Night portrait with noiseless in focus background [duplicate]

Question

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 ?

Answer 1

When shooting in the studio, using 100% flash to light the subject, you quickly learn that your shutter speed is unimportant. Why is it unimportant? Because regardless of whether it's 1/60 or 10 seconds - if the flash doesn't pop, you have only a dark room.

So, how does one control the exposure when the shutter speed doesn't matter? By controlling the power of the flash and the aperture on the lens. More power or wider aperture = more light into the camera = brighter exposure.

Now, take this concept out into your cityscape. Set your exposure for the cityscape as needed. Adjust your flash power until you have a proper exposure of your subject at the same aperture as needed for your cityscape.

Here's a good article on mixing lighting and a good video explaining how to do exactly what you're asking.