I was wondering, and I maybe way off base here but maybe it will work. In doing platnuim prints, it takes a fairly long exposure under the sun or tungston lamp. What would happen if I used strobes? A couple quick pops checking after each pop rather than waiting around in the sun. I'm sure there is a really obvious reason this wouldn't work but I can't figure out what it would be. thanks doug

-- doug (dmcgoldrick@usa.net), February 24, 2000

Answers

An interesting suggestion. I'd be curious to hear the results of any tests (though I would suggest you make them with cheap material, such as cyanotypes, rather than with platinum).

But let me make a thought experiment: you might need quite a couple of flashes, for the intensity of the flash at a distance of a few metres being the same order of magnitude as that of sunlight (and the spectra being similar) you will have to expose for a time at least the same order of magnitude as that the exposure time of a regular platinum print. (see below for the "at least".) The flash reaches this high (for an artificial light source of that size) intensity only during a couple of milliseconds. This means that the print receives the dose of light equivalent to 1 second in the sun after some 100 to 1000 flashes. (Assuming 500 flashes, a readyness time of 6 s for the flash and 1 s for you to react, you could do this in about one hour if you would not wait for the effect of each flash to become obvious.)

You might increase the effective intensity of the flash by choosing as short a distance as possible. This would make for a factor of maybe 10 in intensity. Still about 50 flashes!

There is more to be considered: Reciprocity errors occur at long and short exposures. Regular film being optimised for exposures lasting a few milliseconds such errors usually occur at times considerably shorter than 1 ms. I don't know if the effect exists with platinum prints, but if it does at all, it is safe to expect that the short time a flash takes is way beyond the reciprocity range. This makes things worse.

The next unfavourable effect is intermittence. Up to a certain limit, you can split up single exposures into a couple of additive partial exposures. I have read that you may, e.g., expect eight exposures of 1/8 s to give about the same effect as one of 1 s corrected for reciprocity error. That means that you need not bother to correct the 1-s-exposure, you just split it into eight partial exposures in a time range where this error does not occur yet. Still, I have also read that when using multiple flashes with a weak flash unit to equal one powerful unit for the illumination of remote objects during night shots you need to make an allowance to account for the intermittence effect. The book I read stated that for every six flashes needed, you ought to give one extra flash. This, too, could make things worse.

-- Thomas Wollstein (thomas_wollstein@web.de), February 29, 2000.

I just read this question and the answer and I had to laugh. The reason it would not work is because you are not looking for light intensity but UV radiation. Most flashes come with coatings to reduce UV radiation. The reason you use the sun is not because the light is intense, but because it is a source of UV radiation. You can purchase a UV printing source and make your prints in 5 to 6 minutes!

-- Jorge Gasteazoro (jorgegm@worldnet.att.net), August 22, 2001.