Over the last 12-months there has been a surge of photographers wanting to learn off-camera flash. Regularly I receive emails or Facebook messages from other photographers asking for tips. Over the years I have tried to remember what made learning off-camera flash so easy for me and time and time again it comes down to one simple tip.
The simple tip is to take your flashes off ETTL (or i-TTL if you are Nikon) and put them on Manual Mode. The ETTL (Evaluative Through The Lens) settings are equivalent to putting your flashes on Auto. Each time you fire your flashes on ETTL you might have a different amount of light being thrown on your subject. Sometimes it will be too much, other times not enough and occasionally you might even nail it in Auto, I mean ETTL. But what you miss is the ability to understand how exactly your flash is working, after all you are just spraying light and praying it looks good. You can use flash exposure compensation to add more or less light to the "auto" equation but when using ETTL adjusting your aperture or ISO will continue to yield the same results since your flash will compensate by firing more or less power.
So instead of using ETTL, I highly recommend switching over to Manual Mode and dialing in your flash power yourself. I typically fire around 1/32nd power, sometimes less if my subject is closer, sometimes a little more depending on the distance my subject is from my flash. But it's not very often that I am using flash powers more than 1/8th power unless I am shooting outside before the sun has set.
By switching your flashes from ETTL to Manual Mode you will quickly begin to understand the relationship between the flash power and your aperture and ISO. You will also begin to better understand the inverse square law as you increase or decrease your distance between the subject and your flash. These are all very important things to understand when learning off-camera flash, but you'll be unable to grasp the concept if you are shooting with the flashes on ETTL. Because instead of seeing any difference your flash will continue to change flash power to give you what it thinks you want.
Here's An Exercise
If you are interested in learning OCF go grab your flash and switch the mode from ETTL to Manual. Dial your flash power to 1/32nd. Put your flash about 8 feet away from your subject. Then experiment dialing your aperture up and down and your ISO up and down. Do one at a time and you'll notice the difference each makes. Once you have a recipe that works, move the flash in and out (closer and further away) from your subject and see how the inverse square law affects the amount of light that is landing on your subject. By doing this in Manual Mode with a consistent flash power you'll start to understand the relationship that the flash, aperture, ISO and distance all have with one another and in short time you'll be much better at off-camera flash.