ISO 419,430,400

Often when I get an idea for a post it makes it to a sheet of paper to see if the idea is sensible and sound, every now and then the paper stage is missed out 🙁

My post before Christmas about a Speedlite 580EX II having a range of 90km was based on a light hearted discussion whilst waiting for a sunset over Table Mountain in South Africa to look awesome. Despite the stunning location the sunset was pretty poor and so was the calculation of flash range, a touch of sun to the head i’ll say.

So to show the working and correct the figures.

  • At ISO 100 the flash has a guide number of 53m with the flash head set to the 85mm zoom position. source: Canon Speedlite 580EX II manual
  • If you increase the ISO to 200, then you would be able to close the aperture one f-stop from say f/2 to f/2.8 and the flash result would look the same as it had at ISO 100. This is due to the fact that f-stops change by the square root of two each stop then 1.4x (or the square root of two) is the factor change in the guide number, not the ISO multiplier of 2x as in the original post.
  • The inverse square law is out there just waiting to catch out the hasty photographer and blogger alike!
  • EOS-1Dx ISO in full stops goes ISO 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600, 51200, 102400 and finally ISO 204800 for a total of 11-stops of increase from 100 to 204800
  • So in effect the flash “in a perfect environment” will have a guide number of 53m at ISO 100, 75m at ISO 200, 106m at ISO 400 and so on up to 2399m at ISO 204,800
  • Now taking the EF 85mm f/1.2L II USM lens and dividing the guide number 2399m at ISO 204,800 by f/1.2 gives me a theoretical maximum distance of 1999m (still that’s almost 2km or 1.2miles)

Many thanks to a few folks for pointing this one out to me, it’s really great to have people reading the site wherever they come from. Though I am somewhat surprised that the original post was called in to question on a forum at, the home of the Minolta / Alpha-mount dSLR photographer. Thank you gentlemen for pointing out the error.

ISO 419,430,400

With ISO performance increasing with new cameras it will only take three more stops increase until we see ISO 1,638,400, and I can’t help but think how long it will be until the cameras come with ISO 419,430,400 and then the 90km flash range may be back on. I would offer it as a “stretch objective” for the R&D folk at Canon. One thing, they will need to change the LCD displays to get 9 digits for the ISO scale!

More fun to come in 2012 for sure, as EOS celebrates it’s 25th Birthday


About the author

Full-time photo tutor and photographer. I love to share my knowledge and skills to make photos, videos and teach others. I write books and articles for photo magazines and I always have at least one Speedlite flash in my camera bag