Confusion on Guide numbers ? ? ? studio lights/speedlights

[Orange_crunch]

Sorry if this sounds a dumb question
If the Guide Number (GN) of the studio light is 45 am I right in thinking that it would be f/45 @ 1 metre, or f/4.5 @ 10 metres ? ? So if it was used for a lastolite hilite it would have to be used at around 1/2 power ? to give a reading of around F/22

 

[G.K.Jnr.]

Hi, If the guide number is in metres then divide the "FLASH" to subject distance in metres to get the F stop. ie
guide number 45 in metres, flash to subject 1 metre = F45.
guide number 45 in metres, flash to subject 10 metres = F4.5.
guide number 45 in metres, flash to subject 15 metres = F3

And so on.

Hope this helps.,

George.

 

[Orange_crunch]

Thanks for the reply

 

[HoppyUK]

Guide numbers can only be used for exposure calculation when the flash is fired direct. If it is bounced off a ceiling, or used with any kind of modifier (which is what a HiLite effectively is) like a softbox or umbrella, then they will not be accurate.

Many years ago, guide numbers were the only way of calculating flash exposure, but thankfully that time has past. Today, guide numbers are mostly used to indicate the power output of the gun or studio head, but there are a lot of variables involved (that are often not stated) so beware.