I have asked this before on another list and not recieved a definitave answer sp please excuse andy duplications

Does Reciprocity law hold true for B&W printing?

In other words if my exposure time is 10 seconds at f11 would it be true that f16 for 5 seconds would produce the same results.

And therefor if I wanted to shift all zones by one zone darker I could expose as 20 seconds at f11 as opposed to 10 seconds at f11

I cold use test strips but i am looking for control and pridictability. to my mind we are still dealing with silver halides so the efect should be silimar to film but i am just not sure how to qualtify it.

I do realise that I could achieve a simlar efect by changing paper grade , but to my understanding, changinf paper grade either compressses or expands the scale and does not shift it sideways

Julian

-- Julian Young (Julian_young@nl.compuware.com), February 08, 1999

Answers

10 seconds at f/11 = 20 seconds at f/16.

-- Sean yates (yatescats@yahoo.com), February 08, 1999.

opps, well spotted. But does this still hold the way i think it does

Julian

-- Julian Young (julian_young@nl.compuware.com), February 09, 1999.

As an expeiment, try the following in your darkroom:

Place a neg in the enlarger and focus on the easel.

Place a sheet of paper in the easel and have a cardboard ready that will cover the image area of the easel. Set your timer for about 5 seconds and make a test strip by uncovering about an inch wide strip of paper at a time. Move the card to open about two inches and make another exposure at the same timing. Keep going until the whole paper is uncovered at the last exposure. Put the paper into the paper safe.

Set up again as before, but this time, dont make individual exposures. Instead, start the timer running, set for one or two minutes. Use the card and open up inch wide strips, just sliding it over at each 5 second interval.

Process both sheets at the same time, fix and compare the results.

Whether this illustrated reciprocity or fluctuations in lamp brightness as it is turned on and off, I dont know for sure, but there will be a difference.

I will let you make your own conclusions.

-- Tony Brent (ajbrent@mich.com), February 09, 1999.

Yes, there is reciprocity failure in B&W papers. This is easily proven with a few test strips and a densitometer. The good news is that it is not very extreme and hard for the eye to distinguish within "normal" print times, but nevertheless, does occur.

-- Frank Andreae (fandreae@mich.com), March 15, 1999.

I've found that reciprocity falure starts arround one minute of exposure. Keeping exposures in the range of 10-30 seconds(except for buring) seems to work best and visual interpolation of exposure times in these ranges is linear.

-- Gene Crumpler (nikonguy@emji.net), March 15, 1999.