Guide numbers explained – the maths behind the mystery

When flashes first started to use Guide Numbers, they were a fairly reliable judgement of how one flashes power stacks up against another. But as flash technology has evolved, the humble Guide Number is often exploited as a marketing gimmick to make flashes sound a lot more powerful than they actually are.

In this video, Gerald Undone breaks down the maths behind the Guide Number. He explains exactly how it’s calculated, why it isn’t always an accurate measurement of power, and how you can make sense of it all.

There are a lot of misconceptions surrounding the Guide Number – many of which I’ve seen regularly repeated across forums and social media over the years.

Many photographers still cling to that magical Guide Number without really fully understanding what it means. They just think a higher Guide Number is a more powerful flash without looking at the other elements from which it is calculated. As you can see from Gerald’s video, there’s a lot more to it than that.

There used to be some kind of consistency between manufacturers when it came to creating Guide Numbers. They’d use the same settings, not because they had to, but because it made sense. Over the last decade or so, though, I’ve seen plenty of them fudging the settings on the Guide Numbers to make them sound more powerful than they really are.

Zooming flash heads to their maximum and raising the ISO will both produce a higher number, but that doesn’t mean they’re more powerful. And you’ll often have to go really digging through manufacturer websites to find out exactly how they’ve actually calculated it.

Hopefully, Gerald’s video arms you with a little more information so that you can see through the inflated numbers and make more informed decisions when you purchase your next lights.

There’s a lot of maths and complicated explanations in there, so you’ll probably want to watch parts of it several times to really wrap your head around it. We’ve covered Guide Numbers before in this post, and it’s worth a look if you want to see a slightly different explanation.

Personally, I’ve always preferred just getting hold of the light and trying it for myself vs another light I’m considering replacing. It’s a far less intense headache than trying to decipher the settings the manufacturer used to determine the Guide Number and then try to equate it to your existing kit.

And even with all the maths in the world, there’s still no substitute for just using and really knowing your gear.