how to shoot night portraits with background in exposure

thenextguy wrote in post #12963759
Won't the ambient light from the long exposure infect the flash exposure?

Well, yeah, to some extent. If there's a significant amount of ambient light falling on the subject, then it can certainly add to the flash's exposure of the subject. I alluded to this a little bit in my next-to-last paragraph, and it's certainly different for every situation, and may take some trial-and-error to get right. If the subject is standing under a streetlight, you wouldn't use the exact same settings as if the subject is standing in an unlit area. But if the background is very dim, you wouldn't put the subject under a streetlight anyway.

From the original description, with a cityscape in the background, I would think that the city lights and the sky would probably be much brighter than the subject.

And it depends on how long the "long" exposure is. Relative to most of the photography that I do, 1/2 or even 1/5 second is a "long" exposure. But it's not like a 30-second long exposure.

Of course, if you put your subject under a streelight and shoot a longer exposure, the streetlight will add to the flash expsure because light is additive. In reality, this is one of those times where no one will EVER be able to give the settings to use unless they are actually at the shoot, looking at the camera. If one understands how flash exposure, ambient exposure, ISO, Aperture and shutter speed interrelate to make the final single image, this scenario shouldnt be difficult at all. This is not a shot at you, OP, but just a fact. Here is what needs to be done.


Put camera in manual mode. Find an ISO and an aperture that will allow your background to be properly exposed while underexposing your subject while maintaining the depth of field that you desire. Now, turn on your flash and either use a light meter (the best/fastest way) or rear LCD (not the best way) or the histogram to get proper subject exposure. If you are using on camera flash, ETTL will probably over expose the subject at first since the scene is dark. You may need to dial in some -FEC (flash exposure compensation) to reach that exposure. If youre using manual flash, youll need to adjust the power of the flash up or down until you reach proper subject exposure. Keep in mind that if you change any of your other settings, mainly the aperture or ISO, after the orginial settings are found, you will need to change your flash power. There is a good bit more to understand here but this is the basic starting point.