# Why does the Guide Number have a distance given with it? [duplicate]

I am learning photography and looking to buy a flash. I learned the basics of the flash, and as far as I saw everywhere, the Guide Number had a feet unit with the ISO number; e.g. 160' at 100 ISO.

I came across this format and I don't know how to interpret it — it displays the feet, ISO and another distance. See for example https://www.bhphotovideo.com/c/compare/Yongnuo_YN685_vs_Yongnuo_YN-560-IV/BHitems/1247484-REG_1101196-REG

I don't understand what it means by 196.85' (60 m) ISO100 at 200 mm position or 190.29' (58 m) ISO100 at 105 mm position. Can someone please help me interpret this?

## 2 Answers

I don't understand what it means by 196.85' (60 m) ISO100 at 200 mm position

You're looking at a unit where the flash head zooms to change the area that's illuminated. The head moves toward the front to light a wider area, and toward the back to light a narrow area. Changing the area that's illuminated means that the intensity of light per unit area changes, so the guide number is different depending on the position of the head. The positions indicated (105mm and 200mm in your example) correspond to focal lengths on your lens.

Guide Number is the product of distance and aperture, so a guide number of 60m tells you that the flash will provide enough light to properly illuminate a scene 15 meters away when the camera is set to f/4, or 30 meters away when the camera is at f/2. If you zoom the flash head to a different position, like 150mm, then the guide number will be lower (because the light is being dispersed over a larger area).

Some DSLRs can communicate with certain flash models to automatically adjust the flash's zoom to match the focal length that a zoom lens is set to, so that the whole frame is lit by the flash. For example, Canon's EOS cameras can send zoom information to Canon EX speedlights that support zooming.

Guide Number is about the Inverse Square Law, which describes the way the direct flash intensity falls off with distance. At 2x distance, the flash is reduced to 1/4 intensity. At 3x distance, reduced to 1/9 intensity. At 4x distance, to 1/16 intensity. Like your flashlight or car headlights also fall off with distance. So distance does overwhelmingly affect direct flash.

The Guide Number is a simple way to describe this easily, uncomplicated.

Supposing if the flash exposure is considered "correct" exposure at say 10 feet and f/8, then that defines this case of Guide Number as 10x8 = GN 80 (for whatever ISO being considered). So then we know 20 feet is properly exposed (assuming direct flash) at 80/20 = f/4, and 5 feet is exposed at 80/5 = f/16. Makes it easier than computing squares. Can use meters instead of feet. GN in feet is 3.28x GN in meters, simply because there are 3.28 feet in one meter.

The flash zoom from 105 mm to 200mm concentrates the wider 105 mm beam into a smaller 200mm beam. Concentrated into smaller area becomes a bit brighter, so it has a bit higher guide number (but the area coverage is smaller). This is a variable for the specific reflector design.