Paper reciprocity?

[Craig]

Why is it that film has reciprocity and needs adjustment to the metered exposure for exposures longer than ~1 second, yet papers seen to to have no reciprocity effects? Is the paper emulsion that different, or is there actually a reciprocity correction I'm not aware of?

 

[Peltigera]

Because you always expose the paper in the middle of the log intensity/density curve. Reciprocity only kicks in when you are to the left of the heel of the curve.

 

[NedL]

I think paper might have a little, but making paper negatives I ignore it even with exposures > 1 hour.

I'm not sure I completely understand what Peltigera is saying... is it that on the left side of the intensity/density curve the slope is decreasing, and then it becomes essentially constant? Which is kind of strange if you think about it.. because some initial exposure is needed before anything happens at all. And aren't we in that long straight part of the curve when we are exposing film? Seems to me there must be more to it than just where on that curve we are, but I don't know much about it.

There's a ( coincidental? ) similar thing during development of paper... for the first 15 or 20 seconds nothing is visible at all, then pretty quickly a lot of image is visible, and then it visibly develops more slowly after that for the next couple minutes.

 

[Photo Engineer]

Paper has reciprocity, but you rarely go beyond the limits of what is normal for the emulsion. A paper emulsion is balanced for about 1" to 10 or 20" and thus there is no problem. Also, it is assumed that once you are close, the final tweaks are done by using the diaphragm of your lens and thus time remains constant.

Just don't worry about it.

PE

 

[RalphLambrecht]

Why is it that film has reciprocity and needs adjustment to the metered exposure for exposures longer than ~1 second, yet papers seen to to have no reciprocity effects? Is the paper emulsion that different, or is there actually a reciprocity correction I'm not aware of?

paper has reciprocity too

 

[Ian Grant]

Paper reciprocity can affect contrast slightly as you increase enlargement and exposure times. The owner of my local pro-lab (now closed down) had been a technician for Durst UK I had a couple of turrets full of lenses off scrapped enlarges given to me. Each lens had a grub screw in the aperture ring locking it to a set aperture and Terry explained that was done to standardise the exposure, so regardless of the print size the actual exposure times with colour paper were identical. This ensured that prints of differing sizes matched.

Ian

 

[Craig]

Interesting, thanks all!

 

[ic-racer]

B&W photo paper has excellent reciprocity in the light intensity range frequently used for darkroom exposures.

Failure of reciprocity for B&W paper usually only needs to be dealt with when doing safelight tests. That is, since the white light exposure can be very low intensity, the paper needs to be pre-exposed to do the test.

I never ran into reciprocity failure when I used to print color, but many color enlarger heads have non-diffracting apertures to control the light intensity, so that exposure time can remain constant when adjusting print dentisy.

 

[takilmaboxer]

I have experienced paper reciprocity when working with unusually dense negatives. It kicked in at about 30 seconds exposure time. Burning in highlights on these negs required opening the diaphragm all the way; I had the diaphragm at f/8 so that the entire (curly) negative would be in focus. Nowadays my negatives are properly exposed and developed and reciprocity does not enter into the picture.