Flash to Subject distance calculations with an FL-700WR

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tenderlove
tenderlove New Member • Posts: 2
Flash to Subject distance calculations with an FL-700WR

Hi,

I am new to flash photography, and I'm trying to understand flash to subject distance calculations. According to this website, the Guide Number = Distance * fstop. This formula seems easy enough, but I wanted to check my math. My goal was to recreate the flash-to-subject distance that the flash says, but use the GN values from the table in the flash manual.

(My true goal is that I'd like to do wireless flash shots, but the flash doesn't give me a flash-to-subject distance in wireless mode so I want to make sure I'm calculating it correctly myself)

I have an Olympus micro four thirds camera with a 17mm lens, the ISO set to 200, the flash set at 1/1 power, and the zoom corresponding with my lens at 17mm.

According to the chart in the manual, at ISO 100 with a 17mm lens, the GN is 24.0. At f/2.0, the distance should be 12 meters (24.0 / 2.0), but I'm using ISO 200. If I understand correctly, that means I need to go down one stop to f/1.4. So my flash-to-subject distance would actually be ~17.142 meters (24.0 / 1.4).

I decided to test my math against what the flash says, but at f/2.0 with ISO 200, the flash indicates my distance to be 15.3 meters! Additionally, this would indicate that the GN is actually 21.42 (15.3m * f/1.4 = 21.42), not the 24.0 that's in the manual!

Is the manual wrong? Am I doing something wrong? I feel like I'm missing something but I'm not sure what.

Thanks in advance.

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Olympus E-M5 III
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