Reciprocity and multiple flash pops

[SteveInNZ]

I had a situation last night where I wanted to light paint during a long exposure but once I took reciprocity into account, it was going to be very long. So I started thinking about 'next time' and decided that multiple flash pops might be a better solution.
I would assume that within a few stops, I can just add flashes so that 2 pops gains me a stop of light, 4 pops, 2 stops, etc.
Would there be a point where I couldn't just keep adding pops and how do I account for that ?

Hypothetically, if I determined that I needed 128 flash pops to get the light I needed, I would expect that the light output from a single pop wouldn't be enough to get on the linear part of the response curve.

Steve.

 

[halfaman]

Reciprocity failure in modern films is tipycally found with exposure speeds below 1/8,000-1/10,000 second or above 1 second. There shoud be no problem with a flash at normal power, exposure will be well within that range.

 

[Andrew O'Neill]

I light painted using the strobe effect with my flash when I was making my Stars Over Manda image. I painted so that the buildings would be more noticeable. The yagura (tower) was hit from close quarters. I had to climb a ladder to get close to it... The total exposure was about five hours. I spent around a half hour painting.. I didn't bother with reciprocity. It turned out well.

 

[foc]

Do you intend to use multiple flash units in a series of flash pops or just use the one flash unit and move it each time?

 

[MattKing]

With respect to low light levels, reciprocity failure is actually related mostly to light intensity, not duration.
It is only because we tend to deal with it by lengthening the exposure time that we tend to identify it as being related to exposure time.
This means that if your individual flash pops are enough to give you a reasonable portion of the necessary exposure, using several pops should do the trick.
Eight pops (three stops) should be fine with most films. If you are using colour, that may lead to colour shifts.
Sixteen pops (four stops) might be worth trying.
Thirty two pops (five stops) would probably be a stretch.
The most I've ever tried is eight pops.
These were done with four pops (IIRC)

 

[SteveInNZ]

Matt,
The intensity aspect is what got me thinking about it. If I need to use many pops to build up the image, then the number of photons arriving at the film for each individual flash would be low.
My situation was similar to Andrew's example except that I couldn't get closer.
I haven't seen it mentioned anywhere so I wasn't sure if there was a standard correction method. You don't know if you don't ask.

Steve.

 

[alanrockwood]

With respect to low light levels, reciprocity failure is actually related mostly to light intensity, not duration.
It is only because we tend to deal with it by lengthening the exposure time that we tend to identify it as being related to exposure time.
This means that if your individual flash pops are enough to give you a reasonable portion of the necessary exposure, using several pops should do the trick.
Eight pops (three stops) should be fine with most films. If you are using colour, that may lead to colour shifts.
Sixteen pops (four stops) might be worth trying.
Thirty two pops (five stops) would probably be a stretch.
The most I've ever tried is eight pops.
These were done with four pops (IIRC)
View attachment 290647

Thanks for posting this Matt. I was going to explain that reciprocity effects are a result of light intensity and that shutter speed (or flash duration) enters in via the back door so to speak. You explained it, so I don't have to, and you went beyond this by adding some good practical advice.

 

[MattKing]

Thanks for posting this Matt. I was going to explain that reciprocity effects are a result of light intensity and that shutter speed (or flash duration) enters in via the back door so to speak. You explained it, so I don't have to, and you went beyond this by adding some good practical advice.

You are welcome Alan.
In the interest of completeness, it should be mentioned that there is another form of reciprocity failure - when the light levels are quite high, and the exposure duration is extremely short. One can sometimes run into this with high power flashes that allow the power level to be decreased by reducing the flash duration to very, very short flashes.

 

[alanrockwood]

You are welcome Alan.
In the interest of completeness, it should be mentioned that there is another form of reciprocity failure - when the light levels are quite high, and the exposure duration is extremely short. One can sometimes run into this with high power flashes that allow the power level to be decreased by reducing the flash duration to very, very short flashes.

Yes.

 

[Doremus Scudder]

You won't get additive exposure if you're moving the flash to a different locations and illuminating a different area for each pop. This is what "painting with light" means to me. You need to overlap, of course, and take that into account, but you may want to rethink your calculations.

Doremus

 

[SteveInNZ]

Doremus,
The "calculation" is where I am trying to get to. Is there one ?
As Matt says "Thirty two pops (five stops) would probably be a stretch".
If we were to take an extreme example (for the sake of discussion) and pop the same flash 128 times with full recycle time between flashes and from the same location at f/32. I wouldn't expect to get the same result as a single pop at f/2.8.
So the $64000 question is how many pops would I need to use to get the same result ? How do I determine that ? Is there some way to use the single exposure reciprocity tables/graph/tools and correlate that to the multiple flash situation.

If the answer to the first question (Is there one ?) is no, then so be it.

Steve.

 

[Doremus Scudder]

Well, if you are moving your flash around and overlapping by half the area every time, then everything is getting the equivalent of two flash pops. Base your exposure on that, do a test or two to refine your technique and exposure and you should be good to go. Guide numbers are your friends.

Doremus