Understanding Flash Guide number

dosdan · 10-06-2009, 04:46 PM

The Sigma EF-530 DG Super has a GN of 53 , so at ISO 100 & F1 it can correctly illuminate a subject at 53m when used with a 105mm lens on a FF camera (due to a zooming flash reflector head).

Varying Aperture. Does this mean that changing to F2 would drop the max. distance to 26.5m & F4 would max out at 13.25m?
Varying ISO. Would using ISO 400 on the same setup reach to 106m & ISO 1600 to 212m?
Varying FL. I presume a 70mm lens on an APS-C would have the same FOV as a 105mm on a FF, so the flash with a 70mm lens on an APS-C with an F1 lens at ISO 100 would reach to 53m. A 200mm lens on the same camera would still max on at 53m because the flash refector can't focus the beam any tighter.
Combined. The flash used at F2, ISO 400, with a 70mm lens on an APS-C, would max out out at 53m?

Dan.

timk · 10-06-2009, 05:36 PM

Hi
Using the flash calculator on my iphone seems to agree with your figures.

Eruditass · 10-07-2009, 09:34 AM

I don't know much about flash, but yes, that seems right. Two stops = factor of 2 in distance, 1 stop is sqrt(2). F2 and ISO 400 cancel out, and FL doesn't have an effect on range.

dosdan · 10-07-2009, 02:17 PM

Originally Posted by Eruditass

FL doesn't have an effect on range.

This would be correct if the Flash head didn't "zoom", but it does. The flash reflector moves relative to the flash tube, or perhaps the reflector is fixed and the tube actually moves.

This means that the GN changes but the flash power level is still the same.

GNs for this unit at different FF FLs. 100% power level. APS-C equiv. FL shown in brackets:

20 @ 17mm (11mm) with pullout wide-angle diffuser panel
28 @ 24mm (16mm)
29 @ 28mm (19mm)
31 @ 35mm (23mm)
40 @ 50mm (33mm)
45 @ 70mm (47mm)
48 @ 85mm (57mm)
53 @ 105mm (70mm) or greater

So the GN increases because the flash angle-of-illumination is decreased (the cone of light becomes narrower) at increasing FLs to match the decrease in FOV as lens FL increases. Once you go over 70mm FL on an APS-C camera, there is no further narrowing. This means, for example, if you use a 135mm lens with the flash, some of the flash's light output is wasted because it falls outside (is wider than) the len's FOV.

Dan.

Eruditass · 10-07-2009, 02:19 PM

Originally Posted by dosdan

This would be correct if the Flash head didn't "zoom", but it does. The flash reflector moves relative to the flash tube, or perhaps the reflector is fixed and the tube actually moves.

This means that the GN changes but the flash power is still the same. (This is with the flash running at 100% power level).

GNs for this unit at different FLs. FF FL shown - APS-C equiv. FL shown in brackets:

20 @ 17mm (11mm) with pullout diffuser panel
28 @ 24mm (16mm)
29 @ 28mm (19mm)
31 @ 35mm (23mm)
40 @ 50mm (33mm)
45 @ 70mm (47mm)
48 @ 85mm (57mm)
53 @ 105mm (70mm) or greater

So while the max. power output is the same, the GN increases because the flash angle-of-illumination is decreased at increasing FLs to match the decrease in FOV as lens FL increases.

Dan.

Right, i was refering to simply changing the len's focal length as I assumed the OP was refering to based on his #3, not the flash head's FL setting, which may not change if he is doing manual.

djbrown · 10-13-2009, 07:29 AM

This means that the GN changes but the flash power is still the same?

dosdan · 10-13-2009, 01:49 PM

Originally Posted by djbrown

This means that the GN changes but the flash power is still the same?

This is at the maximum power setting. Yes, the GN increases up to a certain limit (in the case of this flash it's GN 53 @ 70 mm FL on an APS-C) because the beam of illumination becomes tighter/more concentrated as the flash tube moves in the reflector.

Same power - tighter beam - more illumination at a distance - higher GN

Dan.

*Canada_Rockies* · 10-17-2009, 04:01 PM

Dan, you have the idea, but : one thing that the flash manual mentions only in the microscopic fine print is that the guide numbers are usually rated as in a light coloured room. The numbers are fudged a bit to allow for the reflection from the ceiling and walls. Using a flash outdoors will reduce the effective guide number.

Myself, I either use a metered flash (auto/TTL,p-TTL, etc.) rather than manual, or if in manual take some test shots in the environment you will be using, and adjust as needed.

Watch out for zoom lenses with variable apertures, also. The change in f/stop from wide to tele can cause no end of grief.

dosdan · 10-17-2009, 04:52 PM

Originally Posted by Canada_Rockies

The numbers are fudged a bit to allow for the reflection from the ceiling and walls. Using a flash outdoors will reduce the effective guide number.
....
Watch out for zoom lenses with variable apertures, also. The change in f/stop from wide to tele can cause no end of grief.

Albert, thanks for that info.

Luckily, my most used lens with flash at the moment is the DA 16-45/F4.

**Damn Brit** · 10-17-2009, 08:41 PM

Moved to Camera and Studio Accessories, people searching for this kind of info will find it easier there.