Flash guide number?

Hi!

Ive done a quick search and couldnt realy find anything on this, im going to be buying a flash but am unsure of its power...can somebody explain what the guide number is?

Im looking at a flash with a guider number of 46ft/ iso100

I assume this means the flash can illuminate a subject just right from 46ft away when the camera is set to iso100?

Any help would be great!


River Cottage

Try reading over this it will explain it better than I can.
http://www.naturephoto​graphers.net/articles0​703/jm0703-1.html

Eric

thanks for that lots of maths!..ill just let the camera do the maths for me!, slap it on and hope for the best!..should be okay, 46ft seems quite low, but could always boost the iso or lover the apature



River Cottage

It's pretty small - look at the 580EX stats here on the specs tab, but remember it has a zooming flash head.

You can say that again, Tim. My 10D has a built-in flash that has a GN of 43 (ft).. That's just 3 short of 46.

Now, if that were 46 (m) instead, that's be a different story.

"lots of maths"?!?! 'Arithmetic' from grade school, not 'math'!
Yes you can let the camera do the thinking while you're shooting, but it is very valuable to know that your GN 46 flash reaches out to 11' when f/4 is set on the aperture (46 / 4 = 11), but a GN160 flash would let you use that f/4 aperture setting and properly illuminate a subject that is 40' away (160 / 4 = 40)!

Understanding the concept is important. To test whether you're figuring it right, do the math, set the camera and then look at the distance scale on the back of the flash unit. If your flash has a GN of 160, then you should get a distance of 20 feet at f/8 and ISO 100. Turn the flash on, set the camera, half-press the shutter and look at the scale. It should be at 20 feet or at least close. Now change the ISO to 400. Now it should be at 40 feet.

You'll make better pictures if you can do some rough calculations in your head and prepare accordingly. It takes practice, but it's worth the effort if you want pictures that are better than "okay".

Gee, you kids. I grew up being taught GNs at ASA 25 in feet. Based on that a GN of 46 would be decent indeed.

Don't buy a flash just based on it's guide number. Make sure it's compatable with your camera. A good feature is also a 'manual setting' which alot of the older canon flashes (like the 420ex) don't have.