Steve's Digicams Forums

Steve's Digicams Forums (
-   Flash (External) (
-   -   Guide numbers for reduced frame DSLRs (

elinzer1 Jun 3, 2005 3:59 PM

I am using a Canon Rebel XT -- with focal length conversion factor 1.6. Do external flashes know about the focal length coneversion? In other words, for a given lens focal length, will the flash divide by 1.6 to figure out how to focus the light beam, increasing the effective guide number? The flash I am considering is:;navigator=1

VTphotog Jun 8, 2005 12:42 AM

Focal length conversion factor only applies to the field of view of the camera. Unless the flash is equipped with an equivalent lens, there should be no difference in the guide numbers.


elinzer1 Jun 8, 2005 5:14 AM

But I thought the reason that the guide number increases with the focal length pf the lens is that the flash is more concentrated in the center, so when a longer focal length = more narrow field of view is used, the flash range is increased. For example if I use a 50mm lens the field of view is the same as a 35mm camera with an 80mm lens, so the flah range should be the same as a 35mm camera with an 80mm lens, right?

Or is there a different reason that guide numbers increase with focal length?

PeterP Jun 8, 2005 10:01 AM

Some flashes like the Canon 580ex are aware of the camera's crop factor they are mounted on and will adjust accordingly, if the camera is compatible and provides the information.

The flash tries to focus itself for the crop adjusted focal length of the lens. Up to whatever its maximum ability is, after about 105mm it stays constant, unless you add something like the better-beamer to narrow down the beam even more.


geriatric Jun 21, 2005 6:37 AM

Hi el 1

You talk about external flash. Well external flash don`t know nothing, only what you instruct them what too do. Aperture setting of the camera, wide normal or zoom. what the ISO is set too on the camera. not being the proud owner of the new breed of flash guns thats as far as I can go.

All times are GMT -5. The time now is 2:32 PM.