During a recent individual training the topic of flash power came up, the photographer I was talking to was saying his flash wasn’t powerful enough, even though he had a Speedlite 580EX II. I suggested that he try something other than ISO 100 for his flash pictures.
The Speedlite 580EX II has a guide number of 58m at ISO 100 that is acheived when the head is zoomed to 105mm, it drops to 28m at ISO 100 when set to 24mm and even 15m at ISO 100 if you pull out the extra diffuser for 14mm wide coverage. This is all based on ISO 100. You can make the Speedlite more powerful by increasing the ISO to say ISO 400 or more. In fact at ISO 400 the flash will have a guide number of 232m, so should in theory reach 58m with the EF 24-105mm f/4L IS USM lens.
Seeing the specs of the EOS-1DX I got the calculator out, and calculated that ISO 204,800 is 11-stops more than ISO 100…So this means that if I put an EF 85mm f/1.2L II USM lens on the camera then the flash will zoom to 85mm and it’s power would be 53m at ISO 100 and reach a subject some 44m away if I shot with the aperture at f/1.2. So now if I increase the ISO to 208,400 to get more range the humble little Speedlite has an effective guide number of 108,544m at ISO 204,800. Which means it should light a subject over 90km away. Only issue is I have no way to tell if the subject in my picture is 90km from me, and if there isn’t any other light source affecting my calculations. So going even crazier I elected for the biggest lens I can get, the EF 800mm f/5.6L IS USM, put on an EF 2x Extender III and redid my calculation to find that at f/11 wide open I can still reach nearly 11km with the Speedlite flash.
More effective flash power when you increase ISO
This is a bit of extreme Christmas fun, but as a top Speedlite tip it will often be helpful to make use of ISO 400 or more with flash, your flash will not only reach further, it will recycle faster too and most likely your batteries will last longer. Actually most of the recent EOS cameras with AUTO ISO setting will default to ISO 400 when a flash is used unless the scene is already very bright.
Happy Christmas to everyone, i’m looking forward to a great 2012. If you did get a camera for Christmas, don’t forget to check out my post on preparing your camera for taking pictures not just playing.