Does photo paper have an ISO rating?

[ciniframe]

Well the last photo I took had pretty decent exposure at 15 seconds.
Difference between f/16 and f/300 is four stops slower lens=less light.



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I believe the difference is between 8 and 9 stops; ie.
f16-22-32-45-64-90-128-180-256-360
So about 8.5 stops
Of course calculations can be useful but in the end "cut and try" is the likely best way to arrive at a exposure solution. Just be sure to make notes.

 

[tonyowen]

Now I'm wondering if there is a certain amount of time during the image taking exposure that it takes the paper emulsion to "wake up" and start reacting - in other words a reciprocity factor is needed for exposures faster than 1 second or so. /QUOTE]

A late comment, but his would explain why exposing paper negatives (rated at ISO6) in a darkened room with an electronic flash gun whose GN was corrected from the ISO100 rating to and ISO6 rating DID NOT yield any image when the Multigrade paper was developed.
ps the flashguns give good images when used with film.
regards
Tony

 

[Raghu Kuvempunagar]

BTW, Ilford MGIV or indeed any paper can be reversal processed to give some very nice reversal prints or paper positives in-camera. In this case, you do lose a lot of speed though. You don't need a special reversal paper.

Sadly I didn't get a chance to ask PE what causes this loss of speed when reversal processing paper. In my experiments earlier last year, I too found that there's a significant loss of speed and my exposures were running long and requiring even more exposure to address reciprocity. Does anyone know and would be able to explain the scientific reason behind the loss of speed?

 

[Donald Qualls]

I've seen a slight increase in speed when reversing B&W film (at least with Tri-X) using a highly active first developer and a small amount of thiosulfate added in the first dev. The combination of development to completion and very high silver solvency (neither one usual for film) result in reducing the amount of halide remaining for the reversal and second developer, thus lightening the final slide (effectively an increase in speed, if it isn't at the cost of shadow detail).

With paper, we'd normally develop (nearly) to completion anyway, so there's nothing to gain by increasing the level of first development -- so in order to lighten the final positive, we'd need to expose more. Adding silver solvency in the first dev might help a little, but a print emulsion also has a narrower dynamic range than film and starts with less halide (which is more or less the same thing), so there's less room to push the image up the curve than would be the case with a camera film like Tri-X, FP-4 Plus, etc. In the end, all you have available is to give more light.

At least, that's how I read the situation.

 

[Raghu Kuvempunagar]

I wonder if the loss in speed has got anything to do with the fact that the speed of paper is designed for mid-tone reproduction at a density of bf+0.6 (as per ANSI standard for paper speed). This mid-tone density for a neg-pos process translates to a density of 1.5 in reversal (assuming DMax of bf+2.1). To peg the mid-tone density at bf+0.6, additional exposure is needed which varies with the contrast grade of the paper. Is this reasoning correct? Anybody who is familiar with sensitometry side of paper, like @Stephen Benskin or @Bill Burk, can throw light on this issue?

 

[wiltw]

Reading this thread reminds my how very much I miss the very insightful posts by Photo Engineer!