When setting up your photobooth, make sure that your backdrop is 65" to 72" away.
You can set it further back than that, but that will cause the light to be further away from the subject, thus, creating a darker image. You can adjust this by setting Av: to 8.0 or lower and/or increasing the ISO to 800 or 1600. Increasing the ISO will cause a loss of quality in your images. We don't recommend going higher than 800 if you're using the Canon T3i.