Results 1 to 10 of 10

Thread: What's a Flash Guide number?

  1. #1

    Default What's a Flash Guide number?

    What's a Flash Guide number?

    For example: Guide Number 22 (ISO100)

    & does all canon speedlites have Flash Exposure Compensation?
    Also what's TTL, E-TTL ?
    Thanks...

  2. #2

    Default

    Quote Originally Posted by AReality
    What's a Flash Guide number?

    For example: Guide Number 22 (ISO100)

    & does all canon speedlites have Flash Exposure Compensation?
    Also what's TTL, E-TTL ?
    Thanks...
    hmmm... the same qn abt guide number had been puzzling me too ... some one please help !

  3. #3

    Default

    Here's a good guide to your questions.

    http://www.shortcourses.com/how/guid...idenumbers.htm
    1stJournal - Painting your life journals since 2008
    Email / Facebook / Instagram

  4. #4
    Moderator ziploc's Avatar
    Join Date
    Jan 2002
    Location
    Snoopyland
    Posts
    4,584

    Default

    Guide Number (GN) indicates how powerful your flash unit is.

    GN = subject distance * f-stop

    GN can be given in imperial or metice measurement and is normally used in manual flash mode to calculate the f-stop required for a particular subject distance (or vice versa).

    For example, for a flash of GN=32 (m) at ISO100 firing at full power, let's say the subject distance is 4m:

    f-stop = 32 / 4 = 8

    So you will need to set your aperture to f/8 for proper exposure on the subject. For ISO other then 100, just multiply 1.4 (root 2) to the GN for each stop. So for example, for ISO400, the effective GN for the above flash unit is:

    GN = 32 * 1.4 * 1.4 = 32 * 2 = 64

    Assuming the ISO is constant, a flash unit's GN is weaker at the wide angle than telephoto since the flash will need to disperse the light at a wider angle. Hence most of the time, manufacturer will quote the GN at the telephoto end to make it looks good.

    Hope that helps.
    Last edited by ziploc; 25th September 2003 at 02:55 PM.

  5. #5

    Default

    Thanks...

    Do all canon speedlites come with flash exposure compensation controlled on the flash? (Make it more simple: 300D does not have manual FEC, which speedlite can buy?)

    Thanks...

  6. #6

  7. #7

    Default

    hrm i think only canon 550ex and the sigma 500 super

  8. #8
    Member
    Join Date
    May 2003
    Location
    Singapore
    Posts
    984

    Default

    Quote Originally Posted by ziploc
    Assuming the ISO is constant, a flash unit's GN is weaker at the wide angle than telephoto since the flash will need to disperse the light at a wider angle. Hence most of the time, manufacturer will quote the GN at the telephoto end to make it looks good.

    Hope that helps.
    this is assuming of course that the flash is has a zoom head. lots of flash dont...
    some might be confused to thinking that this is the lens zoom setting...

  9. #9
    Member
    Join Date
    Jul 2003
    Location
    Singapore, North Area
    Posts
    507

    Default

    Then what if the flash is for ttl? do we still need to consider the GN when using ttl? Or do we use the Gn only in manual mode?

  10. #10
    Moderator ziploc's Avatar
    Join Date
    Jan 2002
    Location
    Snoopyland
    Posts
    4,584

    Default

    Hi showtime,

    Thanks for the correction.

    Hi Kira,

    In TTL flash you don't need to do the calculation as the camera is doing it for you by metering the reflection of the flash light off the film. In this case the GN of the flash is used only to indicate the max distance it can reach. Same for Auto flash mode. Hope that helps.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •