Some problems that you might encounter and solutions:
Here is a brief description of some potential problems with their solutions.
There is no exposure automation since you will be using non-dedicated strobes. The exposure has to be set manually. So, first set your camera to manual exposure mode. Hence, choose a suitable shutter speed to control the ambient light. Once you set this, select an aperture and take test shots. Adjust the aperture using histogram and highlight warning tools. Never judge the exposure by simply looking at the playback screen! You can and also should adjust the output level of your built-in flash (in case you are using it as a part of illumination) and the slave strobes to achieve the lighting you want.
Setting Exposure: Unwanted light from built- in flash:
If you do not want the built-in flash to contribute any illumination, keep the output level of your builtin flash at minimum. D-SLRS allow built- in flash to fire at 1/ 64 (or even 1/ 128) of full output level and this is normally more than enough to trigger slaves. However, even low output can cause unwanted shadows. To eliminate all light from the built- in flash while triggering you can place a baffle or gobo ( like a black card) in front the built- in flash. This will usually eliminate the light from built- in flash. There is also a device (SG-3IR) available for this purpose ( Picture 3). Though made by Nikon it can be adopted ( with some modification) for use on Canon D-SLRS too.
Positioning of slaves, the line of sight issue:
You can have a problem if there is no “line of sight” (due to an obstacle) between built-in flash and the slaves. You can try moving the slave till there is a line of sight path between it and the built-in flash. This is why, a slave strobe must have a tilt and swivel head for flexible placement.
Your D-SLR does not have the right type of built-in flash or does not have a built-in flash at all:
Your D-SLR’S built-in flash must work in manual mode for proper operation. If it cannot, then it will give out preflashes and these will trigger the slave strobes prematurely. One way to overcome this would be to use digislaves that are capable of detecting and ignoring the pre-flashes. If you don’t have digi-slaves or your D-LSR does not have a built-in flash at all, then you can do one of the following:
Use an infrared trigger ( IR) on your D-SLR. With this you don’t need any special receivers on the strobes since most strobes with an optical sensor will trigger with IR as well.