Designed for automatic operation only, the 220EX Flash Unit excels at providing natural-looking balanced flash exposures. When attached to the EOS IX, EOS Elan II/Elan IIe/IIe, or EOS Rebel G, the 220EX incorporates the superior technology of Canon's unique E-TTL evaluative pre-stored flash exposure control system, FE Lock for a new dimension of creative control and FP Flash(High Speed Sync) for flash photography up to maximum shutter speed. When used with other EOS cameras, or the T90, it operates as a TTL autoflash (off-the-film flash metering.) When in E-TTL operation, flash exposure is measured and controlled by the camera in combination with Canon's AIM system. Subject position, ambient light level, evaluative metering and pre-flash data are instantly analyzed, including heavy backlighting or reflections from background windows or mirrors.