Interesting Reciprocity Failure Observation
I've been taking some pics with a pinhole camera I made, and since the exposures are long, have been trying to correct for reciprocity failure. I forgot to do so on the first few shots, and remembered on the final few.
The interesting thing is, the shots where I extended the time for reciprocity failure are way overexposed - unusable basically. The ones shot with no adjustment are fine.
I'm using Ilford Delta 400 film and the chart that Ilford provides for RF.
Do you know accurately the relative aperture of the pinhole?
Delta 400 has a smaller reciprocity correction than most other films. There is an article in www.unblinkingeye.com called "LIRF is Lurking at Your F-Stop" that may help. Pardon my prejudice, but I wrote the article which is based on data by Howard Bond.
One of the early paragraphs in the Howard Bond article that Gainer references reads as follows:
Both Kodak's and Ilford's generic reciprocity charts are based on general guidelines for films that were made 30-40 years ago. They are no longer close to the mark, especially for delta or t-grain films, but the outdated curves continue to appear in many film data sheets. If you want an explanation for that, ask the manufacturer. I don't have one.
If you use the chart from my article
“Adjusting Effective Film Speed,” (PT,
Nov/Dec ’02), please discard it. It was for
Kodak films manufactured in their old facilities.
Also, Ilford’s 400 Delta is no longer
offered in sheet film sizes, and HP–5 has
become HP–5+. The corrections for the
Ilford films in that chart were based on an
obsolete curve Ilford supplied.
reluctance—I feel this work should have
been done by the manufacturers—I spent
well over a month and about 300 sheets of
film to produce a new chart.
In any case, your uncorrected times are probably much closer to the needed adjustments than the major adjustments in the outdated curves in the Ilford tech sheets. For Delta 400, the truth is somewhere between uncorrected and the outdated generic corrections, and closer to no correction. I can't find any Delta 400 reciprocity tests or information among my usual suspects. Maybe I'll remember to test the next time I shoot Delta 400, which I rarely use.
Based on the charts I got from Bond's data for the Kodak new emulsions, I would expect the correction for Delta 400 to be close to the same as for Delta 100. I found the Ilford data to plot parallel to Bond's data the other films, but shifted quite a bit from experimental values. Even their corrections for the old stand bys were much too great.
I am so happy that someone has finally brought in some fresh thinking to this topic. Every article that I have read here and elsewhere recommends using the published reciprocity failure tables. As Fred Picker said "Try it". Well I did for TXP 320 and I made my own reciprocity failure table with my cameras and my developer. I can say (because I have been using these tables for a couple of years now) that the published tables will overexpose this film and make the negatives unprintable.
Originally Posted by Lee L
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I see, well that makes more sense then. With a pinhole camera, especially the first roll of film, it seems everything you do is sketchy because you did all the numbers yourself and you don't know if you screwed up somewhere. At least now I know that I was pretty close to being on the mark.
Originally Posted by Lee L
The early, uncorrected photos are underexposed, but only a little. This all makes sense now. Thanks.
The chart I have says that for a 30-second exposure you're supposed to adjust it up to 200 seconds, which is a heck of an adjustment. Sounds like they are old indeed.
Now back to the problem of measuring sub-millimeter holes...
Reciprocity failure occurs in the shadows. Just because your shadow detail goes into reciprocity failure doesn't mean that the entire sheet of film is in reciprocity failure. The highlights usually don't for example. Instead, the highlights expose and process just fine.
Originally Posted by mrdarklight
Increasing exposure to compensate for reciprocity failure in the shadows also increases exposure for the highlights. Which as you have found out can make the highlights so dense as to be unprintable. Unless you do something about it.
There are a couple of things you can do. First, use a film with better reciprocity characteristics such as Fuji Acros, 100Tmax, or Delta 100. This may mean that you don't have to increase exposure at all which solves your problem with no further action.
Alternatively you can follow the old Kodak advice, which is to reduce development time to compensate (at least in part) for the over exposure of the highlights.
Of course, you could use a combination of the first two. Or, you could just let your shadows go black. One of these four options will probably work for you.
How about using Diafine to even out the shadow/highlight differential?
Before trying to solve problems that no longer exist, another observation from the Bond article:
Many others have also found that the increased contrast found with long exposures in older films is much less evident to nonexistent with t-grain, delta grain, and revisions of older style emulsions. You need to test for yourself, but with the 'worst' behavior in the 5 tested films being a 2/3 zone increase in density at zone VIII, it's not going to make your highlights so dense as to be unprintable with common darkroom techniques.
In the past, films typically yielded
increased density ranges with long exposures.
The extra exposure that rendered
Zone III as planned was less needed in the
high zones, so they were elevated, increasing
the density ranges.This situation is now
much improved.At 240 seconds indicated,
T-Max 400 and 100 Delta showed no elevation
of Zone VIII. Tri-X was up slightly, but
only slightly more than the typical variation
from one trial to another. The Zone
VIII densities from HP-5+ and T-Max 100
were elevated about 2/3 zone. Stating an
elevation in terms of zones is very approximate,
since the width of a zone (expressed
as a range of negative densities) varies
greatly with development.
Here's the Bond article, well worth reading:
There are many ways of doing that, on way is to take pictures, 2. way is to take pictures via digital camera, 3. way is via microscope -comparing to a known size, 4. way is to use enlarger and compare, 5. way is to use a slide viewer to compare, 6. way is to use a scanner, I prefer the 7. way, measure light with a 50mm at 22, replace the 50mm with the pinhole and count stops.
Originally Posted by mrdarklight