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| p.1 #4 · how to black out the background? |
The flash should auto-compensate as needed as you get closer. ETTL on Canon flashes can be a little finicky though so if it starts to under- or over-expose, just apply a little positive or negative Flash Exposure Compensation (FEC).
Since you're using off-camera flash, you can also try moving that light high (maybe 45-60 degrees to the camera-to-subject line, aim down, and add your own home-made snoot/gobo/flag to block the light as much as possible from the garage wall.
The flag/snoot can be out of black construction paper or something similar. Google "black foamie thing" and you will get the idea. A gridded soft-box would probably be ideal for this scenario; I am not suggesting you go out and buy one - just that the combination of large, diffuse but directed/controlled light would be a good way to think about this.