multiple strobe pops

[Thom Bennett]

So, I'm photographing a painting and, apparently, I figured my exposure, via multiple strobe pops, incorrectly. The method I used was that after the initial pop (f8 through polarizing gels and compensating for the polarizing filter on the lens) I hit the strobes two more times for each additional f-stop i.e. two more hits would give me f11, two more f16, etc. until I got to f32. I even "overexposed" by additional pops but my chromes were under by at least a stop. Any advice on accurately figuring stuff like this? Now I have the experience of seeing the chrome and going from there but I know there must be a better way.

[Jim Michael]

If 1 pop is normal, then 2 pops for 1 stop, 4 pops for 2 stops, ... 2^n pops for n stops.

[BrianShaw]

Counting pops only works (as far as I know) if the strobe is putting out the same output and at the same distance every time. So if it is a TTL system on SLR, etc. the strobe must be used in "manual" rather than TTL and must be fully or equally 'recharged' before every pop.

My preferred way is to verify Jim's general technique with a flash meter that will integrate multiple pops, like Sekonic L-558.

[IanG]

It's not a straight "2^n pops for n stops", you need to add additional flashes with multiples exposures.

So while 1 stop is 2 flashes, 2 stops is more like 5 flashes, 3 stops 11 or 12 flashes and so on. The same is true with any method of multiple exposures.

Ian

[BrianShaw]

My preferred way is to verify Ian's general technique with a flash meter that will integrate multiple pops, like Sekonic L-558.

[Jim Michael]

Ian, why do you say that? I don't think we are necessarily in the realm of reciprocity failure. How is this different from a constant source of the same quantity of light, e.g. 30 ms of 100 units vs. 2 x 15 ms of 100 units vs. 10 x 3 ms of 100 units?

Re Brian's comment on the light output, I would assume the flash is fully recharged between pops.

[Thom Bennett]

Thanks for the replies thus far. I'm using a Dynalite 1000K pack w/two heads in a darkened room and I am keeping the shutter open and, with the modeling lights off, popping the strobes a few seconds after the pack recycles. It seems as though my methodology was correct but, as IanG states, real world results demand a more "adjusted" approach. For my 4 stops I should be popping ~15 times rather than the 12 I was using as a base. From the look of the chrome I got back that would be about right. Probably will go to 18 pops as well.

[IanG]

Ian, why do you say that? I don't think we are necessarily in the realm of reciprocity failure. How is this different from a constant source of the same quantity of light, e.g. 30 ms of 100 units vs. 2 x 15 ms of 100 units vs. 10 x 3 ms of 100 units?

It's a well known phenomena and borne out by many peoples experiences, including my own. There's been long threads about this on APUG.

It can be seen easily when printing, a test step of 2 second steps up to say 32 seconds (so the 2 sec is 16x individual 2 secs exposures) doesn't give the same density as one continuous exposure of 32 secs. It's to do with threshold energies.

Ian

[Vlad Soare]

Ian, why do you say that? I don't think we are necessarily in the realm of reciprocity failure. How is this different from a constant source of the same quantity of light, e.g. 30 ms of 100 units vs. 2 x 15 ms of 100 units vs. 10 x 3 ms of 100 units?

This is known as intermittency effect.
While in practice we tend not to regard it as reciprocity failure per se, technically speaking it is a form of short exposure reciprocity failure.

Because it's most often experienced in the darkroom, some people blame this on the amount of time the enlarger's light bulb takes to reach its full output. This is wrong. It's actually caused by the way the emulsion reacts to light, and not by the characteristics of the light itself (though I guess the light bulb may contribute to it, too).

[David R Munson]

Testing is necessary to determine how many extra pops will be needed for correct exposure with a given film and exposure range. The shot below was done on Provia. Eight pops on the strobes to get the exposure called for by my exposure meter. No extra pops provided for intermittency effect, and the exposure of the transparency is spot-on. Others experience will differ from mine, of course.


Soy by David R Munson, on Flickr