Flash and metering for Night Portrait

[Abbot Man]

you understanding of front and rear sync flash is incorrect, flash is able to freeze action, since the duration of flash much faster than most camera shutter speed. and flash does not increase camera shutter speed.

anyway, for the situation here, the flash is use to illuminate the subject, the long exposure is to record the background.

That is to say a flash photograph actually consists of two separate exposures; one for ambient light and the other for flash - the camera shutter determines outcome of the ambient exposure and the flash lights up the subject.

 

[catchlights]

Ok, here is the spec of SB900

Flash Duration

• 1/880 sec. at M1/1 (full) output
• 1/1100 sec. at M1/2 output
• 1/2550 sec. at M1/4 output
• 1/5000 sec. at M1/8 output
• 1/10,000 sec. at M1/16 output
• 1/20,000 sec. at M1/32 output
• 1/35,700 sec. at M1/64 output
• 1/38,500 sec. at M1/128 output

given the short duration of flash, flash will freeze action,
when the flash mixed with ambient light, and the ambient light is well exposed but shutter speed is not fast enough, it will create ghost image, a sharp image (recorded by flash) with light trails (recorded by ambient light).