External images are important for the consultant reading the images because they can communicate a variety of potential issues such as pterygium (scarring of the cornea) and cataract. It is important for the external image to contain the entire eye, meaning that the eye takes up as much as the screen as possible without cutting off the corners of the eye.
Once the eye is properly centered and taking up the appropriate amount of space on the screen, the image needs to be brought into focus. Twist the blue focusing knob until the dot becomes as thin a ring as possible. If the dot never becomes a ring, but instead appears as an opened ring, the pupil might not be in the exact center of the image. Try moving the camera slightly in the direction of the ring opening; now the opening should close. For example, if the ring is open on top, twist the knob on the joystick to raise the the pupil to the center of the screen. If the pupil is now centered, the ring should be fully formed.
Occasionally, I will have a difficult time turning the dot into a ring no matter what adjustments I make. If a patient's eyes are dry or he/she is wearing contact lenses, it may be difficult to see the dot. In these instances, I rely on the sharpness of the iris on the monitor. I find it easiest to determine sharpness of the eye by first twisting the focusing knob away from me to make the image very blurry. Then I slowly twist the knob back toward me until the central eye details become focused. If you have any questions about taking external images, please contact us at firstname.lastname@example.org.