EXIF Data Flash Guide Number

[lefarmer]

Can someone interpert the "Flash EF Guide Number" for me? I have just bought a 550 EX flash. I took over 200 shots one day using the flash on most of the shots.

In the EXIF data where the flash was used all of them images have the same data listed. It is "Flash EF guide number: 2047.97". The photos were shot outdoors under very different conditions and indoors.

Does anyone know how to interpert this number or where to find documentation on this?

Thanks,

Lynn....

[DetlevCM]

Can someone interpert the "Flash EF Guide Number" for me? I have just bought a 550 EX flash. I took over 200 shots one day using the flash on most of the shots.

In the EXIF data where the flash was used all of them images have the same data listed. It is "Flash EF guide number: 2047.97". The photos were shot outdoors under very different conditions and indoors.

Does anyone know how to interpert this number or where to find documentation on this?

Thanks,

Lynn....

As far as I know the guide number is how far the flash reaches at ISO 100 if I am not mistkane.
For the 580EX that would be 58m

Check the manual, as it should be listed in it somewhere.

[Lowner]

The 58mts translates to around 2260", Your 2048" works back the other way to 52.5m. It seems close enough to be logical.

That makes the guide number 52 or 53, not 58 as Canon would suggest, but maybe they have been a bit ambitious? Using the "550" as a hint, I'd expect it to have a guide number of 55 so quite where the flashes name comes from I have no idea.

[Roy Mathers]

There is an explanation here (http://en.wikipedia.org/wiki/Guide_number)

[Roy Mathers]

As far as I know the guide number is how far the flash reaches at ISO 100 if I am not mistkane.
For the 580EX that would be 58m

Check the manual, as it should be listed in it somewhere.

I think you'd be a bit disappointed if you expect the 580 to reach as far as 50 metres (63 yards) :D

[apersson850]

That's no problem.
Put an EF 85 mm f/1.2L USM on a crop camera. The flash will zoom to the max setting, 105 mm, since the angle of view is equivalent to 136 mm on FF. Thus we have guide number 58 at ISO 100. 58/1.2=48.3 meters, so there's not much remaining already at ISO 100. Increase the ISO a notch, and you are home.

It's another thing that the manufacturer's specifications often aren't fully met by their flashes. The guide numbers also assume typical indoor shooting, where reflections in walls and ceilings redirect some light otherwise lost back on target.

[DetlevCM]

I think you'd be a bit disappointed if you expect the 580 to reach as far as 50 metres (63 yards) :D

Actually, it easily will, depending on ISO ;)

I did a test with my 430EX II at ISO 3200 - heck, it goes well over 150m - I could see the trees at the other end of the Wroclawska Fontanna...

And that's what the guide number of a flash ist - the distance.