How to control brightness of the background when using flash

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By just changing your ISO when using your flash you can change the look of the background. With the camera on a tripod and using Aperture priority mode all I did between these three photos was change the ISO from ISO 100 [far left], ISO 400 [center] and ISO 2000 [far right].

I have the flash off camera to the left as you see in the diagram below. The flash setting is normal mode. The flash is in TTL model so that it is adjusting as needed to the scene.

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In these three I did the same thing and just changed one setting. The flash mode is set on "slow sync mode".

As the ISO goes higher the odds of you blowing out the subject with the flash will go up. As you lower the ISO the darker the background.

As you change the ISO the only other setting changing since I am in Aperture mode is shutter speed. Now unless you want a blurred image you need to keep that shutter speed close to the focal length of the lens. If you have a 35mm lens then try to shoot at 1/30 or faster. If you are shooting with a 200mm lens then you need to be at 1/200 or 1/250.

For these photos I have the flash on the camera with white dome and bounced. The reason the photos are so similar is the shutter speed adjusts to make the background match the foreground subject.

If these were not on a tripod you would see much more blur with the first on shot at 1/6 shutter speed.

ISO 100, ƒ/1.8, 1/6

ISO 400, ƒ/1.8, 1/25

ISO 6400, ƒ/1.8, 1/500

My suggestion is to do a few test shots in a room with your eye paying close attention to being sure background is the ratio of brightness compared to the subject and that the shutter speed is high enough to give me a sharp image. This is example here where I moved the camera during the 1/3 shutter speed. You may want this look.

Are you controlling your camera or is it controlling you? The more you understand how the camera works the more creative you can be and decide yourself what the look will be in the final product.