Flash Photography Tips
Part 7: Off Camera Flash Exposure
In previous article I discussed fundamental techniques of Off Camera Flash Photography. In this article we are going to look at some principles of off- camera flash Exposure and ideas for how to use these techniques to set the off camera flash exposure correctly.
How do I set the flash exposure in off camera mode?
If you use a TTL Flash trigger, basically you don’t need to do anything different from TTL with on camera shooting. Just set your shutter speed, aperture and ISO and your camera and the flash will take care of the rest for you.
If you don’t have a TTL flash trigger or you don’t want to use the trigger, then you may want to pay close attention to this next part!
You will need to measure and adjust the exposure according to the output of the flash gun. There are three easy ways to correctly set the exposure:
1- Using a Flash meter
If you have a flash meter, simply read the exposure from the flash meter and set the exposure.
2- Know the Guide Number for your flash
Guide Number is the measurement for the flash gun’s power. The guide number for an electronic flash measures its ability to illuminate the subject to be photographed at a specific ISO sensitivity and angle of view. A higher guide number indicates a more powerful flash. Here is the formula to calculate the exposure using the guide number:
GUIDE NUMBER divided by DISTANCE (in feet) equals the F STOP or Aperture Value.
You can also find the guide number of your flash in the Owners Manual or on line. It is usually marked at 100 ISO. If you increase the ISO by one stop, it doubles the guide number value.
For instance if I use a flash gun with Guide number 60 and set the flash four feet away from my subject, at ISO 100, the proper F stop is f15 or f16. The equation looks like this:
@ISO 100 60/4= f15 or f16
Now if I increase the ISO to 200 (one full stop increase), then the guide number increases to 120, and the calculation will be as follows:
@ISO 200 120/4= f30 or f32
So let’s say I don’t have f32 on my lens, and we know that the shutter speed won’t change the flash exposure, but I need to shoot at ISO 200 and f5.6 to balance the ambient light. What can I do to make the right exposure? The answer is the distance from the flash to subject! I simply move the flash to 20 feet away from the subject and here is the new calculation:
@200 ISO GN 120/20 (feet) = f6 – f5.6
Using a Guide Number to calculate flash exposures is fun and rewarding because you get correct exposures every time. Here is one assignment to help you better understand this method: I want to use my flash gun (Guide number 60 at ISO 100) to shoot my subject at f8 and my flash can’t be any closer than 15 feet, what ISO should I use to achieve a correct exposure?
3- Adjusting Flash output
This is probably the easiest way to set the flash exposure, with instant feedback from digital camera technology! Almost all flash guns allow you to set the output power from full (1/1) to all the way down to 1/128th power. Thanks to digital camera’s image review, you can take advantage of the ability to review your shot and fine tune the exposure. Here is a little tip which often works for me if I don’t have my flash meter or I am too lazy to use the guide number calculation.
I usually set my camera shutter speed in sync zone, set the aperture to f5.6 or f8, and then select ¼ power for my flash output. I typically set my flash 5 to 6 feet away from the subject and take my test shot. By reviewing the test shot I am able to find out if I need more or less power, to change the flash to subject distance, etc.
That’s all for now. Stay tuned for more photography techniques articles!
Ted and the Omnilargess Team