The Optics of Vision – Part 9: The Crystalline Lens
If you thought the way the eye works was cool before, this is going to blow your mind. You’re about to learn about the crazy wonders of the the cyrstalline lens.
This lesson builds on all of the previous lessons in this course, so please make sure you are up to speed before tackling this one.
This is the ray diagram of the eye that we are familiar with now.
The truth is that this diagram doesn’t actually show the complete picture of how the eye focuses light. This diagram depicts the cornea as being the only lens that focuses light for the eye. In fact, there are actually two structures that act as lenses.
The cornea
The crystalline lens
Here is a diagram that depicts both the cornea and the crystalline lens.
The crystalline lens is hidden just behind the iris. You cannot see it on your own as it is transparent and it is too dark in side the eyes to see.
What is the Power of the Crystalline Lens?
In past lessons, I have used +50D to represent the strength of the cornea.
This is not actually the case. In reality, the total power of the eye is closer to +60D. The cornea is about +40D and the crystalline lens is approximately +20D (at rest… more on this a little later on).
Here is a more accurate picture of the power of the eye. Keep in mind this is still a little simplified, but this is more than enough for you to know.
Review of Light Vergence & Distance
Before I reveal why the crystalline lens is so fascinating, we need to do a brief review of the vergence of light emitted from different distances.
Light that is emitted by objects is always divergent.
As the light travels away from the object it becomes less divergent.
Eventually, when the light has traveled more than 6 meters (20 feet), the light becomes parallel and stays that way.
This diagram illustrates these key points:
Have you noticed that whenever I have drawn a ray diagram of the eye, the incoming light has always been parallel?
Every ray diagram of the eye in this course so far has had parallel light entering the eye.
Why is that?
The answer is because we have always assumed that the eye was at least 6 meters (20 feet) away from the object producing the light.
So…
Q: What happens when the eye is closer than 6 metres (20 feet) away from the object it’s looking it?
A: The light rays entering the eye will be diverging (not parallel)
We are now going to study 3 diagrams that illustrate the effect of distance (between the object and the eye) on the vergence of light entering the eye and focal distance.
*Note: These are all emmetropic eyes i.e., not nearsighted, farsighted and no astigmatism.
This first eye is looking at an object situated over 6 meters (20 feet) away – hence the light reaching the eye is parallel.
This results in the focal point forming on the retina.
This means vision is clear
The eye looking at a distant object.
This second eye is looking at an object that is less than 6 meters (20 feet) away – hence the light reaching the eye is divergent.
This results in the focal point forming behind the retina.
This means the vision is blurry.
The eye looking at a near object.
This third eye is looking at an object the very close to the eye – hence the light reaching the eye is very divergent.
This results in the focal point being formed even further behind the retina.
This means the vision is very blurry.
The eye looking at a very near object.
Wait… What?
Am I trying to say that anytime we look at something up close the vision is blurry?!
If you are an emmetrope (you don’t wear glasses), you know that’s not true. You can see well in the distance just as well as you can see up close, right? But how can that be if these diagrams are accurate?
Because the crystalline lens is flexible and automatically adjusts its power depending on where you are looking!
This is called accommodation
Yes, let that sink in…
So check it out;
When looking into the distance, the image already falls on the retina, so the crystalline lens can just relax. In this state, it is said to be at rest.
When looking up close, the crystalline lens ‘knows’ that the light entering the eye is divergent, so it becomes more curved in order to increase they eye’s converging power (to offset the incoming diverging light). This keeps the focal point on the retina and the eye sees clearly.
When looking really close, the light entering the eye is very divergent, so the lens becomes extremely curved in order to compensate. Again, by doing so, the focal point remains on the retina and the eye sees clearly.
When the eye is finished looking up close and switches back to looking into the distance, the crystalline lens goes back to being at rest (which means it gets flatter again).
Accommodation
The constant adjustments that the cyrstalline lens makes in order to keep the vision clear is called accommodation.
Speed of Accommodation
Accommodation is very fast. So fast in fact that we’re not even aware that it’s happening. When we change the distance at which we are looking, the cyrstalline lens accommodates to that distance before we have time to notice any blur in the vision.
What Controls Accommodation
Accommodation is all controlled by a muscle that surrounds and suspend the lens in place. This muscle is called the ciliary muscle.
Here is a more in-depth explanation of the mechanism that controls accommodation:
Limits of Accommodation
Accommodation has it’s limits. Generally speaking, the limit is based on age. As we get older, our ability to accommodate gets worse. You can test the limit of your accommodation right now by looking into your palm and moving it closer to your eyes until it becomes blurry. The distance at which you are no longer able to see the details on your palm clearly is the limit of your accommodation.
If you were to measure and record this limit every year, you would find that it gets further and further out every year.
Eventually, accommodation runs out. But more about that in the next lesson!
