Clarification needed (unZone contact sheet stuff)

[pdeeh]

This query falls awkwardly between "Enlarging" and "General Exposure", but as the main aspect specifically concerns darkroom exposure, I've put it here.

I use mostly roll film and while I hold the Zone System "in mind" when taking photographs (in the sense of being mindful of where I'm expecting my tones to "fall"), I'm not a "scientific" photographer, and any one roll is likely to have a wide variety of lighting and contrast scenes on it.

But I'm trying to improve the consistency and printability of my negatives, and I've read Barry Thornton's "unZone" article a few times, and find it largely clear and potentially very helpful (Dead Link Removed).

On p.4, he says the following:

... using this minimum time for maximum black exposure, make your full contact sheet processed in the standard way. Be careful how you give the exposure. If, say, you found the time on the test strip to be 5 two second steps, do not give a ten second contact sheet exposure. This will give significantly greater exposure. Give the same 5 two second bursts of light.

Now I think I understand why he might say this (is it to do with the way the paper will respond as if being "flashed"?) - but I am likely to be wrong in this, and if so would like to understand better what's going on here.

So, in sum - why might a single 10-second exposure give greater exposure than 5 2-second exposures?

 

[polyglot]

The bulb takes a good fraction of a second to heat up and cool down. While doing so, the light is dimmer and redder, therefore exposing the paper less-effectively. So what matters is not just the total exposure time, but how many times the bulb was switched on and off to make the exposure.

If you have a mechanical shutter on your enlarger or it has a solid-state (LED) light source, then this effect should not occur because there is no warmup/cooldown effect, the light just goes on/off.

 

[pdeeh]

Ah-ha.

Thank you polyglot.

So, if I am producing test strips under any circumstances (not just Thornton's methodology), if I am switching the light-source on and off for each exposure-step, then I must do the same when making a full print; or, if I leave the light-source on but use a mask to cover (or uncover) steps then I must expose a full print for the total time of my chosen step. Correct?

The corollary being that mixing the two methods will lead to discrepancies.

 

[Bill Burk]

Hi pdeeh,

Yes, I always enjoy these a-ha moments when there is a difference between an author's methods and your own, and you discover what specific parts of their method need to be modified to adapt to your own way of working. And you can do this without faulting the author, you just don't take that part of their advice "because it doesn't apply".

I like polyglot's explanation of the idea that 5 two second exposures, given "Barry Thornton's way" might not equal a single 10 second exposure. (There's intermittency effect too, but light bulb warmup is more significant in this case).

So, assuming you pass a sheet of cardboard over the test strip exposure, your method really gives a single 10 second exposure when you proof (at 10 seconds) so your print exposure will be a single 10 second exposure.

I'll assume you don't have a fluctuating-light-output (fluorescent bulb) type enlarger, these make the synchronizing of test exposures and print exposures less certain... That's what I have, in my case I need to be mindful of whether my 10 second exposures on my test strips got the first or the last 10 seconds of a 40 second exposure...

 

[pdeeh]

Thanks also Bill.

Yes, especially an "ah-ha" for me as the last couple of times I've printed I've thoughtlessly (and completely unrelated to Thornton's article) changed from doing test strips using a mask to turning the enlarger on and off (it seemed to make counting against the metronome easier) ... but then have continued to timing the full print just by counting the full desired exposure; and was starting to feel puzzled as to why what was looking perfectly good on the test strip was looking quite wrong on the full print. It's a Kaiser colour head enlarger with a halogen lamp, by the by.

 

[polyglot]

Also when you're doing test-strips, you often want to see the effect on one small critical part of the print, e.g. a face or a particular shadow so a strip which distributes the measurement steps across the image is often not appropriate. What I prefer to do is make a bunch of test-tiles and expose each of them individually at the test times. The time taken to do this is obviously longer than incrementally exposing a strip but IMHO it gives you much more-useful and more-consistent results.

 

[Nicholas Lindan]

It is possible to compensate out the effect of lamp turn-on delay:

http://www.darkroomautomation.com/support/AppNotePH212LampDelay.pdf

But even without compensation, using a PH212 bulb, 20 1-second exposures are within 5% of 1 20-second exposure. The turn-on delay is in the order of 50 milliseconds - 1/20th of a second.

Although 'intermittency effects' are bandied about as the cause of errors, the real problem is in trying to use analog timers and metronomes as precision exposure controllers. Pseudo-digital timers - those with knobs to set the time, like the ZVI and Beseler Audible Timer - are little better.

The less said about cold-lights and this issue the better.

 

[polyglot]

Absolutely you can calibrate it out with a feedback system. However it you're not, my enlarger (EJC bulb in a DeVere) is nowhere near within 5%; it takes a good 300ms to warm up and nearly a second to cool down. Some of that will cancel, but there is a distinct difference in both density and grade. I'm using a microcontroller-based f/stop timer so I know that that's consistent within 10ms (half cycle AC); the problem is the thermal lag in the bulb.

 

[Nicholas Lindan]

Absolutely you can calibrate it out with a feedback system.

You don't need feedback. The warm-up/cool-down behavior of an incandescent bulb is repeatable, you just need to find the correction time. Then have the timer add/subtract it from the exposure time. It only takes a few minutes to run the test described in the application note. Of course, there aren't many timers that have a correction feature. I'm sure you can add the feature to yours.

The issue of timer repeatability has nothing to do with the warm/cool issue but with timer problems being ascribed to paper intermittency effects.

If the timer is repeatable, and the warm/cool time is corrected then suddenly all the 'intermittency' effects vanish.