Reciprocity issues for paper?

[JCJackson]

Working on big enlargements and wanting to keep the image sharp, I find I wind up with some exposures that measure in minutes, not seconds. It's also convenient to simply take the exposure required for an 8 x 10 work print and multiply by 4 to find the exposure for a 16 x 20 on the same paper using the same enlarger aperture. Despite the logic of this approach, the results are sometimes less than perfect. Do paper emulsions experience a breakdown of reciprocity in long exposure times as do most B&W film emulsions?

 

[Photo Engineer]

All emulsions suffer from reciprocity failure at long and short exposure times. They change in both speed and contrast. Modern, high quality papers and films suffer far less than those made just 50 years ago or even 25 years ago.

PE

 

[Thomas Bertilsson]

Test strips are good, and every time you change size of the print it's a good idea to take new notes.
If you continuously use the same paper, with experience you will find yourself comfortable moving between paper sizes.

- Thomas

 

[JCJackson]

Thanks to both Thomas and PE for confirmation of suspicions about the paper and the need for careful testing and record keeping.

 

[RalphLambrecht]

Working on big enlargements and wanting to keep the image sharp, I find I wind up with some exposures that measure in minutes, not seconds. It's also convenient to simply take the exposure required for an 8 x 10 work print and multiply by 4 to find the exposure for a 16 x 20 on the same paper using the same enlarger aperture. Despite the logic of this approach, the results are sometimes less than perfect. Do paper emulsions experience a breakdown of reciprocity in long exposure times as do most B&W film emulsions?

JC

The inverse-square law to calculate old and new times (t1, t2) only works with the old and new paper-to-lens distance (u1, u2). For old and new magnifications (m1, m2), you need to use a different equation. See attached files.