# Thread: NEed some Help in Flash knowledges!

1. ## NEed some Help in Flash knowledges!

Hi all,

Hope that anyone out there would be kind enough to help me solve some doubts about flash

wat's does it mean by saying :

GUIDE NUMBER (ISO 100, M/FT.)
38 / 125 (at 35mm) to 56 / 184 (at 105mm)

Guide number (ISO 100, m):
36(with Zoom head set at 35mm)

I understand is mentioning something about guide no.. but.. can't understand..

hope some one can elighten me.. THAnKS

2. guide number (GN) is a value representing the "power" pf the flash

its unit is in "metre" or "feet"

in the olden days, people use this equation: GN = A x D

A: aperture (numerically, no units)
D: distance from camera to subject (numerically, unit is either "metre" or "feet")

so if ur flash has GN = 40m, u prepare to shoot @ f4, then if u position your model at 10m away from the camera, the flash at 100% output will then supposedly give u a "nice" flash exposure. (18% reflectivity) because u follow the equation. (40=4x10)

for your flash, the GN is variable and it increases as u zoom the zoom head on the flash.

so at 35mm, your flash GN is 38m which is 125 feet. (one feet = roughly 0.333m)
likewise at 105mm, your flash GN is 56m which is 184 feet

it says the GN is 36 when zoom head is set at 35mm...this is a bit redundant which is ok, since GN=36m is practically the same as GN=38m both @ 35mm

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