Power (actual and GN) and size

[Sympa]

I'm now owner of two flashes - the Godox TT350 and TT685 respectively.

The TT350 has a GN of 35 meters at 100mm zoom. The TT685 has a GN of 60 meters - but at 200mm zoom!

Now I'd think that the illuminated area is roughly equal at the respective zoom settings and distances. Which makes me wonder why the TT685 is so massive when compared to its smaller brother. Is this because the zoom doesn't affect all the light? Or because the bigger flash lights more even, needing more energy to get the center at the correct brightness?

Any ideas about this? The big falsh is twice the weight of the small one, so I'd expect the actual power output in wattseconds to be double as well... which is 1.4 times the GN. With the zoom that should then be 2.8 times GN, right?

 

[fotowbert]

I'm now owner of two flashes - the Godox TT350 and TT685 respectively.

The TT350 has a GN of 35 meters at 100mm zoom. The TT685 has a GN of 60 meters - but at 200mm zoom!

Now I'd think that the illuminated area is roughly equal at the respective zoom settings and distances. Which makes me wonder why the TT685 is so massive when compared to its smaller brother. Is this because the zoom doesn't affect all the light? Or because the bigger flash lights more even, needing more energy to get the center at the correct brightness?

Any ideas about this? The big falsh is twice the weight of the small one, so I'd expect the actual power output in wattseconds to be double as well... which is 1.4 times the GN. With the zoom that should then be 2.8 times GN, right?

In simple terms a speedlight's light output (GN) is related to the energy stored in the capacitor (watt-seconds) and width of the beam the speedlight's zoom head creates.

The difference in light output starts with the capacitor size. The TT350 capacitor would need to be smaller than the TT685 otherwise recycle time and battery life would be unacceptable since it uses 2 AA cells instead of 4. The physical size of the TT350 is also a constraint on the capacitor size.

• John

 

[BAK]

You write: >> I'd expect the actual power output in wattseconds to be double as well. <<

Watt seconds is not a measurement of "power" related to how bright the light is.

And...

The zoom setting is important, but perhaps confusing.

If you had the bigger flash set at 100mm, and the little flash set at 100mm, side by side, they'd illuminate the same amount of a wall.

If you zoomed the big flash to 200mm, and left the flash in the same spot, it would illuminate less of the wall, but it would be brighter in any illuminated spot or area than the illuminated area at 100mm.

BAK

 

[rumple]

I'm now owner of two flashes - the Godox TT350 and TT685 respectively.

The TT350 has a GN of 35 meters at 100mm zoom. The TT685 has a GN of 60 meters - but at 200mm zoom!

Now I'd think that the illuminated area is roughly equal at the respective zoom settings and distances.

No. 100mm zoom will spread the light more widely than 200mm zoom, so all else being equal the 200mm zoom will have a higher guide number.

Which is why manufacturers quote the tightest zoom their flashes will do for GN.

"Standard" GN is at 100iso, 35mm zoom, and most manufacturers will provide that somewhere so you can compare. If not, try to find comparable zoom distances.

 

[Ellis Vener]

I'm now owner of two flashes - the Godox TT350 and TT685 respectively.

The TT350 has a GN of 35 meters at 100mm zoom. The TT685 has a GN of 60 meters - but at 200mm zoom!

Now I'd think that the illuminated area is roughly equal at the respective zoom settings and distances. Which makes me wonder why the TT685 is so massive when compared to its smaller brother. Is this because the zoom doesn't affect all the light? Or because the bigger flash lights more even, needing more energy to get the center at the correct brightness?

Any ideas about this? The big falsh is twice the weight of the small one, so I'd expect the actual power output in wattseconds to be double as well... which is 1.4 times the GN. With the zoom that should then be 2.8 times GN, right?

Stop paying attention to published guide numbers.

Instead do direct A to B tests with the flashes setup exactly the same way: same distance to the same target and if the flashes have a zoom feature set them to the same beam angle - ideally if the beam angle of the flash is marked in focal length, either 35mm or 50mm, and the flashes set to full manual power , and of course if when you are shooting the tests, using the same camera, and if a zoom lens, set to the same focal length and of course aperture setting , ISO and shutter-speed.

Ideally you use a flash meter but real world testing works best.

Do you know how to calculate guide numbers? The equation is simple: Guide number = f-number × distance.

 

[Sympa]

That seems to be the most logical - the guide number being a marketing number.

I'll have to get to know these flashes anyway, so comparing is definitely in order.

 

[Ellis Vener]

That seems to be the most logical - the guide number being a marketing number.

It means something, you just do not know the context. was it derived by looking at the component and engineering specs or was it derived from actual testing, ang if by testing how was it tested?

That they have equal guide numbers but one is for a 200mm beam angle and the other for a 100mm (much wider beam angle) should indicate that the model with a GN for 100mm is more powerful

I'll have to get to know these flashes anyway, so comparing is definitely in order.

Trust, but verify.

 

[tugwilson]

Light power out is roughly proportional to electrical power in.

The 685 will fire about 230 full power flashes when powered by 4 X 2500 mAh batteries

The 350 will fire about 210 full power flashes when powered by 2 X 2500 mAh batteries so we would expect it to fire 420 if it had 4 X 2500 mAh batteries.

So dividing 420 by 230 gives us 1.8. We would therefore expect the 685 to be about 1.8 times more powerful than the 230 at the same power and zoom settings.

You could measure it and see how close this approximation is.

 

[Sympa]

Seems there is about 2 stops of difference: with the zoom setting similar (and similar spot size on the wall) I need 1/16th power on the smaller flash and 1/64th power on the large one.

However, the small flash produces 210 flashes from 2 batteries, while the big one gets 230 flashes from 4. (all spec'd at 2500MAh).

So... the flash energy is somewhere between 2 and 4 times larger for the big flash.

 

[BAK]

There's a lot of math here. Why?

Can you give us a reason for your semi-questions?

Why did you buy those particular flash units?

BAK

 

[Ellis Vener]

Seems there is about 2 stops of difference: with the zoom setting similar (and similar spot size on the wall) I need 1/16th power on the smaller flash and 1/64th power on the large one.

However, the small flash produces 210 flashes from 2 batteries, while the big one gets 230 flashes from 4. (all spec'd at 2500MAh).

So... the flash energy is somewhere between 2 and 4 times larger for the big flash.

You need to do this at full power, not a ratio powered setting.

 

[Sympa]

There's a lot of math here. Why?

I like math to solve problems. GNs are about math.

Can you give us a reason for your semi-questions?

Having insight into actual performance when used at equal zoom setting. Or when used with a light modifier. I'd think the amount of light is a primary parameter of a flash, but it seems there is not too much information available.

Why did you buy those particular flash units?

I got one to use on-camera, found it a bit weak as fill in sunlight, so I got a larger one as well with a compatible radio built in. Oh yes, and the price.

 

[S Castle]

Some more muddling and confusion. But maybe not.

My Canon 600ex ii-rt has a GN chart in the manual, showing the GN for various outputs and zooms. It does not have one for 100mm, but does for 105mm, full power: 54. (Its GN of 60 at 200mm is the same as the TT685.)

Your TT350 has a GN of 35 at 100mm full power. If we assume my Canon and your Godox TT685 perform similarly at the zoom distances, and we guess 52 as the GN at 100mm, that means the TT685 at 100mm zoom has a longer reach by 17m (52-35). But that's not what we want to know. We want to know relative flash power.

Now, by the inverse square law, a light source twice as bright (one stop brighter) will have the same brightness at a distance 1.414 (SQRT(2)) times that of the lower brightness*. 35x1.414=49.5. So if the flash is twice as bright it would have a GN of 50. Close enough.

Conclusion: in general, the TT685 is twice the power of the TT350, or maybe a bit more. I suppose one could get more nitpicky and use fractions and decimals, but this seems like a good figure to use.

*This is a handy thing to know, if you need more light and your flash is already at max, but you can move it closer. If you want one more stop, move your flash closer by dividing the distance to the subject by 1.4, and putting it there. Dividing by 2 gets you two stops, by 2.8, three...just remember your f-stop numbers.

I am leaving out the reliance of GN on the inverse-square law. You can Look It Up.

--
Shane

 

[Sympa]

Thanks scastle, that clears up a lot. Zooming from 100 to 200 mm doesn't change the GN a lot on that Canon - I suppose that applies to most flashes.

If the zoom was perfect (constant f/number) the GN would be doubled going from 100 to 200mm: the illuminated area is the same size at double the distance and double the focal length.

Thanks for the insights!