Very nice Mark! Yes it isn't 60km, but still very impressive!! Ah well since I don't have a f1 lens it doesn't really count for me, but still..
I just kept on thinking about it myself and just had to look it up [:P] And yes you're right!
But then the other way around isn't right as well. What I said earlier:
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Originally Posted by Sheiky
That just wouldn't be correct then, wouldn't it? That should be quadratic as well. Or am I missing something important now?
Jan
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But then the other way around isn't right as well. What I said earlier:
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That just wouldn't be correct then, wouldn't it? That should be quadratic as well. Or am I missing something important now?
Jan
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I think a GN of 58m @ f/1.0 would turn into a GN of 14.5m (58/4) @ f/1.4 since@f/1.4 the lens would gather half the light of f/1.0 makingthe flash'sability to illuminate a subjectaffective at only1/4 the distance?
Who thought photography was going to be so math intensive!
Photography = Physics in disguise. [8-|]
I don't think guide numbers translate into lumens, but rather to lumen seconds. Lumens measure light per square degree per unit time, while guide number measures light per square degree gathered up over time. In other words, if flash 1 flashes with uniform brightness for 1/10,000 second, and flash 2 flashes with uniform brightness for 1/100,000 second but with 10 times as brightly (10 times as many lumens), the two flashes will have the same guide number.
According to [url="http://www.flashbulbs.com/basics.htm]this[/url], a guide number of 150 is about 7000 lumen seconds.
[quote=Mark Elberson]
<span style="font-size: 9pt; font-family: 'Verdana','sans-serif'; color: #000000;"]I understand light to obey the inverse square law. What this means is that (all other things being equal) when you double the distance of your light source to your subject you get 1/4 (25%) of its original power. That being said, in order to double the GN (Guide Number) of a flash you would need to quadruple the output. Since with a 580EX you are limited to its GN of 58 in order the only way you can affect its "power" is to adjust either the aperture or the ISO. Since GNs are rated at f/1.0 and ISO 100 we're pretty much limited to changing the ISO. Here's where I take a leap...based on every I just stated, I believe that if you change your ISO from 100 to 400 you will effectively increase the GN of the 580 from 58m to 116m. Make sense??? Someone with more knowledge please correct me where I'm wrong :-)
<span style="font-size: 9pt; font-family: 'Verdana','sans-serif'; color: #000000;"]Another way to understand the GN of your flash is if the GN is 58m @ f/1.0 & ISO 100 then to maintain that 58m GN @ f/1.4 you would need an ISO of 200. To maintain that 58m GN @ f/2 you would need an ISO of 400, etc, etc.
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I'm don't claim more knowledge, but I agree with most of this, though I would say that when you double the distance you get 1/4 the intensity, not 1/4 the power. IIRC, intensity is power per unit area. As you get further away, the light spreads over an area that grows like the square of distance. The power stays the same, but the power per area, or intensity, obeys an inverse square law. (A 100 W bulb is a 100 W bulb no matter how far away you are).
Terrific point! I knew I was forgetting something :-)
Nevermind, somehow I missed the inverse square law had already been brought up.
Anyone know how far a 580exii will reach at102,400? It is still an interesting question. I don't think it will be that amazingly far though.
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