By default, when you open the camera, the pictures you take will have the macula in their center. You can change this setting by clicking the icon at the bottom of the screen.
The circles represent the available targets for fixation. The larger circle, located either on the left or right (depending on the eye being examined), signifies the optic nerve. The green circle indicates the currently selected fixation target.
To select a different fixation target, press the circle that corresponds to the part of the eye you wish to be at the center of the image. The only change noticeable will be the repositioning of the light that indicates where the patient should look.