Multiple bursts of flash to increase exposure?

Discussion in 'Lighting' started by Pasto, May 25, 2012.

  1. Pasto

    Pasto Member

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    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.
     
  2. Newt_on_Swings

    Newt_on_Swings Member

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    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.
     
  3. keithwms

    keithwms Member

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    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.
     
  4. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    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.
     
  5. Two23

    Two23 Member

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    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
     
  6. wiltw

    wiltw Subscriber

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    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
     
  7. keithwms

    keithwms Member

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

    Ian C Member

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    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.
     
    Last edited by a moderator: May 27, 2012
  9. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    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.
     
  10. Pasto

    Pasto Member

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    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 :smile:.
     
  11. MattKing

    MattKing Subscriber

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    IIRC, with this shot I used 4 "pops" of flash to give me an extra 2 stops of depth of field. It's a near life size image on a 6x7 negative (Portra 160C):
     

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  12. Bob Darby

    Bob Darby Member

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    Multiple bursts of flash to increase exposure.

    Just some additional to keep in mind. If the room you are shooting in has lights that are difficult to switch on & off you can meter for the ambient exposure and use it a base exposure. Say f22 @ 1 minute. You can then determine the number of pops you need Say 4 pops. Set your time as 15 sec. ( 15 sec X 4 = 1 minute) Now your ambient is properly exposed as are your strobe exposures. This also works if you have window included. This concept is also useful if you have mixed light sources - say fluorescent & tungsten & strobe. Do the fluorescent turn them off, do the tungsten turn them off & do your strobe work. Alternately you can add the strobe to the individual shots with the others if you have the filtration for your strobe heads. It can get interesting !! To fine tune your exposure you can fire your strobes say 4 times overall & a couple of heads only once or twice as needed.
    Good Luck - Once you do it it will become clear & easier.
     
  13. Derek Simpson

    Derek Simpson Member

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    Thre's something here I'm not getting and could do with 3rd party clarity. I'm relearning 5x4 Sinar Wolf using Fuji Astia from the freezer: on test landscapes exposures are fine. However embarking on a series of still lifes there is a problem of underexposure. My latest attempts have gone as follows. I set up the table-top subject and light it with four flashes until I am happy. I spot meter off a grey card and note result for ballpark reference. I then take shots with 35mm digital (polarizer fitted to match that used on Sinar). I find an exposure that gets the histogram right in the middle with space at both black and white ends. Typically this will be f11. Using quickdisk I then check and get a multiplying factor of say 2:1. So I proceed using a theoretical f8 for calculations. Depth of field and coverage after setting movements can require f32 on the Sinar. I then do maths on my fingers and deduce that I shall require 16 flashes and a couple for the pot ( 32-x2-22-x2-16-x2-11-x2-8). In darkness I open the shutter and give 18 flashes. The resultant transparencies look a good full stop underexposed. I aim to scan and print initially using an old Epson 2450 so a stop under is just begging for noise. As I say I feel there's something very basic I'm missing and as our own mistakes are hardest to see can anybody spot the flaw ?