Yet Another GN Question

guide numbers are meaningless in tryingto figue out problems with TTL controlled

exposure. Guide numbers are good for figuring out maximum power output at a known

zoom setting on the flash (not on the lens) so the guide number will depend on what the

Speedlight's zoom setting is.

 

100 x 2 = 200. Where did you get the 1.4 factor from?

 

GN = distance x aperture at ISO 100

I guess I'm talking about practicing using the flash on manual. Also, the 1.4 factor for adjusting GN is due to doubling the ISO number from 100 to 200. You don't double the GN to go from 100 to 200.

 

If I'm firing in full manual isn't the GN how I figure my exposure (other than by checking the LCD and adjusting, which doesn't teach me anything)?

I've browsed through some of the old forum answers on this and couldn't quite

find the exact answer to this, so here goes. I'm studying guide number stuff so

I can learn what my auto TTL flash is doing in case something goes wrong. I'm

shooting a D40 so my minimum ISO equivalent is 200. So once I convert the

provided 100 ISO GN to 200 ISO (multiply by 1.4, right?), do I have to do

anything to my adjusted GN other than plug in my distance and aperture? I guess

what I'm asking is whether the GN equation of GN=distance x aperture is

independent of ISO? Am I correct when I say that the ISO speed is used only to

determine the actual GN, and not factored into the calculation of aperture, etc?

Thanks in advance.

If you're firing in full manual, with manual flash, then GN calculations can work. Not sure how much you'll learn though, as they are seldom used in real life situations.

 

One thing you will most likely find is that GN ratings for flash units are highly overrated. I think they measure than in gloss white hallways. If checked in a large dark room you will usually find them a stop or more dim. Also, Some manufacturers rate them more optimistically than others. And of course, the zoom position on the flash head changes everything, and they are not nearly as good at concentrating the light as the manufacturer would like you to believe.

 

The other thing you will discover is that you will really want to buy a flash meter.

 

I think you're right though. Going from 100 to 200 ISO, you want to multiply the stated GN by 1.4. Been a while since I did that though, so maybe not.

talking of GN i was thinking of getting a canon ex580 as its got a GN of 58 but when i find the real GN when zoomed back to 35mm is the same as my vivitar 283 at GN 36 (alleged) i saved my cash as i allmost always use a defuser so the zoom is no use Dave

Sorry that is correct: use 1.4 instead of 2x. My bad.

Thanks folks.