# Thread: What's a Flash Guide number?

1. ## 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. 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...

3. Here's a good guide to your questions.

http://www.shortcourses.com/how/guid...idenumbers.htm

4. 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.

5. 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. .

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

8. 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. 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. 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.

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