custom function 14 on eos 1n

in cf 14 of 1n there is to option fill-in flash or full power flash.

which of this function i must set to my camera?

Custom Function 14 does NOT allow selection between fill-in flash and full power (normal) flash. It enables or disables the Auto Fill Flash Reduction program. Whether you are or are not in a fill-flash mode depends on what exposure mode you are in. E.g., in Av and Tv modes, you are always in fill-flash mode as the camera sets exposure for the ambient light conditions. In P mode, you will be in fill-in flash mode only under bright ambient conditions.

All EOS cameras have a built-in program designed to provide natural looking flash fill (and avoid the "deer in the headlights" look)in most bright conditions. The �Automatic Fill-flash Reduction Control� program varies flash exposure according to ambient light level, applying no compensation in dark conditions, (below EV 10) where the flash provides the primary illumination, and proportionately reducing flash exposure in bright conditions as the ambient light level increases. Any flash exposure compensation set by the user on the camera or Speedlite is applied in addition to the automatic fill-flash reduction control program. Although the program generally works well, for occasions when a subject is strongly back-lit by the setting sun or an overhead light source, which may result in underexposure of the subject, the photographer can set Custom Function 14 to cancel the automatic fill-flash reduction control. This will also appeal to users who wish to apply their own exposure compensation settings in order to gain greater control of the results.

My advice is to shoot some pictures in flash fill mode and see if the auto fill program gives you the kind of results you want. If not, then set C.Fn 14 to 1 and experiment with different settings of FEC. However, be advised that the results will depend largely on the ambient light level and how contrasty the light is, among other factors.