View Full Version : Print exposure time vs. contrast

Scott Hamming
31-Oct-2001, 12:27
After reveiwing this sites Q&A from Andre Noble (Oct 10) and Simon Rodan (Sept 2 ) I have a very similar issue, but have already addressed some of the responses posted without luck. Here goes...

BASICS: Film is HP5, Ilford RC paper, Dektol dev.(90sec) Indicator stop (30sec) , and Kodak fixer(120sec). All temps were 68 F. Filtration (Durst max. 130, dic hroic) was set at Filter Grades 2,3,4, and 5 per Ilfords data for said type of enlarger (using both Y & M settings to maintain consistency in exposure times, w hich I will state below worked seemed to work for test strips). Head was set fo r 5x7 print. F stop 5.6, (8 takes way too long for basic exp.)

METHODOLOGY: I started with test strips, exposing a "blank" negative (exposed w ith the cap on for max. density). Grades 2 & 3 were exposed and processed, then 4 & 5 under the "same" conditions. Grade #'s 2 & 3 came out very well achievin g black in 8 exposures for 8 sec each (does that seem like a long exposure??). There seemed to be an "appropriate" contrast gradation between the two as well. However, in comparing Grades 4 & 5 (also max. black in 8 sec. by 8 exp) I notic ed the lighter greys were not what I expected in a higher contrast strip, that b eing darker with the increases. In fact the first step on Grade 5 was lighter t han 4. **Had I not separated exposures between 2,3 & 4,5 I too would have tho ught this was a bulb heating/cooling issue as addressed in prior Q/A.**

RESULTS: I decided to run "full size" prints (5x7) using a neg. I know to have a good contrast range (the same neg. used in a darkroom class) at Grades 2 and 5 - running lean on paper at the time. The G2 print came out great, with a full range of tones, max. black, etc... G5 was another story, in part foreshadowed by the test strip. First, the contrast was not there. But MUCH more noticeable a nd vexing was that I had nothing CLOSE to max. black (and therefore unable to re ally evaluate the contrast ranges). Regardless of the contrast, shouldn't I at least get close to a max. black (per the test strip)? ***Again, would the size of the paper in the chemicals make that much difference?*** I understand that t he light "intensity" is different based on the filter setting, but wouldn't the test strip have reflected any necessary exposure changes for max. black? If the re is some type of paper or filter reciprocity at work here, why the differences between my "blank" neg. strips and 5x7 prints to achieve black (or even somethi ng close for that matter). I am hoping to set some BASIC exposure values for pri nting from a "normal" contrast negative as to avoid running test strips for ever y print I make (understanding of course that I will need to fine tune anyway). * **I'm not close to ready to estimate like Michael and Paula*** Any and all help would be greatly appreciated.

Eric Pederson
31-Oct-2001, 14:47
A few points which might help you narrow your problem.

1. Ignore the Ilford dichroic filter settings. Paper companies provide these numbers as an approximation because people ask for them. They are pretty useless in my experience. Get a step tablet and make your own calculations as to what filter settings give what contrast. Redo your tests as things age or if a bulb is replaced. I liked Anchell's description in "Variable Contrast Printing Manual".

2. At least with the older dichroic heads I have used, the filtration settings are far from linear. Y0,M0 may not equal Y60,M60 in contrast. Any estimates which assume that such are equal may be off. Easy to check though. I recommend making an exposure time / filter settings conversion table rather than hoping for a single exposure time for all settings. It's easy enough to change a timer setting on the enlarger control.

3. With one aged dichroic head I tested, higher contrast filter settings did not always translate to higher contrast. Commonly, contrast increased only minimally past around Y0 M100. One head wouldn't let me get past G3! If you are using Y+M to create the same exposure times at different contrast levels, then you will often use values of M (and Y) which are quite high and possibly well beyond the more linear part of the filtration effects.

4. You seem to be assuming that the black of the base+fog negative should be the same as for the darkest shadow areas of your full contrast negative. If not, then you might well get gray instead of black at the same exposure time.

Summary: I wouldn't worry about failure in reciprocity for modern paper for the normal printings speeds you are likely to use. I'm confident you are having problems with uncalibrated filtration.

Happy testing!

Doug Paramore
31-Oct-2001, 14:53
Scott: I may be reading it wrong, but I think you will find that filter grades four and five will take twice or three times more exposure than the lower grades. That is due to the density of the filter and the fact that you are using only one layer of the paper. Try more exposure to get a max black.


Richard Bennett
31-Oct-2001, 16:24
I agree that you probably need more exposure at grade 5. I did this experiment on VC_FB Oriental and found a full stop difference between each grade after #2 or three stops required when comparing #2 with #5

Pete Andrews
1-Nov-2001, 03:40
Ansel Adam's book 'The print' covers this issue. The conclusion Adam's and his co-workers drew was that there is no appreciable change in contrast with exposure time, but that reciprocity effects were noticeable when the exposure time was increased from 16 seconds to 144 seconds (surprise, surprise!).

Scott Hamming
1-Nov-2001, 06:23
Thanks all. If I may, one last question based on Erics point #4. I *am* assuming that max black with base + fog is "roughly" equal to max black on my other neg. In fact, minus some batch variations of a specific film (in this case HP5), shouldn't this hold true for any neg. of the same specific film? Along this line of thinking, if my test strip (using base+fog) reaches black in x-time with y-contrast, shouldn't any other neg. of the same type reach the same degree of black when using the same X-time and y-contrast (keeping all other variables as equal as possible AND the 2nd neg. was exposed so max black is present)? Maybe my assumption is wrong, or my question has already been answered and I'm too narrow focused to see the whole picture right now.

Eric Pederson
1-Nov-2001, 09:06
If your negative's clearest areas (areas of maximum black) have no silver left on them, then they should equal the base+fog of your test strip negative of the same film and development. However it is quite possible that even on a very contrasty negative, the areas you print to be "full black" will have some density on the negative which is more than base+fog. If the deepest shadows of the subject reflected any light at all, it might have been enough to register on the film. Sorry if that wasn't clear before.

Brian Ellis
5-Nov-2001, 13:21
I'm curious about your times. If I understand you correctly, you're saying it took 8 bursts of 8 seconds each (64 seconds total) at F 5.6 to achieve DMax with your #2 and #3 filters. I know nothing about your enlarger, or where your head was set, but this seems like a very long time to achieve DMax through a base plus fog "negative." When I make tests using a step wedge my typcial exposure times are in the 15 second range at F 8. Also, you say you had to use F 5.6 rather than F 8 because things were way too long at F 8. F 8 is a very common aperture to use with a lens that has a maximum of F 4 or F 5.6 (enlarger lenses are usually sharpest at one to two stops from wide open) and certainly should be usable. I'm wondering if there isn't some sort of problem with your light source or condensers. Finally, I've attended Michael and Paula's workshop and know their feelings about test strips. But I've also attended John Sexton's workshops. John makes a test strip with every negative he prints so there's nothing wrong with doing that. Perhaps because of John's influence, I still make a test strip for most of my new negatives despite having printed for about 12 years now. Test strips can tell you more than just a starting exposure time. They can give you an idea of burning and dodging times for different areas of the negative and sometimes can give you ideas about unconventional ways to make the print. I actually don't use "strips" (nor does John), I use whole sheets of paper. I don't think the little one inch or so wide strip tells you enough.