B Notes About Using the Remote Flash Unit
•Set up the remote flash unit in a position close to the subject from the camera to ensure the light from the Speedlight to enter the sensor window.
•Aside from monitor preflashes, the master flash unit emits a minute amount of flashes to control the remote flash unit. When shooting a subject at a short distance from the master flash unit, these minute amounts of flashes may cause adverse effects and the proper exposure may not be achieved. To reduce such adverse effects, use the wide panel for the master flash when the master flash unit is directly facing a subject that is being shot at a short distance.
Section Reference
E112