kenArchi Loc: Seal Beach, CA

The flash GN's are all very different. The more GN the more light output?

PixelStan77 Loc: Vermont/Chicago

When I bought a flash in my film days, I always started by testing their GN's. I always found they were overstated and not real. This enabled me to develop a chart based upon distance what F stop to be used for each film speed.

RWR Loc: La Mesa, CA

Correct.

papa Loc: Rio Dell, CA

YES to your question. Both GN and W/S are relative to the light output. Higher number is more light. My Canon 430 EX II and 600 EX-RT are rated with a guide number, but my Photogenic studio lights are rated in watt seconds, as studio lights all are.

kenArchi Loc: Seal Beach, CA

I have an example. I have sunpac 383 on camera flash, a gn of 120, iso 100, 35mm lens. Then I see flash that rates at 197gn at 200mm. I don't see how that compares.
What is the gn at 35mm?

I hope you understand what am saying.

Bill_de Loc: US

The GN divided by distance give you your F/Stop. So a higher G/N indicating more light would require a smaller (higher #) F/Stop. At one time I could figure it all in my head. Then came automation and I forgot how to do division.

If a flash head zooms, as many do today, they spread or concentrate the light. The concentrated light would most likely be brighter and have a higher guide #.

Nikon SB5000:

Power Output
Rated Guide Numbers at ISO 100:
at 35mm zoom: 113'/34.5 meters.
at 200mm zoom: 180'/55 meters.

--

RWR Loc: La Mesa, CA

The light is concentrated more at the flash’s telephoto settings, resulting in higher guide numbers. At the wider zoom settings the light is spread out more, thus lower guide numbers.

RWR Loc: La Mesa, CA

Didn’t see your edit til I posted - good description.

JohnSwanda Loc: San Francisco

I'm sure there are others there who remember using flashes before they had auto exposure. Then you actually had to use the guide number to determine the aperture based on distance to the subject. Nikon had a lens you could set the guide number on, and as you focused it would change the aperture to match the distance you were focused at. If you wanted to bounce the flash, you had to estimate the distance from the flash to the surface you were bouncing off of and then to the subject.

SharpShooter Loc: NorCal

Stan, how were you checking your GN?
If I recall, GN is based on f1.0 which very few people have the equipment, thus the ability to actually check.
Then there are all the lenses that say, say 1.4, 1.8 etc but are rounded down to appear faster but could be as much as a half stop off. First you have to check that the lens is on, and few are, especially the less expensive they are.
If comparing Flashes, make sure they are all rated to the same f-stop and ISO and always use the same lens for consistency.
If you have a good flash, who cares???
SS

Thomas902 Loc: Washington DC

"Do you unsderstand GN and WS of flashes?" kenArchi Guide Numbers are in theory a value derived from empirical data which determines a relationship between the intensity of light verses distance from the source... A better way to think of this is a flash guide number (GN) is a measure of the distance at which the flash can (effectively) illuminate a subject for a given ASA or ISO. The higher the guide number, the greater the distance at which the light from the flash is sufficient for optimal exposure. The formula for calculating the guide number is:

Guide number (GN) = distance (meters or feet) × aperture (f-number)

You will typically find Guide Numbers written 174/53 at 200mm Zoom

Please be aware that Guide Numbers are NOT related to Watt Seconds in any way whatsoever...
They are two entirely different values that have virtually no relationship to each other

Watt Seconds are simply a measure of "Potential" energy available at the head or mono-block.
Watt Seconds are in fact a unit of energy equivalent to the joule.
Probably the best way to think of this is that a "Watt Second" rating is the amount of energy that can be stored in a units capacitor(s) and released when the flash is fired. You absolutely can not use Watt Seconds to calculate the "effective" distance at which the light from the mono-block or head is sufficient for optimal exposure. You must instead use an incident light meter to determine the correct aperture.

btw, an incident light meter is highly effective for calculating the output of speedlights also...
Trying to balance a multi-light scenario without an incident light meter is at best an exercise in futility...
This can not be done in a timely manner without an incident light meter...

Best Advice? if you are shooting Canon you might start by relying on ETTL Mode rather than setting your speedlites to Manual Mode
Same holds true for Nikon where you can start by relying on iTTL instead of worrying about GN's and Manual settings for your speedlights.

Once you begin to see the amazing potential of off camera speedlights (or speedlites for Canon) you will likely mix them with ambient from then on... At the commercial level speedlights are ubiquitous and for compelling reasons... Simply relying on ambient illumination will drastically limit your creativity and seriously compromise you ability to achieve aesthetically superior imagery...

Bottom Line? Photography at it's very essence is painting with light... the more control you have over your medium (light) the greater your potential to create what you envision with your minds eye...

Hope this helps kenArchi or is at least food for thought...
I wish you well on your journey...

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

GN refers to Guide Number. If the GN of a flash is 80, that means at 10 feet your f stop should be f8 at iso 100. OLD RULE. So if your guide number is 110 that means f11 at 10 feet, so , yes, the higher the GN number, the more powerful the flash.

aphelps Loc: Central Ohio

This may be nitty, but w/s should not be called watt seconds. W/s is watts per second and is a measure of instantaneous brightness. Flashes in photography are rated in watt seconds or WS which is a measure of flash power (watts) over a time period (secs) which relates directly to exposure. I have seen equipment manufacturers use these terms interchangeably but they should know better.

Denny45acp Loc: Sparta,Michigan

Yes they should

twowindsbear

'Instantaneous brightness' pretty much seems to define a flash. What SHOULD the measurement be instead?