are 2 x GN20 flash equivalent to 1 x GN40 flash?
After doing some research I have come to understand that guide number is not necessarily a measure of the power of flash unit and another term used is watts/s. I'm still learning about flash usage and I ask a general question for indoor use. Would the illumination from 2 same flash units of GN20 be similar to the illumination of 1 flash unit of GN40 given all set for the same fov, say 28mm (35mm eqv)? Im having some difficulty understanding luminosity vs intensity.
If I wanted to illumiate a large room with 10 foot ceilings, would the GN40 flash produce a brighter composition than 2 x GN20 flashes? Would 2 x GN20 units pointed up and away from each other be a more favourable or equivalent option to one GN40 pointed directly up for bounce flash if given the limitations of those two options?
No, combining two equal flashes as one (2x) would give 1.4x GN of one of them.
Combining N equal flashes would give square root (N) x GN of one. 1.4 is sq root (2).
This assumes the flashes are combined aimed at the same coverage. In not aimed at the same coverage, they would not combine as effectively, and 2 might not be 2x and 1.4x GN then. If one were pointed oppositely at 180 degrees, they may not combine at all, in any degree.
Watt seconds is the electrical input of energy into the flash. Output light depends on the energy of conversion (flash is more efficient than incandescent, etc). But this WS total energy does not discuss how that light is distributed, into a narrow spot, or into a wide area. The narrow spot will be brighter than the wide area, with greater GN within that spot.
Guide Number is the simple product of f-stop x distance, which gives the proper exposure (for direct unmodified flash). Supposes the GN is 100 (feet). That would imply a correct exposure was 10 feet at f/10, or 20 feet at f/5, or 5 feet at f/20. This takes into account the inverse square law.
But the angle of flash coverage affects intensity, which affects GN within that coverage. To compare power of two speedlights, we can compare GN, only so long as it is specified at the same zoom (same coverage) for both.
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