The 540EZ's circuitry has been designed to accept the new AA-size lithium batteries in addition to the alkaline-manganese or rechargeable Ni-Cd batteries. The lithium AA batteries are recommended when using the Speedlite 540EZ in colder temperature or when you know you will be shooting lots of films with flash in your assignments since they have a longer lasting shelf life compared to alkaline or Ni-Cd batteries. The 540EZ uses four AA-size alkaline batteries. AA-size Lithium batteries can also be used. My preferred shooting mode when the EOS-1N or EOS-1N RS cameras is used with the Speedlite 540EZ is with Shutter-priority AE for fill-in flash in most indoor situations. Programmed A-TTL flash is only used mainly for fill-in flash during bright outdoor shooting conditions for the shadows. The Speedlite 540EZ utilised fill-in flash mode when it is used with any EOS camera in both the Shutter-priority AE and Aperture-priority AE modes, outdoors or indoors. Fill-in flash in the Manual exposure mode is possible with the TTL mode for the subject shot in outdoor conditions. To use fill-in flash with Manual exposure mode indoors, the camera may recommend a shutter speed of 1/8 sec. or slower if the aperture selected is f/5.6 (at ISO 100). Ignored the camera's recommendation and you will not be able to get fill-in flash should you persisted in shooting with your choice of say, 1/60 at f/5.6 using TTL flash in the Manual shooting mode. The camera's suggestion is basically to mimic fill-in flash as though you were using the flash in either the Shutter-priority AE or Aperture-priority AE modes. To get an idea of how the lighting effect differs when the Speedlite 540EZ is used in the Shutter-priority AE (TV), Aperture-priority AE (AV), Programmed AE and Manual exposure modes, below are four photos showing their effects. Photo 1: Shot in TV mode. Photo 2: Shot in AV mode. Photo 3: Shot in Programmed AE mode. Photo 4: Shot in Manual exposure mode. << Back to Main . 1 . 2 . 3 . 4 .