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How to measure GN?
This is not a scientific measurment, just an approximation that may help someone to determine (approximate) guide number of his flash (I know that most of the flashes have all necessary specifications but it is always good to know how to check if they are true). What is needed? A solid background, flash and something to determine the right exposure (in my case it was a cheap grey card). What to do? 1. |
Use a tripod (or a chair if you don't have tripod) to compose the picture - it should consist of as much of a solid background as possible. Make sure you know the exact distance from the flash (usually on the camera) to photographed object. |
2. |
Use grey card (or something else) to determine the right exposure using ambient light (artificial light is a good idea). Set the camera to 100 ISO (you can use diferent one, just remember to recalculate the GN), X time, right white balance (for the ambient light), and aperture. Take the shot. |
3. |
Attach the flash, change white balance to flash preset, turn off the ambient light (now you know why I suggested artificial one), and start taking shots with different apertures. |
4. |
Watch the histogram and compare it with histogram of the first, properly exposed photo. When you find aperture that gives the closest result multiply it by the distance from the flash to the object and you have the guide number! |
Example
Here I measure the GN of a flash that is built into my Konica Minolta Dynax 7D (oficially 12 GN but I had searious overexposures using that). First I set my camera on a tripod against green wall, exactly 2 meters from it. I measure the exposure using a grey card (f/4, 1 second at ISO 100). Lamp from my bedroom was the source of light (typical bulb, bulb white balance preset was used). I took the photo (the first one below).I turned off the lamp, set white balance to flash preset, set the flash to full power (manual flash and 1:1 flash ratio was set on the camera, no additional compensations, full manual mode) and started to take the shots (without changing distance to the wall of course). Below I present four of them that were the closest to the right exposure. I think that the f/8 is the closest, therefore I multiply 8 by 2 (the distance in meters) and voila! Guide number of my built in flash is 16 (for ISO 100).
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This is an ambient light shot, camera settings (ISO 100, f/4, 1 second) were determined by a grey card measurment. |
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Shot taken with flash (ISO 100, f/7.1, 1/160th of a second) |
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Shot taken with flash (ISO 100, f/8, 1/160th of a second) |
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Shot taken with flash (ISO 100, f/9, 1/160th of a second) |
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Shot taken with flash (ISO 100, f/10, 1/160th of a second) |