Alternatively, if your flash to subject distance is a constant, then flashes with higher guide numbers allow you to use narrow lens apertures which helps you achieve more depth in the image and keeps everything sharp :. So there you have it: you can leverage the extra power of a flash with a high guide number in one of two ways:.
Bottom line: look for a flash with a high guide number if you think that you'll often use the flash to light subjects from a distance OR if you don't want your aperture choices to be limited every time you use a flash.
Sometimes the ISO value will be stated, but if it isn't just remember that all guide numbers are calculated at ISO The only value ever reported as the guide number is the flash to subject distance in both feet and meters.
You'll note that the lens aperture used to calculate the guide number is left out of the reported value, which leads quite well into our next topic. We now come to the only drawback of using flash guide numbers to evaluate and compare different electronic flash units. Over time, flash manufacturers have come to rely on guide numbers to sell flashes a lot like camera manufacturers have relied on megapixels to sell cameras.
As such, they "optimize" the environments in which they test their flashes so that they can report a high guide number on the flash spec sheet. Since you probably won't be using your flash to light subjects in a controlled environment, you'll discover that the "real" guide number of your flash doesn't match what the manufacturer states in their promotional literature.
Make sense? Sorry, there's really no way to factor in the magnification on a global scale. There are too many other variables when you bring that into the equation. Hi John, I really enjoyed your article and it's all starting to make sense, thank you. Can I ask, are there any principles that I can follow, in conjunction with your calculations above, when using a soft box and flash for instance?
Many thanks, Yvonne. Hi Yvonne, unfortunately, there aren't any "formulas" for adjusting these when using softboxes. The only "rule" is that a softbox will absorb some of the flash output, so you won't get as much power out of it when using a softbox. Any time the flash has to go through something like fabric or bounce off of something, it's going to lose some power.
I'm a total novice, looking to buy my first flash and this made perfect sense to me with the help of your answers to people's questions about the zoom difference. Thank you very much. First and foremost, thank you for your Service Thankyou on finally getting a very linear explanation to me that my very mechanical mind can grasp with ease All the other places I looked had to much in the abstract that was way to much to understand Thanks for the feedback I've read several explanations of guide numbers, including The Strobist's, but this is the first one that made sense.
The light from these flash units will reach subjects at a greater distance, ensuring that they are correctly exposed. Imaging Products. Flash Flash Mode. Flash Level Guide Number. Flash Level Guide Number In flash photography, the light from the flash is used to illuminate the subject.
Photographs taken at different distances with a built-in flash Subject close to camera. Subject far from camera.
In another word, GN specifies the power of an electronic flash in a way that it can be used to determine the right f-stop to use at a particular shooting distance and ISO setting. When there were no automatic flash units available for photographers, they used to do a manual calculation using the GN and the distance of the subject from the flash unit. GN is usually given in reference of ISO value And in Manual mode, it has a Guide Number of 13 meters or 43 feet.
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