Hi, Asher:
Here's an excerpt from a post I made on the RG forums a couple of years ago that may be helpful to you:
Focusing aids: EOS-1D class DSLRs and the EOS 5D do not have a built-in AF Assist beam. The EOS 10D, 20D and 30D do, but it exists in the form of a stroboscopic burst from the built-in flash. This method can be effective over a limited distance range, but it is annoying to most human subjects, and thankfully unavailable when using an EX Speedlite. For best low-light AF performance with these cameras, we strongly recommend the use of the near-IR AF Assist beams built into EX Speedlites and the Speedlite Transmitter ST-E2. These devices project a red, patterned beam onto the subject and are effective over varying distance ranges depending on the accessory you choose. Speedlite 550EX (or 580EX) has the most powerful AF Assist beam. It is typically effective at distances up to approximately 10 meters/33 feet or more. However, the camera MUST be set to One-Shot AF in order for the AF Assist beam to function. Speedlite Transmitter ST-E2 provides an AF Assist beam without firing a flash, so it’s useful for non-flash photography in low light. If that is your objective, be advised to stay away from Program mode on the camera, since the lowest shutter speed will be limited to 1/60 second, resulting in massive underexposure in low light without flash. Pick any other exposure mode, but stay away from Program in low light unless you are using flash.
Best Regards,
Chuck Westfall
Director/Media & Customer Relationship
Camera Marketing Group/Canon U.S.A., Inc.