Distance could be a factor... but in your camera's flash settings it is possible to set the flash to "disabled".
E-TTL flashes actual fire a couple of times for each shot ... but it all happens so fast you'd swear it only fired once.
In E-TTL flash mode, the camera evaluates how much light is in the scene when no flash is firing (it takes a meter reading). Then it fires "pre-flash" at low-power and takes ANOTHER meter reading (the shuter is still closed at this point) and it compares the difference between these two readings to determine how much power should be used for the shot. It then takes the shot at the calculated amount of power.
IF an external flash is used and the on-board flash is a "master" it communicates optically by having the on-board flash fire the instructions (in visible light) to the external flash and all of this happens BEFORE the shutter opens.
Then the shutter opens and the flashes fire as needed.
It is at THIS POINT (when the shutter opens) that the camera can be configured to NOT fire the on-board flash. This can be confusing to the photographer becuase they visibly saw the flash fire... they just didn't realize that it fired a fraction of a second before the shutter opened. By the time the shutter is open, the on-board flash is off and not contributing any significant light to the shot (we've previously established that if you do this by shooting into a mirror, you'll see the glow of the flash-tube as it cools down... it wont be bright enough to contribute any significant light to the shot (it can't affect exposure) but it wont be completely dark.)
That's a special case where the photographer only wants light from the off-camera flash... but they need a way to control that off-camera flash so the on-camera flash is used to communicate with the off-camera flash.
See page 183 of your manual.
Go into the menu, red-camera tab #1 and at the bottom of that page you should see "flash control" - pick that.
Make sure "flash firing" is set to "enable"
Also check page 198.... in the flash settings under "Wireless func." make sure you see TWO icons of flashes... one looks like the profile of an external flash.. the other looks like the profile of the pop-up flash. You should see BOTH icons. If you only see one of them, then it means only one flash will fire when the shutter is open.
You should read the entire section of the manual from page 181 through page 200 ... possibly a few times through to make sure it all makes sense to you.
I also recommend Syl Arena's book "Speedliter's Handbook" which is extremely good at explain the Canon flash system in a way that is easy to understand.
Unfortunately the 2nd edition is out of print (there's a 3rd editiion expected sometime this year) and those that have "new" copies of the printed version want a ludicrous amount of money (nearly $200). For some reason the ONLY e-book platform that has it is the Apple iBooks store (about $39). It IS a very good book. If you have an iPad, Mac or iPhone... I'd get it on the iBooks platform. If you want a paper copy... you might want to wait until the 3rd edition is released so you can get the latest edition at a reasonable price.
Tim Campbell
5D III, 5D IV, 60Da
