Guide Numbers and Alien Bees

Discussion in 'Beyond the Basics' started by Johnboy2978, Sep 4, 2006.

  1. Johnboy2978

    Johnboy2978 No longer a newbie, moving up!

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    Hello all, I am quite confused about guide numbers and trying to get a properly exposed image. What I have been doing up until now is basically trial and error, b/c I don't have a light meter. I would like to do more w/ my film cameras and my strobes, but I don't want to waste film, thus I have been trying to figure out guide numbers.

    I am working with an Alien Bee 800 and just got a Sigma EF500 DG Super. Primarily my question has to do w/ the AB strobe. As best I can tell from the website, they report a GN of 172 using the 7" reflector at 10' for ISO 100. My digital camera only goes down to 200 and I use that exclusively right now. I read that doubling the film speed increases the GN by a factor of 1.4, thus, at full power the strobe would have a GN of 240 at 10' for ISO 200. Now I also read that using an umbrella basically halves the GN (that correct?) which would leave me with a GN of 120 at 10' (ISO 200). So if I want to shoot at f/8, I would need to move the strobe back 15' from subject. And if I wanted to leave it at f/8 and have the strobe 10', would reduce the power 1/3 from full power. Is that correct? 10.9' for f/11, and 7.5' for f/16. Are these calculations correct?
     
  2. midget patrol

    midget patrol TPF Noob!

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    Couldn't you just adjust your shutter speed?

    edit: oh, sync speed. right.
     
  3. Johnboy2978

    Johnboy2978 No longer a newbie, moving up!

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    Anyone able to check that math? I tried some test shots, got a string and marked it at 5' increments, and it appears that it's pretty much spot on. It produces a nice histogram w/o any blown highlights, so if I'm off it must not be too much. Still appreciate any comments though.
     
  4. Digital Matt

    Digital Matt alter ego: Analog Matt

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    Personally, I would buy a flash meter, but since you are shooting digital, just check it yourself. Take some test shots, and verify your settings by looking at the histogram. I'd photograph a person, and measure your distance to their nose, and then open the image in photoshop. Use the eliptical marquee tool to make an oval over just the face and check the histogram.

    I appreciate wanting to understand the math, but you are not going to want to do that during a shoot. It's terrible for workflow and creativity.
     
  5. mysteryscribe

    mysteryscribe TPF Noob!

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    The history on guide numbers will help you understand. There once were no auto strobes and no strobe light meters. In those days the factory experimented and found that the light was consistant. At ten feet if you used and f/11 and it was perfect. then that relationship was constant. the guide was 110 at any distance.

    Also in those days there were numbers on the lens of the camera giving the distance in feet (usa) so all you had to do was to focus the camera look down and divide the number of feet there into the guide number and wah la a perfectly exposed photo.

    Actually it began with flash bulbs and the guide number also varied with the shutter speed. But that is of no concern to you. Most auto focus cameras have done away with the distance scales on the lens so guide numbers are harder to use now.

    I suppose you could make marks on your floor for five foot increments. A flash meter for film is a good idea. I used a very inexpensive wein for years. Wasn't fancy but there is nothing fancy about strobe light just extremely dependable.
     
  6. JamesD

    JamesD Between darkrooms

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    Everything in photography is calibrated in stops (well, maybe not umbrellas, I dunno).

    Guide numbers are for figuring aperture at ISO 100. Divide the GN by the distance, and that's the aperture you need--if you're using ISO 100.

    If you're using ISO 200, that's one-stop gain, so close down a stop (ie f/8 to f/11). ISO 400? close down two stops (f/8 to f/16).

    Again, I dunno about the umbrellas. If you don't have a flash meter, and don't want to buy one, shoot a test roll at different exposures, first direct from the light, then with the flash.

    Oh, and when calculating the aperture, measure the distance from the flash to the subject, not the camera to the subject.
     
  7. mysteryscribe

    mysteryscribe TPF Noob!

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    for umbrella measure from the back of the umbrella and add one stop it is a good place to start.
     

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