Flash Guide Number


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Jun 18, 2005
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What is flash guide number and what difference does it have from the power of the flash?
 

Snoweagle

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#3
megascriler said:
What is flash guide number and what difference does it have from the power of the flash?
Flash guide number is usually stated in metres at ISO100. A higher guide number have a longer flash range.
 

Jun 18, 2005
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#4
Then why is it that people here say that the flash of the Fujifilm Finepix V10 is stronger than the flash of the F10 when the F10 has a flash that can reach 6.5m(specs), whereas the V10 has a flash that reaches on 3.5m(specs).
 

solarii

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#5
Flash range depends on the selected ISO and aperture. There is no fixed range... the guide no.s are quoted for a given ISO. Also note the unit of measure... some quote in feet others in metres.
 

denniskee

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#6
megascriler said:
Then why is it that people here say that the flash of the Fujifilm Finepix V10 is stronger than the flash of the F10 when the F10 has a flash that can reach 6.5m(specs), whereas the V10 has a flash that reaches on 3.5m(specs).
taken from dpreview

Finepix V10
Flash guide no. 4.4 m (14.4 ft) 4 m

Finepix F10
Flash guide no. 3.0 m (9.8 ft) 0 m

it is correct base on the info stated in "dpreview".

when in doubt, ask "dpreview":bsmilie: :bsmilie:
 

Snoweagle

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#7
solarii said:
Flash range depends on the selected ISO and aperture. There is no fixed range... the guide no.s are quoted for a given ISO. Also note the unit of measure... some quote in feet others in metres.
Usually those from U.S. sites quote in feet.
 

Ian

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#8
megascriler said:
What is flash guide number and what difference does it have from the power of the flash?
The flash guide number is normally quoted in either meters or feet for a given ISO rating and refers to the full output of the flash unit. For Example, a typical high powered flash unit may have a guide number (GN) of 60m @100 ISO. You need to be aware that there are NO standards for specifiying the GN of a flash, so a manufacturer may use 100 or 400 to specify the GN depending on the model of camera.

You also need to be aware that the GN of many flashes is somewhat over optimistic in nature, it's not uncommon for manufacturers to inflate the real GN by up to 50%.

To turn this in to a practical figure that you can use, in otherwords the effective range of the flash unit you need to divide the Guide Number (GN) by the working aperture of the lens. eg: Effective Flash range is GN/f.

For Example, a flash unit with a GN of 60m when used with a lens set at F4 will have an effective range of 60 / 4 = 15m. While using the same GN 60 flash at F2.8 will result in an effective range of 21.4m and so on.

When changing ISO ratings you multiply by 1.4 for each doubling (1 stop increase) of the ISO speed, eg: 100-200 ISO multiply the result by 1.4, 200-400 ISO is another 1.4x and 400-800 is another 1.4x. When going from a higher to lower speed (1 stop) you multiply the result by 0.7 for each halving of the ISO.

Hope this helps.
 

denniskee

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#9
in full flash @ ISO 100
Metz 60 series flash has a GN of 60 @ 35mm

Metz 76 series has a GN of 76 @ 105mm, but only 48 @ 35mm flash head zoom setting.

550ex has a GN of 55 @ 105mm. but only 36 @ 35mm flash head zoom setting.

so it is advicible to read the specification correctly.
 

Jun 18, 2005
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#11
Thanks. I get it now. Then what about the flash distances used by DCResource reviews? It isn't guide number, then what is it? The maximum power of the flash? It says the flash on the F10 reaches 6.5m.

And, what is a flash motion/exposure sensor for?
 

elcap

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#12
A bit confused. Is there a way to convert the flash guide number from 35mm to 105mm for the same ISO no?
 

denniskee

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#13
megascriler said:
Thanks. I get it now. Then what about the flash distances used by DCResource reviews? It isn't guide number, then what is it? The maximum power of the flash? It says the flash on the F10 reaches 6.5m.
ah, i thought you got it??
see, did they quote at what aperture, iso?

taken from dpreivew
Flash
• Built-in pop-up flash
• Modes: Auto, Red-eye reduction, On/Off, Slow Synch, Red-eye reduction + Slow Synch
• Range: (wide) 6.5 m, (tele) 4 m, (macro) 80cm
 

Snoweagle

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#14
elcap said:
A bit confused. Is there a way to convert the flash guide number from 35mm to 105mm for the same ISO no?
I doubt so.

Maybe unless you can try those flash with manual functions which can adjust the output range of the flash to your liking.
 

MDZ2

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#15
Don't forget that guide numbers are also referenced to an aperture of F1 so if your lens has a max aperture of F2.8, then inreality, with the iso fixed at 100, you only get 1/3 the distance. ie GN 60 will probably give you a good 20m to work with.
If you are using a long lens, you can always increase the flash power by adding a fresnal magnifier infront of your flash. This will focus the light into a narrower concentrated beam of light. I have actually managed about 40-50m with my Nikon SB26 @400mm F1/5.6, iso100.
 

Snoweagle

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#16
MDZ2 said:
Don't forget that guide numbers are also referenced to an aperture of F1 so if your lens has a max aperture of F2.8, then inreality, with the iso fixed at 100, you only get 1/3 the distance. ie GN 60 will probably give you a good 20m to work with.
If you are using a long lens, you can always increase the flash power by adding a fresnal magnifier infront of your flash. This will focus the light into a narrower concentrated beam of light. I have actually managed about 40-50m with my Nikon SB26 @400mm F1/5.6, iso100.
Ah something new i've learnt. The flash magnifier i've seen it before, but only in photography books ;)
 

Watcher

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#18
elcap said:
A bit confused. Is there a way to convert the flash guide number from 35mm to 105mm for the same ISO no?
It is not that difficult but needs some work only. Some makers will specify both but the concept to convert is to calculate area of exposure. Since you can calculate FOV from focal length, assuming a 3/2 ratio, you can calculate the rest. I'm sure someone on the net has done it...

Edit: Look from this chart at the bottom of the page from Canon. You can calculate the rest.
 

MDZ2

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#19
Snoweagle said:
Ah something new i've learnt. The flash magnifier i've seen it before, but only in photography books ;)
You can easily make a magnifier yourself (and it can also be used over your zoom flash to increase the reach)
basically, you only need the magnifier which you can buy from most bookshop. Just ask for the map reader. They come in a few sizes from about 4"X 2" to about 1/2 A4 size. Should not cost more than $10 for the largest. I got the smallest for about $2
As for mounting it to the flash, you will have to use your imagination as there are just too many ways to do it to mention here.
 

Snoweagle

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#20
MDZ2 said:
You can easily make a magnifier yourself (and it can also be used over your zoom flash to increase the reach)
basically, you only need the magnifier which you can buy from most bookshop. Just ask for the map reader. They come in a few sizes from about 4"X 2" to about 1/2 A4 size. Should not cost more than $10 for the largest. I got the smallest for about $2
As for mounting it to the flash, you will have to use your imagination as there are just too many ways to do it to mention here.
Hmmm sounds interesting. But i still dunno how to make one hehe
 

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