Even a $2000 sensitometer from ebay won't be adequate for ISO film speed testing. For example the ISO 9001 certified Agfa Avisense cost $24,000 US dollars. (http://www.agfa.com/docs/sp/aerial/a...4-06-15_en.pdf)
Likely you just need to do relative film speed tests and make control strips to manage development times. In that case, a $20 "contact printer" hooked to an enlarger timer should work fine with a little modification. These show up on ebay all the time. I'd run it at one second. That is the exposure time the Wejex sensitometer uses.
Last edited by ic-racer; 09-18-2012 at 06:32 PM. Click to view previous post history.
There's always tradeoffs...
Originally Posted by Rafal Lukawiecki
Flash is closer to daylight in terms of spectral distribution, though it is "discontinuous" meaning it has spikes of certain wavelengths instead of a smooth curve. That brochure from IC-Racer shows the zig-zag curve and describes how they dealt with that issue.
At the low settings, your flash duration might be very short, less than 1/1000th second - so your results from too short a flash might be affected by Reciprocity Failure. (Recommend brighter flash and find a way to block some light).
Tungsten doesn't have as much blue light as flash - an 80B blue filter can compensate.
For tungsten, I think you can easily rig a reliable shutter for repeatable times (The best time is between 1 second and 1/1000 second to avoid Reciprocity Failure). Think about jerry-rigging a camera body to the enlarger lens mount.
As IC-Racer says - all you really need speed testing for is to see how one film compares to another. Consistent exposure will give you a relative speed test - where the results give you an EI to believe. When you get a value that is within 2/3 of a stop of what you expected... That is good enough. We can write pages of threads trying to find that last 2/3 stop - You would have to follow every ISO/ASA specified procedure to get absolutely certain film speed results (including taping film to a plate of glass inside a Thermos bottle).
My favorite short-cut is to develop to the "ASA Triangle" and then mark that down as if it corresponds to box speed. My shortcut is within 2/3 stop of reality.
I'm trying to calculate the needed exposure to place step 28 of my 31-step Souffer, 2.6D, just above fb+f. Could someone confirm if I got my numbers right? I gather that:
Opacity of a density of 2.6 is 10-2.6 = 0.002511
Exposure necessary at film plane is 0.8/ISO so 0.8/320 = 0.0025 lux s
Exposure metered above just above the Stouffer needs to be 0.0025/0.002511 = 1.13 lux s
Looking at the earlier given formula to convert a light meter f/stop reading to lux s:
Number-of-stops-to-right-of-f/1.0 = log2 (exp/2.5)
so, log2 1.13/2.5 = -1.15
which would suggest my meter ought to indicate an f/stop of 0.7, a surprisingly low amount of flash exposure, which it may not be able to read, I would think. I suppose I could just keep reducing down to f/1.0 and then fiddle with a filter of some sort to reduce the flash output even further and perform a test with film.
I plan to have a go at it today, so please shout if you feel I got it wrong. Thank you.
Without doing calculations, since it's Saturday morning and I have to head out...
I added the 2 stop ND filter (it's the filter that gets my exposures to be 0.1 near the last few steps) over my flashmeter and metered the EG&G with meter set at ISO 400.
It recommended f/2.0 at ISO 400, you might have set the meter at ISO 100 so that you can get EV readings. If so... That accounts for the two stops difference in the plan.
So this is the light "without" the Stouffer scale... Just at the glass of your contact frame...
If you choose to aim for f/0.7 instead of f/1.0 that isn't significant... It validates your calculations.
Once you get a strip developed, you can always "count steps" to adjust exposure up or down. On your scale 0.1 density is 1/3 stop, so for each step you count from the step you wanted to be on... is just 1/3 stop more or less depending whether you want to go up or down the scale.
Last edited by Bill Burk; 09-22-2012 at 10:43 AM. Click to view previous post history.
Thanks, Bill. Here is my Irish "sensitometer", foot pedal operated. Balancing the higher output of the flash, for repeatability, with an ND cut from Lee lighting ND gels, I am sure to get some interesting spectral distribution, but it still beats my enlarger head (500H) for this purpose.
I measured some 400 pops over the film area, placing the meter in different positions, and it is repeatable within 0.1 f/stop at the surface. The central area might be 0.05f/stop higher than the outskirts. Ideally, the light source would have been further away, to eliminate this fall-off, but this will have to do for this test. I'll do some pre-tests, to check the exposure, and then I'll expose the test sheets. I'll report when I have them analyzed, and, hopefully, plotted out.
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Just a quick update on my progress. The calculation of exposure as about 1.1 lux s, and therefore the assumption to get flash output to f/1.0 at ISO 100 and then to reduce it by a stop or so by reducing the power level has proven quite accurate. I have run the pre-tests, for both 320TXP and HP5+ and the first 2 steps of the 31-step Stouffer are fb+f on 320TXP, while the HP5+ shows 5 steps of fb+f (bit odd to seem slower, reciprocity?). I have now exposed 15 sheets of each film, having slightly adjusted the exposure by -1/3f for 320TXP and by +1/3 for HP5+. Unfortunately, I have also experienced a spouse-related lights-came-on-in-the-darkroom issue (also known as a quite personal, don't-forget-to-tape-that-switch-to-the-wall issue), so I need to do this thing again... I am waiting for a bit more film, hopefully arriving later this week. Who said that film testing was bad for film manufacturers?
While I wait for the film, I am pondering if, and how, to improve my "sensitometer". I am not too skilled in manual construction, but I could consider making some sort of a reflective box or a diffuser for the flash head. I am also going to post a WTB for an EG&G.
IC, do you know if those Airequipt Junior contact printers have anything fancy in terms of providing an even illumination in their enclosure, it is it just a box for a light bulb?
Originally Posted by ic-racer
Are you sure this last step is correct? As far as I understood, the value 'exp' in your equation should be an illuminance value [lux] instead of an exposure [lux s].
Originally Posted by Rafal Lukawiecki
As far as I understand, exposure needs time. Can you point out the 'exp' you mean? I didn't see it in Rafal's formula.
Rafal used the following formula to calculate the EV value he expects to read on his flash meter: Number-of-stops-to-right-of-f/1.0 = log2 (exp/2.5)
The formula actually requires an illuminance (without time, unit [lux]) value where he puts an exposure value (with time, unit [lux sec]).
Please compare http://en.wikipedia.org/wiki/Exposur...ing_conditions
Last edited by freemp; 05-27-2013 at 01:52 PM. Click to view previous post history.