Multiple bursts of flash to increase exposure?

[Pasto]

I'm just about to start photographing interiors with flash and large format. I expect to use my 4x5 most often, but do intend to try some 8x10 as well. I'm begining to experiment with my lights to see what kind of aperture I could expect when I'm on location. With my 2400 w/s speedotron system I can get f22. This is not quite enough for sufficent depth of field on 8x10. Can I increase the effective exposure by using multiple bursts of the flash, thereby allowing smaller apertures? The interior spaces I'm shooting have no windows so it would not be a problem to turn off the lights and keep the shutter open. If yes, how?

Thanks for your advice.

[Newt_on_Swings]

I have done this only on 35mm. Multiple flashes plus long exposures to fill in a room or to light an object with very slow films. The best performance I have had was with an OM4t that did off the film metering, it worked even with films such as kodalith which I shot at ei~3-6.

If you are not shooting people I think a sufficient long exposure under normal room lighting will look the most natural. The darker areas you could possibly paint in with a hand held strobe or diffuse flash light beam.

[keithwms]

Sure Luigi, this is done routinely by photographers specializing in interior architectural shots. The world expert on this is one Kirk Gittings; I remember an interesting discussion with him on this topic on LFPinfo. He may be on here from time to time.

The main technicality is that the exposures don't quite add intuitively, in a simple linear way; reciprocity failure is the culprit. But you can reduce the effect and work with it.

[David A. Goldfarb]

My Minolta Flashmeter III and presumably other models will even measure multiple bursts. As a rule of thumb, add one pop for every four. You can darken the room and leave the shutter open or use the shutter to minimize ambient. The latter is easiest with a self-cocking press shutter.

[Two23]

Yes. I'm assuming you are using a lens with a shutter, and not a Petzval? Calculate how much light you need. Say all you can get with one pop is f22, but you want f45. Simply cock the shutter, pop the shot, cock the shutter & wait for the strobe to recycle, pop again (f32,) repeat pop again, and again (f45.) Remember that you must double the amount of light to gain a stop. Thus, going from f11 to f22 requires 2 pops for f16 and x2 again for f22--total of four pops, not three.


Kent in SD

[wiltw]

Assuming the flash output overlaps the same area each time, you have to DOUBLE the count of flashes for each EV increase.
So if you can get f/22 at one pop, it takes two pops for f/32, four pops for f/44, eight pops for f/64

[keithwms]

Just bear in mind that the pops don't add arithmetically.

[Ian C]

Right. The number of pops must be a positive integer power of 2 for 1-stop increases.

2^1 = 2 pops for +1 stop

2^2 = 4 pops for +2 stops

2^3 = 8 pops for +3 stops…and so forth.

In general, we need 2^n pops for +n stops of exposure increase using multi-pop flash exposures of equal strength.

[David A. Goldfarb]

In theory, yes you can double the number of pops for every stop. In practice you might want to add one extra pop for every four calculated pops. I think this is due to what is called the "intermittence effect," if you want to research the cause.

[Pasto]

Thanks to all for the feedback. I feel a lot more confident about this now. One more technical detail out of the way before embarking on the new project. I've looked up the "intermittence effect" and still trying to figure this out. As the ambient light will be off for all the exposures there is no reciprocity issue with multiple flash pops. This intermittence effect may be something to consider however. I'll keep researching. Just so that you know the context of my next project, it will be to photograph diagnostic imaging rooms (i.e., x-ray, MRI, CAT, etc.) in a hospital. Had to jump through lots of hoops to get access. One more administrative hurdle to jump this week and I'm there .