Reciprocity Failure by breaking the metered time into smaller, additive sequences, because the light intensity is causing the failure, not the time. Time is the compensating factor, not the problem.

Hope this helps. Let us know how your shots turn out!

Cheers,
Geary

 

[Tobik]

Eric,
What you are describing is actually Reciprocity Failure, from the failure of the film to follow the reciprocity law. Low light intensity fails to activate the same amount of latent image in the silver crystals in the same period of time as would occur at "normal" levels of light. One can not avoid Reciprocity Failure by breaking the metered time into smaller, additive sequences, because the light intensity is causing the failure, not the time. Time is the compensating factor, not the problem.
...

Cheers,
Geary

This is right. Film is optimized for certain intensity of light, if it is lower or higher, film looses efficiency. This is very important because impact in shadows differs from impact in highligts.

There is also another effect called intermitency effect. Exposure created as seria of partial exposures does not equal to one uninterrupted exposure unless the delays between partial exposures are very short. It means that 100 x 1/100s does not equal to 1 x 1s. There is something with relaxing or restoration of latent image or something like this, I do not remember exactly. Kodak gives in datasheets corrections for multiple flash exposure.

It means, that attempt to avoid Reciprocity Failure by multiple shorter exposures could lead to even worse result.