No, it's not a magic trick. Just a nifty photography tip I thought I'd share with everyone. If you're ever out doing a model shoot at the local park at evening with an on-camera flash (such as a speedlight) or any form of flash, this could be helpful to you. Let me say as simply as possible. Aperture controls flash, shutter speed controls ambient light. Notice my image below; I kept the same aperture and my subject, lit by flash, remained the same exposure. Changing the shutter speed only affected the ambient light in the background. It's pretty logical if you think about it. Whether your shutter speed is 1/100th or 1/15th, the flash will fully fire in that amount of time. So, adjusting the shutter speed does not affect the flashes output. However, the aperture could be adjusted to let in more or less light. So, aperture can affect flash. It's a pretty simple idea, but sometimes the technical aspects can be more confusing. So here is a video by AdoramaTV, that shows a more in-depth example. So just remember: Aperture = flash, shutter = ambient.