# Embarrassing it is ...

Started Feb 6, 2013 | Discussions thread
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 Re: Embarrassing it is ... In reply to deednets, Feb 7, 2013

I would forget about the guide number and buy an inexpensive flash meter, but…

The GN of 58 metres is at ISO 100 at full power, f/1 (have you ever seen an f/1 lens), and fully zoomed in to 105 mm.

If you assume that constant zoom and full power, then at stop down you reduce the distance by Ö2 (approximately 1.4x). So f/1.4 gives you 58/1.4 or 41 metres, f/2 gives you 29 metres, f/2.8 gives you 21 m, f/4 = 15 m, f/5.6 = 11 m.

Also halving the power gives you 1.4x less distance, quarter power gives you half the distance, etc.

Increasing the ISO from 100 to 200 willincrease the distance by 1.4, to 400 from 100 will increase the distance by two times.

Or you could just take test shots and look at the camera's histogram.

(Sorry I see that DPR has hit a new low and symbols are no longer appearing that Ö2 should be square root of 2)

Brian A

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