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Archive 2014 · Help needed comparing Olympus flash guide numbers

  
 
cameraman72
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p.1 #1 · p.1 #1 · Help needed comparing Olympus flash guide numbers


Hi,

Can someone explain to me how to compare flash guide numbers, specifically Olympus flashes? For instance, here are the specs for some Olympus flashes:

FL-LM2 - 33' at ISO200 at 50mm
FL-300R - 28' at ISO200 at 35mm
FL-600R - 50' at ISO200 at 8mm
FL-36R - 65' at ISO100 at 24mm
Fl-50R - 79' at ISO100 at 24mm

Different distances, ISO and focal lengths are specified in the guide numbers. So what formula do I use to determine the output for each of these flashes for a given distance, ISO and focal length. For instance, how would I determine the F stop to use for each of these flashes at 50mm (35mm format, so 25mm m4/3 lens), 10 feet away at ISO 100? Also, how does shutter speed affect on the situation?

I found the general guide number formula of GN=distance* f-stop but I don't see how this formula works when determining if a flash has enough power to do the job in a specific situation.

Thanks for the help.




Aug 23, 2014 at 10:55 PM
tsilva
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p.1 #2 · p.1 #2 · Help needed comparing Olympus flash guide numbers


On the ones showing ISO 200 divide the guide number by 1.44 to get the approximate ISO 100 number. Sorry, not sure how the focal length affects the calculation.


Aug 24, 2014 at 12:55 AM
cameraman72
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p.1 #3 · p.1 #3 · Help needed comparing Olympus flash guide numbers


Thanks, I think I understand most of it now. The specs for the FL-36R were wrong, confusing me. It should be 36' at ISO 100 at max power. I'm still not sure how the focal length comes into play, but I think GN are usually quoted at 50mm FL be default, whatever that means.


Aug 24, 2014 at 06:50 PM
curious80
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p.1 #4 · p.1 #4 · Help needed comparing Olympus flash guide numbers


cameraman72 wrote:
Thanks, I think I understand most of it now. The specs for the FL-36R were wrong, confusing me. It should be 36' at ISO 100 at max power. I'm still not sure how the focal length comes into play, but I think GN are usually quoted at 50mm FL be default, whatever that means.


Most of the modern (external) flashes have a Zoom head. When you are using a longer FL, they can concentrate the flash light to a narrower field of view. As a result of concentrating into a narrow FOV the light could go farther and thus you get a larger range and a bigger GN. Therefore the GN varies with the FL used (many flashes also allow you to manually set the flash zoom position ).

Unfortunately different manufacturers use different conventions in the FL they use to report GN. For example Canon typically reports the GN at the maximum zoom setting supported by the flash. For example even though Canon 580XII has a GN of 58m, that's at 105mm. At 50mm the GN for that flash is 42m. And at its widest zoom setting of 14mm, the GN is just 15m.

You can roughly compare the power of two flashes by looking at their GN at a common FL such as 50mm or 35mm. However that will not be the complete picture --- two flashes which have the same power at 50mm could still have a different range at a longer FL depending on how much zoom each flash head supports. Built-in pop-up flashes don't tend to have zoom heads.

Hope that helps!



Aug 24, 2014 at 07:30 PM





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