Vision is a very amazing and complicated thing. To try to help make better sense of it all we are going to post a few articles on how different parts of the eye work with the hopes that this will help explain some of the visual process. There are several parts of the eye that contribute to helping you see but we have to start somewhere so we decided to start with the function of the retina. Clearly we can not write comprehensive articles on all the functions but we will do our best to scratch the surface of each topic and give a general understanding of how each component works.
What Is the Retina?
The retina is the photosensitive (light sensitive) tissue that covers approximately 65% of the interior surface of the eye. The retina is not attached to the choroid layer that it lays directly against, but is held in place by the pressure of a jelly-like fluid known as the vitreous humor that fills the chamber of the eye behind the lens.
How Does the Retina Work?
The retina works a lot like camera film. It takes the visual information made by light reflecting off of objects and converts that information into an image. It then transmits that image to the brain through the optic nerve.
When a light enters the eye, it passes through multiple structures like the iris, pupil and lens. These structures bring that light to the photosensitive retinal layer. This layer houses all of the photoreceptor cells, called rods and cones which generate a nerve impulse that when received and translated by the brain gives us the images we see.
Rods are the specialized cells that allow us to see in dim lighting. They are ultra sensitive to dim light (enabling us to see gradations of blacks and grays in low light settings), as well as very sensitive to high-speed movement.
Your retina contains about 125 million rod cells, giving you both a very acute awareness for anything trying to sneak up on you, as well as the ability of twilight sight. Rods are completely insensitive to red light frequencies, so if someone were to shine a red frequency light in your eyes after your eyes had adjusted to the dark, your night vision would be undisturbed. This is why navigational instruments use red lights for night illumination and dark rooms used red lights for developing film.
Cones are the specialized cells that allow us to see color. They are great at sensing bright light and detailed colors but are insensitive to low lights and high speed.
You have three different types of cone cells that are each sensitive to either red, green or blue frequencies of light. And, while there are only about 5-7 million cone cells in the retina (compared to about 125 million rods), when they are excited in various combinations, they allow us to see a range of 7-10 million different shades of color! Cones exist in the highest concentration (about 200,000 per square millimeter) in an area of the retina called the fovea centralis.
The fovea centralis is a small indentation of the retina tissue that contains a very dense collection of cones (no rods) and is responsible for your detailed sight. So when you focus on something by looking directly at it your eyes are centering the light entering your eyes on the fovea centralis.
Here’s a great little 2 minute video for a little more detail on how the retina works.
At Schmidt’s Optical and Hearing™ we want all of our customers to be as educated as possible on their visual health and will continue to do what we can to help them do so. Come in today and talk with one of our many Licensed Opticians about any questions you have regarding your glasses or your vision.