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We know to take a noiseless night landscape/cityscape, we need use a tripod and set a low ISO. For example in a scene I use ISO 100, aperture f/8.0 and exposure time 30 seconds, I get a night cityscape bright enough and very clean.
My question is, in the same scene described above, if I want to take a portrait and have the same image quality for the background cityscape, how to achieve this ? I can use flash, tripod. Don't forget when focus on the person's face and want have a background in focus, I need use aperture f/8.0 or smaller.
I know slow sync flash, but it will set something like shutter 5 seconds and ISO 800, so the background is not as good as the photo taken with ISO100 and the people maybe blurred a little.
Or some ways in post processing ?