The AF-Beam is is used while the camera focuses but before it meters. Otherwise, metering gets affected and significantly so in condition when the AF-Assist beam may help.

Since in any type of Continuous AF this is not usually the case, the AF-beam must be turned off. The camera must also keep focusing, so the AF-Assist beam could create a situation the camera knew were to focus but no longer can if it were to be one at the beginning.

The AF-Beam is is used while the camera focuses but before it meters. Otherwise, metering gets affected and significantly so in condition when the AF-Assist beam may help.

In any type of Continuous AF this is not usually the case, so , the AF-beam must be turned off. A camera must also be able to keep focusing, so the AF-Assist beam could create a situation in which the camera knew where to focus but no longer can, should the AF beam be on but later turned off.