So what the hell is that GN thing ...??? .... Any wild guess ???
Ok lets start from the scratch ....
GN term is basically belongs to the world of FLASH PHOTOGRAPHY. A world which is not liked by many (lets be honest here) .... Don't like not because of the hardware it self or price .... people hate it because they not able to control the flash at the beginning and thats where the turning point it. At the beginning when you see trouble .... exactly at that point either you make it or break it :) .
So i was telling the GN thing here .... Let me get in my comfort zone by copy pasting the definition from the WIKIPEDIA :)
In simple mathematical terms, it is the aperture multiplied by distance, in which your flash can PROPERLY EXPOSE a subject at a given distance (aperture and distance). The standard GUIDE NUMBERS you should be looking for are measured at ISO 100 (film speed), at 35mm flash head position, at full 1/1 power.
Lets take a basic Nikon Flash (yes i am proud Nikonian) for an example. The SB700 has a GN of 92. To make our math easier, we'll around it off to 100. Lets take a look at our diagram to see what this means. We have a flash and a subject.They are 20 feet away in distance.
To properly expose our subject, we would need an aperture of f/5 ... Why?
Because .... f/5 x 20' = 100GN
Now what happens if we move our subject 20' further away ...... !!!!
To compensate for this additional distance, we would need to let in more light! This means we will have to open up our camera aperture to f/2.5
f/2.5 x 40' = 100GN
If we kept our aperture at f/5, we would underexpose our subject. Likewise, if we opened up our aperture too much ..... say for example f/1.4 .... we would overexpose our Subject. Got it?
Note that all of the above calculation are done based on the assumption of ISO100 (already highlighted in blue above). So what if we have ISO more then 100 ????
Note that all values in the "Factor" line is the hundredth of the Square root of the ISO.
Lets have some example here ..... Suppose we plan to use direct flash at f/8 at 12 feet at ISO 400.
I hope we all use this GN thing at some point in our flash photography .... practically speaking.
Happy learning Guys :)
BabarPhotography
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Ok lets start from the scratch ....
GN term is basically belongs to the world of FLASH PHOTOGRAPHY. A world which is not liked by many (lets be honest here) .... Don't like not because of the hardware it self or price .... people hate it because they not able to control the flash at the beginning and thats where the turning point it. At the beginning when you see trouble .... exactly at that point either you make it or break it :) .
So i was telling the GN thing here .... Let me get in my comfort zone by copy pasting the definition from the WIKIPEDIA :)
"The guide number (GN) for an electronic flash measures
its ability to illuminate the subject to be photographed at a specific film or
sensor sensitivity and angle of view. A higher guide number indicates a more
powerful flash."
The guide number is the product of the maximum
flashtosubject distance and the fnumber of the aperture that will correctly
expose film or a digital sensor with the specified sensitivity."
GN = distance × fnumber
Even it took me some time to get what Wikipedia is saying
here .... Lets get practical now gentlemen.
So what is Guide Number?
In simple mathematical terms, it is the aperture multiplied by distance, in which your flash can PROPERLY EXPOSE a subject at a given distance (aperture and distance). The standard GUIDE NUMBERS you should be looking for are measured at ISO 100 (film speed), at 35mm flash head position, at full 1/1 power.
Lets take a basic Nikon Flash (yes i am proud Nikonian) for an example. The SB700 has a GN of 92. To make our math easier, we'll around it off to 100. Lets take a look at our diagram to see what this means. We have a flash and a subject.They are 20 feet away in distance.
To properly expose our subject, we would need an aperture of f/5 ... Why?
Because .... f/5 x 20' = 100GN
Now what happens if we move our subject 20' further away ...... !!!!
To compensate for this additional distance, we would need to let in more light! This means we will have to open up our camera aperture to f/2.5
f/2.5 x 40' = 100GN
If we kept our aperture at f/5, we would underexpose our subject. Likewise, if we opened up our aperture too much ..... say for example f/1.4 .... we would overexpose our Subject. Got it?
Note that all of the above calculation are done based on the assumption of ISO100 (already highlighted in blue above). So what if we have ISO more then 100 ????
Converting Guide Number for ISO
The Guide Number charts are typically always printed showing ISO 100
values, but we can just multiply ISO 100 GN values by 1.41 to get ISO 200 GN
values, or multiply by 2 to get ISO 400 GN values, etc. Or we can divide if
converting going the other way.
ISO

25

50

100

200

400

800

1600

3200

6400

Factor

x0.5

x0.71

x1

x1.41

x2

x2.83

x4

x5.6

x8

Note that all values in the "Factor" line is the hundredth of the Square root of the ISO.
Doubling Guide Number doubles distance range, or doubles f/stop
number, which is two more stops.
Doubling flash zoom
number should theoretically multiply Guide Number by 1.414, but only as an
approximation, because of course, it depends on the individual reflector
design.
Lets have some example here ..... Suppose we plan to use direct flash at f/8 at 12 feet at ISO 400.
So we know we need flash power of f8 x 12 feet = GN 96 (feet) at ISO
400. The charts are always for ISO 100 that comes with the flash manual for any flash. Below is the GN chart of SB800 as an example here where distance of the flash with subject is given in meters/feet.
We know the ISO 400 factor is x2 (from above converting guide factor). So now we need to scale down ISO 400 to ISO 100 to check the flash power from the above chart and that we can do by dividing ISO400 by 2.
Converting this to ISO 100
is GN 96/2 = GN 48 (feet, ISO 100). We search the guide number chart in the
speedlight manual (ISO 100), and maybe we find this value at 24mm zoom and 1/4
flash power to be say GN 49 feet. More than close enough to 48. This SB800 GN
chart below (for an example) says that 24mm flash head zoom and 1/4 flash power
is GN 49, almost exactly the GN 48 that we need.
The charts show GN as both meter and feet values
(as meters/feet), so use either one, so long as you are consistent with units.
Duplicate this situation by setting flash to 24mm zoom, 1/4 power, and set the
camera at ISO 400 and f/8, and you're very close on first try for a 12 foot
flash distance (direct flash). Or, the chart includes several other
combinations of power level and zoom which are near GN 49. It need not be
exact, GN varies with f/stop numbers so that another GN value of about 12%
difference is roughly within 1/3 stop (and 41% is one stop). We don't
necessarily have to match flash zoom to lens zoom values, we can use any wider
flash zoom  which may waste a little light if the flash is wider than the
lens, which is no problem if not at maximum power (some flashes do not zoom
anyway). FX flash on DX cameras is already 1.5x wider anyway. To adjust results
of this method, you can simply adjust the power level by 1/3 stop, or adjust
the aperture by 1/3 stop. Or zooming the flash head makes steps sometimes
roughly about 1/3 stop (no bets, there is no actual relationship).
I hope we all use this GN thing at some point in our flash photography .... practically speaking.
Happy learning Guys :)
BabarPhotography
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BBM Pin: 75DEED40