Flash unit guide numbers.... confused!

[woody]

I'm probably being a bit thick with this question but...

Can someone explain how the guide numbers work? - I found this on the Jessops site:

jessops web site said:

Guide Numbers are used to calculate the correct exposure of a flash unit. The guide number, divided by the subject to flash distance (in metres), equals the aperture required for the correct exposure (but most flashes are automatic).

I've noticed that most are given at ISO 100 - how does this work if you have your camera set to a higher ISO?

The reason I'm asking is I want to get a flash unit for my 300D - but don't want to get the wrong one for what I want to do. I want one that will be OK for taking to Le Mans and using at night by the track.

Any suggestions as to which would be best?

Cheers

Chris

[luca brazzi]

IMO no flash you use will make a difference at Le Mans by the track at night. The distance is simply too far.

Don't use a flash for that, but go for long exposures on a tripod, or set ISO 1600 or 3200 to ensure you get the fastest possible shutter speeds.

A decent zoom with a decent wide-aperture lens will help no end also.

LB
Decent flashes by Sigma and Canon come in about 50-60GN which is very powerful, but won't make a difference at the races if its cars at night you want to shoot. All IMHO.

[simpo two]

You used to need to know about GNs when you calculated exposure manually (after you put the coal in and waited for the steam pressure to rise). However, everything is so automated that I don't think you really need to know how to work with them to get good results. GN is most often used just to give a comparative idea of power.

It can get horribly complex - the Nikon SB800 varies from 1.1 to 56 (ISO 100, m). At higher ISOs the GN will increase - but I think here I might be hitting the inverse square law and so will retreat hastily before I fall foul of Bacardi and co!!

[Bee_Jay]

Depends how close you are to the track...

You could possibly take some nice car flash shots with the 580EX (GN 58 at 105mm) at night with the camera on 1600 or 3200 (dunno if the 300D goes to 3200).

see here: http://photonotes.org/articles/eos-flash/index2.html#guidenumber

So - and please correct me if I am wrong.

1600 ISO means GN gets multiplied by 4, equalling 232m
f4 means dividing GN by 4, bringing it back to 58m

So, with the camera on 1600ISO, at f4 at 105mm you should get a good 50m range out of it.

Still would be rather dodgy results though and a bit hit and miss, though I would personally give it a go for a laugh...