Using Light Modifiers Correctly
This weekend I was attending my wife’s graduation and watched a guy with a high end Canon camera outfitted with the latest 70-200mm f/2.8 L II lens and 580 ex II struggling for half an hour to get a good exposure. The guy had all the right gear and was only 40 feet away from the stage, and well within the range of the flash. I was in the same situation on the other side of the room. Why did I only need a single test shot to double check my exposure and this guy fired close to one hundred shots and never got a good image? It wasn’t because I am just awesome or have magic powers, it all boiled down to understanding your gear.
Before I just jump in and explain what he was doing wrong, let’s break down the situation and explain why this shouldn’t have been a problem.
The room wasn’t very bright and you needed to shoot at around f/5.6 to keep multiple people in focus. Using this aperture and a reasonable shutter speed of 1/60th required an ISO setting (without flash) of 6,400. A Canon 580 ex II zoomed to 105mm at full power easily has a working distance of 150 feet so I was able to get good exposure at a much lower ISO at only 1/4 power. I didn’t even bother to use manual flash settings as eTTL was doing just fine.
Given the ability of the equipment, why was it that the other guy was having problems even though he actually had a better camera? Well, the other guy had something I didn’t, he had a Sto-fen diffuser on his flash. But wait, isn’t a Sto-fen supposed to improve your flash? Why was this killing the shot?
Let’s break this down. At full power he should have had 150 feet of flash range. Just having the Sto-fen on the flash will cut the total light output by two stops (Understanding Light Stops), add to that the diffusion that is throwing the light in every possible direction and you are probably losing 60-70% of your forward facing light. If we start with a practical 150â€™ working range and cut our light by two stops, we cut our working distance down to 37.5â€™ which would still be in range of having a good exposure. Now letâ€™s be really conservative here since I donâ€™t have a Sto-fen here to test and say that its really only wasting 30% of the light and 70% is still going forward, 70% of 37.5â€™ is 26..25â€™ which is too short of a working distance to light a subject 40 feet away. All he had to do was remove the Sto-fen diffuser from his flash and he would have got instant great results.
I am not telling this story to mock him for using a Sto-fen, but you have to understand what different modifiers will do to your light and how it will affect your working distance. The following day I saw people at the final graduation event sitting in bleachers close to 350â€™ away from the subjects and having their flash turned on. Not on the best of days with the wind behind you and finding a four leaf clover will your flash give you an exposure at 350 feetâ€¦this is physics, it just aint gonna happen.
This is why you need to learn how your equipment works, try your different modifiers, figure out what the longest working distance you have is and even write it on the device. If you decide a Sto-fen device is best used at 20 feet or less, write a â€œ20-â€œ on it so there is no second guessing when you really need something to work.
Again, I am not knocking the Sto-fen diffuser here, I am simply saying that there is a time and a place for almost any modifier and knowing that some devices will give you a shorter working distance is absolutely critical to making sure you are setup properly.
Any time you get a new light modifier you need to really practice with it and figure out its strengths and weaknesses, and they ALL have weaknesses. Some are better for individuals, some are better for groups, some are better for soft lighting, some are better for dramatic lighting. The point is, they are all different and donâ€™t assume that you can take one specific modifier, put it on your flash, and never have to worry about it again..