When you're taking a photo, you can control the angle and position of the camera with the 3DS's gyroscope. Various other bits can be controlled through the touch screen. The A Button opens up more options
Zoom: The zoom is controlled via the circular slider in the bulk of the screen
Focus: There's a second slider which deals with the focus of the camera, allowing you to blur out everything you don't want, or to make it crystal clear
Natural Light: Using the Y button, you can control the light levels
Shutter Size: Using the X button, you can control how open the shutter is. This will shift the focus of the photo.
When you're ready, press the R button and the photo is taken. You can then save this photo onto the SD card on your Nintendo 3DS.