how long does it take for the shutter to open / close?

is the time the shutter is set to inclusive or exclusive of the opening / closing?

of course it would never, NEVER have an appreciable effect on the exposure of the film, but it's fun to think about none-the-less.

like dave, i am also curious why the recipricatory factor would differ. wouldn't the samples in aaron's example both heve the same reciprocity factor, as they are both being exposed to the same amount of light?

scott

I believe you would need 8 bursts at f32.

I believe it is 16, I think you forgot f11 David.

If you first calculate your exposure time and account for the reciprocuty departure of the film you are using, and this results in an exposure time of 1 minute, then yes, 4 - 15 second exposures will give you an equivalent exposure. I find the most common mistake with this type of exposure among students is the failure to calculate the reciprocity factor.

I question why anyone whould need to do this instead of a 1 minute exposure. if making a succession of exposures of 15 seconds it is too easy to vary by a second or so on each exposure. One exposure of a full minute would leave less room for error.

As regards multiple flashes, a lot is dependent on the flash unit. Some indicate a full charge when they are in fact only 75-90% charged. I usually suggest that students charge the unit twice as long as it takes for the indicator light to come on. This takes some of the error factor out of the picture.

Only if they are on the same film.

Actually, along with all the other "effects", there is an intermittency effect. Quoting from Todd & Zakia's Photographic Sensitometry, "In general, if a continuous exposure is divided into portions, the photographic response differs from that to be expected from the continuous one, even if the total energy incident upon the emulsion is the same." They then go on to say it's related to reciprocity. My experience is that the effect is serious for printing paper if you're trying to make precision step tablets or something, but less so with film.

Conrad has it. Intermittency. 16 strobe pops will not do it, we have done lots of work this way in the studio over the years, the number of pops you will need ramps up. Something like: 1, 2, 5, 11, 25, this is where you are really happy to have Polaroid film to use. I try very hard not to have to do that many flashes for one exposure if I can avoid it.

Thanks for sharing all your experiences. Appreciate it!

DOH!

"I believe it is 16, I think you forgot f11 David. "

I hate it when that happens. Thanks for the reminder Mark.

at speeds that allow the shutter to be all the way open for more than an instant, the film greater exposure in the center vs. the edges with multiple short exposures ... but this is likely to be an infinitessimal difference.

Enlargers are a different story. The amount warmup time of any bulb is significant. One reason I like using a compensating metronome for printing ... it eliminates any of thess kinds of inconsistencies.

I don't know if reciprocity effects are relevent in this case ... does the film care that the exposure is coming in multiple bursts? I assumed that exposure was cumulative, and that this wouldn't make a difference.

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