Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 69,958   Posts: 1,522,986   Online: 1161
      
Results 1 to 7 of 7
  1. #1
    olleorama's Avatar
    Join Date
    May 2009
    Shooter
    Multi Format
    Posts
    526
    Images
    5

    Quick! Manual GN question

    Two flashes on manual, I know the guide number for both of them, the will both light the same area (more or less, they are both set on wide and will fill more than the angle of view the lens gives), can I just add the guide numbers to get the correct exposure?

    Ex, distance 10 meters, flash 1=GN 45, flash 2= GN 64, hence I would get an f/stop of roughly (109/10)~=f./11 ?

  2. #2

    Join Date
    May 2006
    Location
    Aurora, IL
    Shooter
    35mm
    Posts
    1,939
    no the guide number is

    sqrt( 45^2 + 64^2)= 78 so it's f/7.8 or about f/8

  3. #3
    olleorama's Avatar
    Join Date
    May 2009
    Shooter
    Multi Format
    Posts
    526
    Images
    5
    okay, thanks!

    You don't have to, but if you wouldn't mind, can you explain why the relationship is like that?

  4. #4

    Join Date
    May 2006
    Location
    Aurora, IL
    Shooter
    35mm
    Posts
    1,939
    OK I will try to explain as I am not good at this.
    First I would give you an example just to prove that it works. Take your example but change the flashes to 2 identical units with the GN of 56. Using the formula it came out as 80 for the combined GN. Which give you f/8, 1 stop more than with 1 flash.
    The reason has to do with the inverse square law that states that the illumination at the subject is inversely proprotional to distance squared. And so if we are to keep the same amount of illumination but increasing the distance then the flash power has to be increased proportionally with the distance squared. The aperture number in similar fashion that the intensity of the light fallen on the film is proportional to inverse of the aperture squared because light intensity is proportional to the area of the lens opening not its diameter. With this 2 factors the square can be removed and form a simple relationship between distance and aperture. However, when we deal with power we have to put the square back before we can add or mutiply. I hope it helps because like I said I am not good at this.

  5. #5

    Join Date
    Apr 2004
    Location
    Montgomery, Il/USA
    Shooter
    Multi Format
    Posts
    5,003
    Hi Chan,

    Wouldn't the smaller flash just be ignored as if you were using it as a fill?
    Heavily sedated for your protection.

  6. #6

    Join Date
    May 2006
    Location
    Aurora, IL
    Shooter
    35mm
    Posts
    1,939
    Yes if the smaller flash is significantly smaller. Fill light generally in the opposite direction and fill in the shadow of the main and thus doesn't really contribute much to the main exposure. However in this case, the 2 flashes are in exactly the same position and the smaller flash isn't that much smaller than the larger flash. As you can see with the larger flash alone it's f/6.4 and with the smaller flash it's f/7.8. A differenet of 0.57 stop.

  7. #7
    olleorama's Avatar
    Join Date
    May 2009
    Shooter
    Multi Format
    Posts
    526
    Images
    5
    Wow, thanks, I suspected it had to do with the inverse square relationship. I blasted the living be jeepers out of an empty parking lot during a rain storm last night. I hope it will come out good. It did in the viewfinder at least.



 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin