Gimmie the real skinny on guide numbers

I'm looking to upgrade flashes in the near future and have started to "do my homework". Ideally, I'd love to just whip out the credit card toss a 580EXii into the cart and call it a day. Unfortunately, I can't justify it quite yet. So, I've been looking into the 430EXii and it's counterparts from Nissin and Metz. One of the specs that is always at the forefront is the guide number. Now, I have a basic understanding of what the number means, but I'm curious as to how it can/would impact me in the real world. In other words, have any of you felt that flashes along the lines of the 430EXii are noticeably lacking in power compared to a 580? How important is flash "power" in terms of shooting events (most specifically weddings)? I understand that every situation is different, so I'm more or less interested in answers that focus on "most of the time" or "usually". Thanks in advance.

In my real world experience, between the 580ex2's and my former 430ex's - when I shoot indoors, I never used full power. Outdoors is a different story because you need all the power you can get.

As for the master/slave feature, I only used it a couple of times years ago, and quickly discovered that they are pretty much useless when there is a sun.


If you are doing primarily weddings, then get a flash with TTL as it will be almost impossible to use manual flash the whole time.

If you are going to shoot in a controlled environment (i.e. studio, home-studio, etc), then here's what I would suggest. I would get a cheap TTL flash (popular off-brand) to use during the days that I am lazy and buy cheap manual Nikon/Canon flashes and cheap triggers when I want to be creative.

The only thing that I will be missing from the above setup is high speed sync - which I never use because I'm too chicken to kill my $450 flash!

But if you will just use flashes for weddings, instead of buying a 580ex-2, get two Sigma equivalent for about the same money.

On-camera flashes really make life difficult by the fact that every different FL coverage angle creates a diferrent guide number. If all flashes have 105mm coverage angle, you can do direct comparison. But if some flashes are fixed angle (e.g. 35mm FL coverage) you first have to find out the zoom head GN for the same coverage FL or else your comparisons are apples-vs-oranges.

As for what different GNs represent in the 'real world', simply divide each GN by 4, and that would tell you what the maximum distance that the flash would reach if the lens were set to f/4...the farther, the better.

Then you further complicate comparisons with 'feature list' comparisons...ETTL or not, wireless flash or not, master/slave capable or not, HSS or not, stroboscopic mode or not, number of manual flash power levels available, etc.

Someone asked Joe McNally during one of his tours about guide numbers when working with speedlights and he didnt even know...lol

I've shot weddings with both the 580EX II and 430EX II... I couldn't really tell you the difference. I can tell you, the fact that I can not attach my battery pack to my 430EX II makes it a backup flash and sits in my bag.

A GN of 60 meters on one flash often doesn't equate to a GN of 60 metres on another.

A little like motorcyle manufacturers. Some makers rate their power output at the crank, some at the rear wheel.

GN need 3 additional qualifiers to be meaningful:
- reflector position
- ISO
- unit meters / feet

If all 3 are known then it's easy to compare different flashes! Many flashes have a guide number table in the instruction manual with GN for different zoom reflector positions, and ISO is mostly set to 100 - although there are some "smart" marketing people who use GN for the ISO-200 setting which lets their flash appear 40% more powerful.

The 430EXII is only slightly less powerful than the 580EXII, especially when used with a modifier. The Nissin Di866 is slightly more powerful than the 580EXII:

http://www.robgalbrait​h.com …p?cid=7-10050-10598-10599

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airshaq20 wrote in post #12361106
The only thing that I will be missing from the above setup is high speed sync - which I never use because I'm too chicken to kill my $450 flash!

I don't understand your statement. Why do you think HSS is hard on your flash?

It is if your using a 580exii with a TT5.
Also HSS has longer flash durations which sucks battery power, and in turn probably generates more heat.
Aside from using it with a TT5, I wouldn't be scared of HSS though...