Anyone Measure Their Flash GN?

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Eternal Student Junior Member • Posts: 39
Anyone Measure Their Flash GN?

I have a Good V860II which is a great, usable battery powered portable flash. I have just measured its guide number to check how much power I actually have available.

To do this I simply set up an incident light flash-meter in a dark room 4 metres away from the flash. I then fired it at full power using a shutter speed of 1/250th of a second (because it is the fastest shutter speed my camera can use without high-speed sync). My meter is only precise to 1/10th of a stop. I found that the variation I saw when repeating the test were at most 1/10th of a stop. I re-tested the measurements at about 15 degrees off-axis and confirmed the values were the same - within the 1/10 limit I already mentioned.

Here are my measured values;

Focal Length - measured Guide Number (GN) in metres @ 100ASA

200 - 31

135 - 29

105 - 29

80 - 25

70 - 25

50 - 23

35 - 21

28 - 18

24 - 18

20 - 18

The claimed GN at 200mm is 60 - about two stops better than I was measuring. I am sure my meter is not perfectly calibrated but also sure that it is not off by two stops. I knew zoom functions are not as useful as they would claim but again was surprised at how very little difference the focal length setting on the flash makes.

Has anyone else done this kind of check for their flashgun?

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