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tonyowen
7-May-2016, 08:31
Iím trying to get a feel for using a flash gun to expose paper negatives.
Rather than trial and error Iím initially using logic to give me a start point!!!!
Therefore, assuming:-
1] the stated GN of the flashgun is correct and repeatable
2] the ISO value adopted for the paper used for the paper negative is correct and constant.
3] the flashgun GN decreases by the square root of 2 for each halving of the ISO number. (ie GN 33 for 100 ISO becomes GN24 for iSO 50 etc Ė light falls off at root2 times the increase in distance)
Therefore assuming unity for the (any flash gun) GN at 100 ISO, then the correcting factors are:
0.707, 0.500, 0.354, 0.250 and 0.176 for ISO value of 50 down to 3.
So for a GN of 48 [meters @ 100 ISO], the corrected GN for 3 ISO is 9 which gives 1.6m @ f5.6
Is my logic correct or am I missing something that effects the practice of using a flashgun for paper negatives?????
Regards
Tony
PS Mod, move if in wrong forum

Jac@stafford.net
7-May-2016, 08:36
I’m trying to get a feel for using a flash gun to expose paper negatives.
Rather than trial and error I’m initially using logic to give me a start point!!!!
Therefore, assuming:-

2] the ISO value adopted for the paper used for the paper negative is correct and constant.

Paper does not respond cumulatively to a number of short exposures.

Randy Moe
7-May-2016, 09:47
Paper does not respond cumulatively to a number of short exposures.

Why is that Jac?

I have used multiple flash strobe to expose X-Ray using pinhole as lens in studio.

Maybe apples and oranges, but pinhole is slow...

tonyowen
7-May-2016, 11:42
Paper does not respond cumulatively to a number of short exposures.
My question did not refer to multiple exposures, I'm only considering a single exposure.
regards
Tony

Doremus Scudder
7-May-2016, 11:43
Paper does not respond cumulatively to a number of short exposures.

Sure it does. Remember Fred Picker and his cumulative 3-second bursts of light printing technique? Heck I switch my enlarger lamp on and off, cover and uncover parts of the print and sometimes flash a print hours before I make the negative exposure; everything comes out just fine. The Beseler-Minolta computer head used a rapidly pulsing light source IIRC.

Now, whether the short duration of the flash will be in the reciprocity area of the paper is a different issue, but I see no reason not to try multiple pops and see how it works. If there is reciprocity failure, the flash exposures will accumulate, just not linearly.

Best,

Doremus

Randy Moe
7-May-2016, 11:45
Let us know how it develops.

tonyowen
7-May-2016, 11:59
Heck I switch my enlarger lamp on and off, cover and uncover parts of the print and sometimes flash a print hours before I make the negative exposure;
Have I confused people - My question referred to the taking of an image using (say) multigrade paper.
I'm not referring to pre-flashing of paper before an exposure.
Simply using a flashgun as on film except I'm using paper with a low ISO rating.
regards
Tony

jnanian
7-May-2016, 12:17
hi tony

vc paper's iso is between 6ish and 24 depending on the paper
and the light being used. and it will be slower if you use a yellow (contrast) filter

have fun!
john

LabRat
7-May-2016, 23:20
Why is that Jac?

I have used multiple flash strobe to expose X-Ray using pinhole as lens in studio.

Maybe apples and oranges, but pinhole is slow...

Look up "Claydon effect" in an old photo reference... This is why multiple flash pops require more and more pops to progressively build up more exposure...

For the OP, allowing more time with continuous light might be easier than a small flashgun...

Steve K

tonyowen
8-May-2016, 01:32
For the OP, allowing more time with continuous light might be easier than a small flashgun...Steve K
Sorry people, I'll rephrase my question:
All things being equal, the GN of any flash gun vary with the ISO number of the film/paper being (singularly) exposed. Also, the (normal) series of ISOs from 400 down to 3 halves or doubles for two adjacent numbers (ie 100 to 200 ISO doubles, whereas 100 to 50 ISO halves).
Consequently are my 'correcting factors' valid - as a both as a theory and for use in practice?
Or for very low (sub 25 ISO) paper/film exposures just something else have to be considered for (flashgun) exposures of around 1/60th sec?
I'm thinking of something similar to the correcting factors used for (say) bellows extension of very long exposures >10s
regards
Tony

Doremus Scudder
8-May-2016, 03:02
Have I confused people - My question referred to the taking of an image using (say) multigrade paper.
I'm not referring to pre-flashing of paper before an exposure.
Simply using a flashgun as on film except I'm using paper with a low ISO rating.
regards
Tony

Tony,

I'm not confused, just responding to something else than your original question. Totally off-topic; my apologies :)

To answer you: Aperture scales and flash guide numbers are based on the inverse-square law. Since whole-stop apertures correspond to doubling or halving the ISO, you can therefore use the same factor to calculate flash distance for a given ISO instead of a given aperture; the factor, the square root of 2, is the same.

You can simply repeatedly divide the GN by the square root of 2 to get a distance for an ISO sequence in whole stops, or, as you have done, use the reciprocal values as multipliers. (1 / square root of 2 = 0.707 approx., etc.). As far as I can see, your calculations are correct.

The thing that may affect flash exposure for your paper is the short duration of the flash itself. I'm not sure there won't be some reciprocity failure. Most people know reciprocity failure only for long exposures. The same thing happens with very short exposures as well. Films are designed to be exposed by flash, with exposures sometimes shorter than 1/1000 of a second. I don't know about paper emulsions. You may have to add exposure if the short exposure of the flash does, indeed, elicit reciprocity failure. Even if you are setting your shutter to 1/60 second or whatever, it is the actual duration of the flash that determines length of exposure. Tests will tell the tale.

And, don't be afraid to use multiple pops if you find it helps :)

Best,

Doremus

tonyowen
8-May-2016, 03:36
Tony,As far as I can see, your calculations are correct. You may have to add exposure if the short exposure of the flash does, indeed, elicit reciprocity failure. Even if you are setting your shutter to 1/60 second or whatever, it is the actual duration of the flash that determines length of exposure. Tests will tell the tale. And, don't be afraid to use multiple pops if you find it helps :) Best, Doremus

Thanks Doremus - no need to apologise.
So, my maths are correct and I have a starting point given my assumption of paper ISO.
Therefore all things being equal - If my "natural light" paper negatives' are okay; and the "flashgun" paper negatives are okay then my assumptions become facts.
If the flashgun paper negatives are NOT okay then I need to apply some fudge factor when using a flash gun to expose a paper negative.
regards and thanks again
Tony