By Sunny Chir

For the benefit of all I have written it out how I use Evaluative metering and Fill-Flash for my shots.

Evaluative /Spot Metering

Evaluative metering compute for an overall exposure for the ambient light whilst spot metering is metering emphasising a small percentage of the overall scene ,where your active aim point is pointed. If spot metering is used , it will reduce ambient light to come through as the metering is likely on the butterfly which is a small percentage of the overall scene. This could lead to an overall darker picture. I always use evaluative metering. Fill- flash/ 2nd curtain flash/ Rear syn Flash

When the shutter button is half depressed:
Firstly , evaluative metering compute the overall scene( including background) the camera metering system compute what is the proper exposure for the overall scene. At the same time the flash also meter the subject in the foreground and pre-compute the amount of flash needed for the foreground subject .

When the shutter is fully depressed :
the 1st curtain opens and allows the computed duration of ambient light by the metering system to be registered on the sensor. For the fill-flash . As the shutter is depressed fully, a pre-flash is fired , the reflected light of this PREFLASH is used to further refine the appropriate amount of flash to be fired ( on the Canon flash system) to illuminate the subject in front. After the ambient light is registered , and just before the 2nd curtain of the shutter closes, the flash is fired and illuminate the foreground subject. That is why some call fill-flash as 2nd curtain flash or Rear Syn flash.

Fill-flash/ 2nd curtain/rear syn flash photography is effectively a two exposure process.

I hope the above explain better how the fill-flash and evaluative metering works together to give a brighter backdrop.