Figure 1: A young emmetropic eye with an efficient focusing system. Swipe right and left on the image to see how this eye is able to maintain a tight pinpoint of light on the retina at all distances. The gauge shows how much focusing effort is required at each distance, ranging from 0 - 8 untis of focus. This eye can see clear at all distances, with only a moderate amount of focusing effort required for the closest distances.
As far as creating accurate, simple, and easy to remember terms to describe common eye and vision related concepts, the eyecare world has failed miserably. Farsighted vs nearsighted, glaucoma vs cataract, and transitions vs progressives are just a small sampling of common confusions that I hear every single day (don't even mention the topic of astigmatism, which is on a whole other level of confusion). But the confusion that comes with defining a person's eyesight (i.e. nearsighted vs farsighted), seems to be the most prevalent mix-up, so let's see if we can help clear things up.
One concept in the eyecare world that is almost universally understood is that 20/20 signifies very good vision (although when people start changing the numerator in an effort to describe their acuity, things go off the rails fast: "I used to have 15/20 vision when I was a kid". No you didn't, stop). But the idea that 20/20 is a term to describe good vision is not only well accepted, but it is actually pretty accurate. There are numerous ways to define what 20/20 actually means, but one way that I find the easiest to comprehend is:
[the maximum distance I can just barely see an object at] / [the maximum distance an average, good sighted person can see just barely see an object at]
So if I have better than average vision, I might be 20/15, which means an object that I can spot at 20 ft away, an average good sighted individual would have to take a few steps forward, and only begin to see the same object when they are 15 ft away. It's pretty easy to realize in this example that my vision would be superior to theirs if the numerator is larger than denominator, since that would imply that I can see something further back then they can. Conversely, if I have really poor vision, I might be 20/400, which again means that an object I can see at 20 ft, a normal sighted individual would've been able to see at 400 ft. But what is going on inside the eye to determine who can see good and who can't?
In the absence of any pathology that could be affecting vision (like cataracts), the primary dictator of if you can see clearly at a certain distance or not is how tightly your eye can focus the light that originated at the target onto the retina (the retina is analogous to the camera film of the eye). Just like a well-positioned magnifying glass on a hot sunny day can start a fire if it perfectly condenses the sunlight into a tight pinpoint onto a piece of wood, a well-focused eye can condense an image originating at a long distance perfectly onto the retina, allowing the eye to see the image clearly. If the magnifying glass is too close or too far from the wood, then it will be very difficult to create enough heat to cause combustion. And similarly, if the retina is too far or too close to the front of the eye, then instead of a pinpoint of light being shone on it, a larger diffuse circle of light will hit. This is what causes images to blur.
Now before we go any further, there are 3 concepts involving light and optics that need to be made clear:
Figure 2: Light originating from long distances away (top image) tends to be travelling parallel by the time it hits an eye 20 ft or further away. Light from closer distances tends to still be diverging (spreading apart) when it meets the eye (bottom image), and therefore is harder to corral and focus.
- The closer the object is, the more focusing power that is required to keep the object in focus. The further away an object, the less focusing power that is required. This can be explained based on the direction the light rays are travelling in relation to each other as they leave the object. When positioned very close to an object reflecting or emitting light, the light rays can be described as travelling away (diverging) from each other. When positioned further from an object, the light rays that make it to this distance and location must more or less be travelling in the same direction (and parallel to each other), or else they never would’ve made it there it in the first place. Think of the spray pattern a can of whip cream exploded next to a wall would create. Not only would there be whip cream everywhere, but the spray pattern on the wall would show globs of whip cream in a splatter formation that radiates out, all pointing back to their origin (the can), signifying that each glob was spreading away from each other glob. Now think of how the spray pattern would differ if that can was 20 ft away from the wall. At that distance, any whip cream that managed to hit the wall would have to be more or less travelling in the same direction, with each glob travelling relatively parallel to all the globs around it, creating a spray pattern that would show very little difference in the splatter appearance of each of the globs. It should make intuitive sense that in general, it's easier to corral and redirect any objects that are travelling towards a target that are parallel to each other than it would be to corral objects that are spreading away from each other as they move towards a target (not only light rays but this goes for whip cream, hockey pucks, cattle, etc…)
- At distances greater than 20 ft, the light rays that are originating from an object are essentially now travelling parallel to each other, and any further distance won't make the light rays travel “anymore parallel” and become any easier to focus. This is why 20 ft and further is termed “distance vision” and why the number 20 is used so frequently in the eyecare world (i.e. 20/20). One can assume that if you see an object clearly at 20 ft, then you will likely (if the target is large enough) see it equally clearly at a 100 ft or more. Any closer than 20 ft and the light rays start to show increasing signs of diverging from each other (see # 1 above), and the closer the object gets, the more diverging (and therefore harder to focus) the light rays become (Figure 2).
- For the eye to see clearly, the light rays it corrals need to be focused into a tight pinpoint that falls precisely on the retina. The smaller and tighter the pinpoint of light is when it hits the retina, the clearer the image will be. If the light hasn't fully finished focusing before hitting the retina, then a diffuse circle of light will hit the retina instead of a pinpoint, causing blur. If the light focuses to a pinpoint too early and prior to hitting the retina, then it will begin to invert and start to spread again, ultimately leading to a blur circle by the time it does hit the retina.
With a basic understanding of these three concepts, it becomes much easier to understand and remember the basic properties of the three major vision classifications: Emmetropia (perfect eyesight), Nearsightedness (myopia), and Farsightedness (hyperopia).
The Emmetropic eye
It's all relative. Depending on where the light focuses in relation to the retina, one can say either the eye has too much or too little focusing power, or conversely, the eyeball is either too long or too short. Both methods accuratley describe the relationship between the lights focal point and the retina.
The emmetropic eye (perfect sighted eye) is an eye that has just the right amount of focus power to focus parallel light rays (remember, parallel light rays come from any object 20 ft away and further), into a tight pinpoint of light placed exactly onto the retina. I like to think of this as a movie theater where the projector is the exact distance away from the movie screen, allowing for a high definition image to be projected onto the screen. The emmetropic eye can view objects 20 ft and further for hours on end, and will not suffer any fatigue, since this is the optimal distance for this eye and it is naturally tuned to it, therefore exerting no effort to keep the light in focus. Things start to change however, when the target object begins to move closer than 20 ft. Knowing that objects that are closer than 20 ft begin to emit light rays that diverge or spread away from each other, the emmetropic eye will tend to be unable to focus light into a tight pinpoint onto the retina at any of these closer distances. At first, the light rays will only be slightly out-of-focus with the light cone beginning to focus slightly behind the retina, but the light rays will become exponentially harder to focus the closer the target gets, and ultimately any object within about 3 ft will be very out of focus (Figure 4). Because of this phenomenon, the eye needs to be able to increase its focusing power on-demand to allow the eye to see objects closer than 20 ft. The system the eye utilizes for this is termed the "accommodative system" and it is composed of a muscle (the ciliary body) and a flexible lens (the crystalline lens) which the muscle can squeeze and contort to create a temporary increase in focusing power. In a young eye (i.e. age 5 to age 18) this system is very efficient due to the lens being very flexible and therefore requiring the muscle to only have to exert a small effort to affect a big change in focusing power. Without caring too much about what the units of measurement are, we can say that the young eye can increase its focusing power up to about 10 units. These 10 units are more than sufficient to allow the emmetropic eye to keep an object clear and the light cone in-focus on the retina from distances from 20 ft all the way up to about 6” away from the eye (Figure 1). The use of this system does require energy though to keep the muscle working, and sustaining focus at very close distances can begin to deplete it, leading to eyestrain even in a young, efficient accommodative system.
Things begin to change as the eye ages, as the crystalline lens becomes progressively less flexible, making the muscle used to manipulate it exert progressively more force to perform the task of increasing the eye’s focusing power on-demand. This phenomenon eventually leads to a condition termed presbyopia (starting at around age 40 and ending at around age 55), in which the focusing system has weakened to the point were routine near tasks like reading a book become too blurry to manage (Figure 3). Instead of the robust focusing system of 10 units seen in the young eye, the early presbyopic eye has only around 3 - 5 units remaining, while a late presbyopic eye can be assumed to have zero focusing units remaining (Figure 4). Besides the obvious fact of the presbyopic emmetropic eye being unable to focus at any close distances that require more focus than it can muster up, the eye also begins to suffers from increased frequency of eyestrain at more normal distances (like the computer) since to keep these distances clear, the eye is having to call upon all of its power, leaving nothing in reserve .
Figure 3: An aging (early presbyopic, age 40) emmetropic eye with an inefficient focusing system. Swipe right and left on the image to see how this eye is still able to maintain a tight pinpoint of light on the retina at all distances further than 1.5 ft. Closer distances than 1.5 ft require more focusing effort than the eye is able to generate, and as a result these distances are blurry. The gauge shows how much focusing effort is required at each distance, with the max focus effort being reduced to only 4 units at this age (compared to 8 units in the young emmetropic eye).
Figure 4: An aged (late presbyopic, age 55) emmetropic eye without any remaining focusing system. Swipe right and left on the image to see how this eye is still able to maintain a tight pinpoint of light on the retina at all distances further than ~ 13 ft. Closer distances than this require increasing amounts of focusing effort that this the eye is unable to generate, and as a result these distances are blurry. The gauge shows that no focusing system is even available.
The Nearsighted Eye
Figure 5: A young nearsighted eye (approx. prescription -4.00) with an efficient focusing system. Swipe right and left on the image to see how this eye only begins to achieve a tight pinpoint of light on the retina at ~ 1.0 ft. The gauge shows how much focusing effort is required to keep the image clear at distances closer than 1.0ft. This eye can see clear at all distances closer than 1.0 ft, with only a moderate amount of focusing effort required for the closest distances. Distances further then the optimal distance of 1.0 ft are blurry, and there is no changes the focusing system can impart to help improve vision at these distances.
The defining feature of the nearsighted eye (myopia) is that even in its relaxed state it focuses light too strongly. This ultimately leads to the light rays focusing in the eye sooner than they would in the emmetropic eye. When looking at a target at 20 ft or further, where the emmetropic eye would focus these parallel light rays perfectly onto the retina, the nearsighted eye would focus these light rays too quickly, ultimately causing them to focus into a pinpoint before the retina, at which point they begin to slowly diverge before hitting the retina in a blur circle. Said another way, a nearsighted eye viewing objects at 20 ft or further will see blur, since it basically has too much focusing power for this distance (Figure 5). This trait, however, can be used beneficially since extra focusing power is exactly what is needed to keep objects that move closer and closer to the eye focused onto the retina. In the emmetropic eye, this is accomplished by harnessing the accommodative system (and expending energy in the process), but in the nearsighted eye at the right distance, a close up object will focus perfectly onto the retina without expending any energy. In fact this “optimal” distance will be the inverse of a nearsighted person's prescription, so if the eye is determined to be a -3.00 prescription, the distance of perfect vision of this eye with no extra focus required (or glasses) would be 1/3.00, or 0.33 m. And just like in the emmetropic eye, the young nearsighted eye can also harness the focusing system to keep objects closer than this optimal distance also clear. Even when the focusing system begins to fail in presbyopia (Figure 6), the nearsighted eye will always be able to see clearly at its optimal distance, similar to how the emmetropic eye will always be able to see at its optimal distance of 20 ft and further. Its only the areas that lie closer than these optimal distances that will diminish in clarity with age (Figure 7).
Figure 6: An aging nearsighted eye (approx. prescription -4.00, age 40) with an inefficient focusing system. Swipe right and left on the image to see how this eye still only begins to achieve a tight pinpoint of light on the retina at ~ 1.0 ft. The gauge shows how much focusing effort is required to keep the image clear at distances closer than 1.0ft, with an age reduce range of only 0 - 4 units of focus available. This eye can still see clear at all distances closer than 1.0 ft, but at a significant increase in focusing effort as compared to the young nearsighted eye. Distances further then the optimal distance of 1.0 ft are still blurry.
Figure 7: An aged (late presbyopic, -4.00 prescription, age 55) nearsighted eye without any remaining focusing system. Swipe right and left on the image to see how this eye is still able to maintain a tight pinpoint of light on the retina at its optimal distance of 1.0 ft. Closer distances than this require increasing amounts of focusing effort that this eye is unable to generate, and as a result these distances are blurry. The gauge shows that no focusing system is even available, and this eye can only see clearly at its optimal distance of 1.0 ft.
The Farsighted Eye
Figure 8: A young farsighted eye (approx. prescription +4.00) with an efficient focusing system. Swipe right and left on the image to see how this eye is able to achieve a tight pinpoint of light on the retina at distances greater than 0.8 ft. Due to a farsighted eye having no optimal focusing distance, this eye requires a focusing effort at all distances, with the closer distances requiring more effort than the further distances. The gauge shows that even at 20 ft+, the eye needs to harness 4 units of focus to see clearly, with distances closer than this requiring an eventual extreme, unsustainable level of focus, which would lead to eyestrain. Distances closer than ~ 0.8 ft require a level of focusing effort (> 10 units) that not even a young, efficient focusing system can achieve, and therefore these very close distances are blurry.
By far the most difficult of the common eye configurations to explain is farsightedness (hyperopia). A farsighted eye is an eye that is unable to focus even the easiest, most parallel light rays onto the retina (again, a final reminder: light rays from 20 ft or further require the least amount of focusing power). Because the eye lacks sufficient innate focusing power, it subsequently focuses light rays too weakly, and the pinpoint of light required to see clearly begins to form behind the retina instead of on it (contrast this to nearsightedness where the light rays focus in front of the retina). If a farsighted eye had no additional focusing power to call upon (i.e. in late presbyopia), then it would see all objects blurry regardless of distance (with the further objects being less blurry than the closer objects, Figure 10). In contrast, the young farsighted eye doesn't suffer from any of this blur, but is required to rely heavily upon the focusing system to make up for what it lacks in innate focusing power in an effort to keep objects clear (Figure 8). Whereas the emmetropic eye only slowly begins to call upon the focusing system at distances closer than 20 ft, the farsighted eye needs to call upon the focusing system at 20 ft plus, and it increases its need as the object moves closer. As mentioned above, if the young eye has about 10 units of focus available, then the young farsighted eye will begin to eat into these 10 units far quicker than the emmetropic eye because of this requirement. For example, if an eye has a prescription of +4.00, that suggests that if they aren't wearing their glasses, they will be required to steal 4 of the 10 focusing units from the accommodative system in an attempt to see far away, leaving it with only 6 units in reserve to be utilized for objects closer than 20 ft. As an object gets closer and closer eventually all 6 of these units will get called into action, leaving the eye under tremendous stress. If the object continues to move closer still, the farsighted eye will be unable to muster up any further power, and the object will be blurry. This tremendous focusing effort farsighted individuals exert is one of the reasons why a not insignificant number of farsighted people have “lazy eyes”, as the eyes start to cross under the focusing pressure. Farsighted eyes are also trickier to diagnose, because they do have the ability to keep objects clear without glasses for as long as there is enough focusing system available. It's the strain and cracks that begin to show in the focusing system, especially as the eye enters early presbyopia that typically tips off the optometrist that perhaps this eye is working too much (Figure 9).
Figure 9: An aging farsighted eye (early presbyopia, approx. prescription +4.00) with an inefficient focusing system. Swipe right and left on the image to see how this eye is only able to achieve a tight pinpoint of light on the retina at distances greater than 7.0 ft. Due to a farsighted eye having no optimal focusing distance, this eye requires a focusing effort at all distances, with the closer distances requiring more effort than the further distances. The gauge shows that even at 20 ft+, the eye needs to harness 4 units of focus to see clearly, which in early presbyopia is almost the entire amount of focusing ability the eye has available. Therefore at distances closer than this require a level of focus that this aging eye is unable to muster, and are therefore blurry.
Figure 10: An aged farsighted eye (late presbyopia, approx. prescription +4.00) lacking any remaining focusing system. Swipe right and left on the image to see how this eye is unable to achieve a tight pinpoint of light on the retina at distance. Due to a farsighted eye having no optimal focusing distance, this eye would require a certain level of focusing effort at all distances, which it is unable to accommodate due to having no focusing system left to call upon. At this age, all distances to a farsighted eye are blurry, with closer objects being more blurry than distance objects.
In summary, it can be said that the main defining features of each optical prescription is that nearsighted eyes focus light from long distances too much, farsighted eyes not enough, and emmetropic eyes focus light just right. Or said another way, nearsighted eyes are too long in length (since light focuses in-front of the retina), farsighted eyes are too short, and emmetropic eye are the perfect length. Since the focusing system degrades at a fairly predictable rate regardless of type of prescription, all the eyes experience a decrease in their range of clear focus as they age, with farsighted eyes experiencing the greatest reduction (since they rely on the focusing system the most) and nearsighted eyes experience the least. Finally, the goal of wearing glasses is to make each eye, regardless of the underlying prescription or age, have the range of the young emmetropic eye, so a +4.00 hyperope would be given +4.00 glasses in effort to reduce the focusing demand, while a -4.00 myope with presbyopia will be given -4.00 glasses in effort to nullify the extra focusing power of the eye and allow it focus clearly at long distances, in addition to a bifocal in effort to replace some of the lost focusing system.
Dr. Burke is an optometrist practicing at Calgary Vision Centre. All images have been created for himself in attempt to make this topic easier to understand. Opinions above do not constitute medical advice, and readers should consult with their optometrist if they have questions or concerns about their eye health.