Adding Guide Numbers -- please confirm my math

Discussion in 'Digital Photography' started by Pat, May 1, 2006.

  1. Pat

    Pat Guest

    This newsgroup has a few people who like physics and other things that
    are more math-intensive than I am comfortable with. So I need you to
    make sure I am doing this right (and am using the right formula)..

    I just picked up a second flash and want to use them for night sports
    photography. One is a Canon 550EX with a guide number of 180. The
    other is a 580EX with a guide number of 190.

    If memory serves me right, to add guide numbers, I do the following:

    sqrt(180^2 + 190^2) = 262 +/-

    Please let me know if I am using the right formula.

    Thanks.

    Pat.
     
    Pat, May 1, 2006
    #1
    1. Advertising

  2. Pat

    Bruce Hoult Guest

    In article <>,
    "Pat" <> wrote:

    > This newsgroup has a few people who like physics and other things that
    > are more math-intensive than I am comfortable with. So I need you to
    > make sure I am doing this right (and am using the right formula)..
    >
    > I just picked up a second flash and want to use them for night sports
    > photography. One is a Canon 550EX with a guide number of 180. The
    > other is a 580EX with a guide number of 190.
    >
    > If memory serves me right, to add guide numbers, I do the following:
    >
    > sqrt(180^2 + 190^2) = 262 +/-
    >
    > Please let me know if I am using the right formula.


    Flash guide numbers tell you how far away the subject can be from the
    flash unit (not the camera!) using ISO100 film and an f1.0 lens and be
    illuminated for a correct exposure. The guide number is therefore
    proportional to the square root of the light output (because the area
    illuminated is proportional to the square of the flash to subject
    distance), so your formula is correct -- convert to light output, add,
    convert back to guide number.

    --
    Bruce | 41.1670S | \ spoken | -+-
    Hoult | 174.8263E | /\ here. | ----------O----------
     
    Bruce Hoult, May 1, 2006
    #2
    1. Advertising

  3. Pat

    2 Guest

    In article <>,
    "Pat" <> wrote:
    >
    > This newsgroup has a few people who like physics and other things that
    > are more math-intensive than I am comfortable with. So I need you to
    > make sure I am doing this right (and am using the right formula)..
    >
    > I just picked up a second flash and want to use them for night sports
    > photography. One is a Canon 550EX with a guide number of 180. The
    > other is a 580EX with a guide number of 190.


    The numbers are close enough FAPP. You gain 1 stop using two. If you want
    some really depressing news, calculate how many you need to gain four stops.
     
    2, May 1, 2006
    #3
  4. Pat

    Pat Guest

    I thought about that. It doesn't take very long to realize that very
    quickly you need to start carrying around something of the magnitude of
    the sun.
     
    Pat, May 1, 2006
    #4
  5. Pat

    Pat Guest

    Thank you.
     
    Pat, May 1, 2006
    #5
  6. Pat

    Roy G Guest

    "Pat" <> wrote in message
    news:...
    > This newsgroup has a few people who like physics and other things that
    > are more math-intensive than I am comfortable with. So I need you to
    > make sure I am doing this right (and am using the right formula)..
    >
    > I just picked up a second flash and want to use them for night sports
    > photography. One is a Canon 550EX with a guide number of 180. The
    > other is a 580EX with a guide number of 190.
    >
    > If memory serves me right, to add guide numbers, I do the following:
    >
    > sqrt(180^2 + 190^2) = 262 +/-
    >
    > Please let me know if I am using the right formula.
    >
    > Thanks.
    >
    > Pat.



    Hi.

    Why bother getting into advanced maths, Square roots and things like Pie R
    Squared.

    The 2 guide numbers you quote are so similar that they can be treated as
    identical. Using both guns together, at the same distance, gives double the
    light & double the light equals one Stop less for the aperture setting.

    Roy G
     
    Roy G, May 3, 2006
    #6
  7. Today Roy G commented courteously on the subject at hand

    [snip]
    > Why bother getting into advanced maths, Square roots and
    > things like Pie R Squared.

    [snip]

    "cake are square, pie are round"

    --
    ATM, aka Jerry

    "Gentleman, you can't fight in here, this the War Room!" - From
    the movie 'Dr. Strangelove or How I Learned to Stop Worrying and
    Love the Bomb'
     
    All Things Mopar, May 3, 2006
    #7
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Evil Uncle Chris

    Confirm my wireless understanding please?

    Evil Uncle Chris, May 1, 2005, in forum: Wireless Networking
    Replies:
    1
    Views:
    580
    Sooner Al [MVP]
    May 1, 2005
  2. Dribbler

    Math help please

    Dribbler, Oct 28, 2005, in forum: Computer Support
    Replies:
    8
    Views:
    608
  3. mort

    Frame Numbers vs. JPEG Numbers With CF Cards

    mort, Feb 14, 2005, in forum: Digital Photography
    Replies:
    3
    Views:
    620
  4. =?Utf-8?B?U2FsYWh1ZGRpbg==?=

    Please confirm this possible Marvell Lan x64 bug

    =?Utf-8?B?U2FsYWh1ZGRpbg==?=, Aug 9, 2005, in forum: Windows 64bit
    Replies:
    5
    Views:
    532
    John Barnes
    Aug 10, 2005
  5. Breedo

    Re: Numbers and more numbers...

    Breedo, Dec 2, 2005, in forum: A+ Certification
    Replies:
    0
    Views:
    564
    Breedo
    Dec 2, 2005
Loading...

Share This Page