Flash as Sensitometer
On another thread I am discussing aspects of my film testing experience. As I am about to run the tests again, I would like to ask for any advice regarding the use a small electronic flash unit as a precise source of exposure, to contact print a 31-step Stouffer tablet onto the film I am testing. I would like to explore this option in more detail before I fall-back to using my enlarger, which I would prefer to avoid, due to its longish lamp warm up and start-up times, and the blue/green colour of light (Ilford 500H). At my disposal I have a few units that let me adjust their flash power. I am thinking of using a Nikon SB-800 as it has always performed well, and I have a reliable, and calibrated flash meter, Sekonic L-508, which I have used for the last 12 years. I suppose a studio flash head would be less precise, but I could use it, too.
Please share your experiences of using a flash for this purpose, any gotchas, suggestions and so on. Ideally, I would like to find a fixed position for the flash unit, and a set of relatively fixed parameters, so I could use this set-up for ongoing film testing.
I am also curious about the math of calculating the exposure for this application, though I realise I can arrive at that through additional testing.
PS. It is hard to find second-hand EG&G sensitometers in Europe.
The most difficult thing is making some kind of box that will give even illumination. You can flash the film from across the room but that might not always be repeatable, but it certainly is a good way to get an even exposure across the stepwedge.
These little "contact printers" usually go for little money on ebay. You could mount the flash inside one of those.
Yes I agree with flash it from across the room for evenness. I would use a flash meter and check to see if the illumination is repeatable. For calculation of the exposure I would also use the flash meter to measure lux-sec.
Set flashmeter to ISO 100 and retract the dome or use flat disc.
Time 1/30th (irrelevant but you have to set something).
Try to get the flashmeter to read f/16.0... That will give you same as an EG&G.
Now you want to reduce the intensity because that's too much for testing ASA 400 film.
I have a 1.84 filter, and I also add .60 on top of that.
So reduce the intensity of the flash or add filters to add up to density 2.44 (give or take 0.30).
Is that about 8 stops? If so, you could try for f/1.0 at ISO 100
If you have to use a filter, a physical filter is preferred over a neutral density filter. For example sheet metal with holes drilled or slots cut in it, placed over the front of the flash.
If you process some film like Ilford Delta 100 to ISO conditions then the exposure required to get a 0.1 density on the film is 8 millilux seconds (by definition). If you take the log of this (0.9) then you can simply add in the density of your step wedge at that point (like 2.8 log d or whatever that step is) and that gives you the output of your sensitometer light. In the example it would be 2.8 plus 0.9 = 3.7. If you take the antilog of that you are back to millilux seconds which would be 5011 millilux seconds.
Originally Posted by Rafal Lukawiecki
My plan, so far, was to have a fixed mount for the flash unit on the wall in darkroom, next to the enlarger, about 1-1.5 m above a simple glass/foam contact printing frame. I would hope that the illumination would be quite even this way, not needing a box at all.
That little printer looks very cute, though.
Thanks, Bill. What is the relationship between a reading on the flash meter of f/16 at ISO 100 and the flash output of X lux seconds, if I may ask? I am curious what I am aiming for at the exposure plane.
Originally Posted by Bill Burk
As for reducing the intensity, couldn't I just dial the flash down until I read a suitably smaller f stop number, instead of using a filter?
IC, thanks for explaining how to calibrate this set-up with a known film. Are ISO conditions for development simply what the manufacturer states or something more exotic?
It would be helpful if you could state your purpose including your film application's exposure time, spectral characteristics, illuminate levels etc.
Even without this information I would suggest following approach KISS: keep it simple.
The most accurate and reliable mechanisms can be very simple: ½ to 1/100th of a second use guillotine drop shutters. Gravity is quite constant in one location. For 5 seconds and greater use synchronous motors in timers i.e enlarger timers. They can be very repeatable. Accuracy is not as important as repeatibility if you are only using one time duration.
It seems to me that a common electronic flash is far too variable. EG&Gs are good but they are specifically designed for the purpose. Typical flash units are designed to perform to very different criteria.
Your Ilford light source has its own problems.
A simple solution assuming you are using materials that do not have RLF in the 5 second range: use a low wattage light bulb controlled by a electro-mechanical timer that is repeatable in the 5 second range. Many enlarging timers can satisfy this need. Put the lamp in a housing like a safelight. Add neutral "filter material" if the light is too bright. Bond paper provides about a stop of density per sheet or increase the distance (start at 10x the diagonal of your exposure plane) between the light and film.
To check out the performance expose ten pieces of film and process them identically. Processing at the same time would be best.
see my website: www.makingKodakFilm.com
Unfortunately, my flash meter, L-508, only shows f stop value, there is no readout in lux-sec.
Originally Posted by Chan Tran
Thanks for the guillotine suggestions, Bob. I think it would be a bit out of my mechanical skills comfort zone at the moment. It might be easier to try and locate a used EG&G in the future, but I'd love to have a solution sooner.
Originally Posted by laser
The purpose of the device is for re-testing 320TXP to establish my N, N-1, N+1 development times, at this stage, and perhaps to evaluate speed too. I describe my issues, including my flawed attempt at deriving CIs, on this thread. Longer-term, I would like to use this set-up to continue testing other materials, but all would be fairly standard B&W film.