How near vision long-term activity causes myopia & eyestrain
Natural Sight to far distance: eyes are parallel
(no convergence) and IOL is flat
The best for our eyes health is to gaze to far distance so that the intra-ocular-lens (IOL) is in flat state (no accommodation) and object is fixated by fovea (central retina location with most dense population of cons)
Human eye, similar to video camera, uses optics in order to focus objects on the retina (similar to image sensor). Brain controls focusing and processing visual information in a same way a camera processing unit does. We recognize objects via tiny sensory area on the retina - named Fovea.
Optics of an eye, cornea and intra ocular lens (IOL) are responsible for focusing an object through the fovea. The cornea has fixed optical power, while IOL optical power is variable. When an object is approaching us, brain activates accommodation mechanism that applies IOL muscles to changing a shape of IOL from flat state into thick state. This occurs in order to get more optical power and keep the object focused on the fovea.
Eyes are parallel while observing a distant object. Each eye fixates on an object under slightly different angle enabling 3D (binocular) vision by the visual c aortex of the brain. When an object is approaching, in addition to accommodation, brain activates convergence mechanism that applies Extra Ocular Muscles (EOM) of the eye which turn eyes inward in order to keep an object fixated to each eye fovea.
Natural Sight to short distance:
each eye accommodates for short distance (IOL is thickened) =>
not healthy state.
The eyes also are converging (rotating inside)
Fixating on a close object:
=> eye accommodates
=> IOL is thickened
=> vitreous body pushed back => retina pushed back
=> eye elongates
=> myopia develops over time
It is very important for the object not to approach closer than convergence comfort zone-a threshold that is about 20cm from the eyes. Moving an object closer than 20cm is possible for limited period of time, however, this would create eyestrain. Also, after returning gaze to normal distance this could create crossed eye vision side effect. Moving an object closer than 10cm will pass the convergence distance limit and will create double vision since the object will miss the fovea of the each eye as shown on the illustration.
Convergence thrreshold
When the distance is too short, eyes are strained for maximal accommodation, however, can’t converge (can't fixate the object) what creates double vision
The eyes convergence comfort zone ends-up at about 20cm. The optimal distance to work with screen is 30cm – 40cm distance
And if the distance is too-close:
Double vision starts from less than ~10cm
The amplitude of accommodation declines with age leading to presbyopia (single focus distance) by age of 50 or so. In opposite, a young human eye can change focus from a distance to as close at 6 cm. That is equivalent to about 15 diopters. In addition, convergence distance for young eyes is significantly shorter. Such an ability provides tremendous advantage for quick near vision tasks, however, long-term engaging in prolonged short-distance vision activities is hazardous for the young people.
For example, watching a movie on tablet from 20cm activates 5 diopters of accommodation This is considered as strong. Movie watching can engage kids for hours per day. As depicted on the figure above, accommodation is achieved by thickening the IOL that pushes the vitreous body back. The vitreous body applies extra pressure on the back of an eye. Since children's eyes grow alone with other organs, the extra pressure affects elongation of the eyes. The elongated eyes are myopic because for far vision tasks (with no accommodation) the optics of an eye focus an object before the retina. In general kids are trying taking any object closer to the eyes to get more immersive feeling or “zoom-in”. They do it out of natural curiosity. Near-vision kids’ activities should be limited to prevent development of myopia. To avoid myopia development the prolonged near-reading or near-observation tasks should be performed through optical devices neutralizing monocular eye accommodation. For distances up to 20cm regular reading glasses will do the function, however, for closer working distance (and for bigger magnification), biocular or binocular magnifiers should be used.
Optical magnifiers and limitation of regular eyeglasses
For variety of visual tasks there is a need to “zoom-in” on an object while eye or eyes are relaxed. This “zoom-in” is done by employing optical magnifiers. The magnifying glass (or loop) is the simplest monocular magnifier that usually is a form of a simple positive lens. Such a magnifier is held close to an eye, while an object to be viewed is brought to the focal point of the lens. The x6 monocular magnifier has focal distance of about 8cm and is equivalent to +12 Diopters. Monocular magnifiers could have very strong magnification. At the same time, 2nd eye required to be closed (please see part-A of the illustration). This creates discomfort while preforming longer task. The long-time near objects observation is requiring “zoom-in” for both eyes. However, what happens if two strong loops such as very strong reading glasses (e.g. +12D) are used at once? The effect will be inability to converge eyes (rotate eyes inward) resulting in double vision, headaches and nausea (please see part B of the illustration).
The issue described above can be resolved by employing a biocular magnifier. The principle of biocular magnifier is to use a lens of a diameter equivalent to distance between the eyes (average is 63mm), so that each eyes vision would be assisted by an offset from the lens center. This offset creates “prismatic effect” that virtually shifts an observed object to the left and right eye correspondingly (please see illustration labelled Part-C). As a result the angles of sight are less steep and new eye convergence distance results in a meeting point of the lines of sight. The goal is to make “prismatic effect” strong enough to move eye convergence to the comfort zone. The single lens biocular magnifiers are limited in the magnification because of physical size. In 2014 MOGO-Oculars were invented which are used as a conjugate pair and outperform single lens biocular magnifiers (see Part-D of the illustration). The MOGO-Oculars are prismatic aspherical lenses providing strong magnification while keeping in focus all points of the near and wide objects and directing eye convergence to a comfort zone. The details are described in the next sections of this page.
Relaxed near vision with MOGO-Oculars
MoGo-Oculars “transfer” very close object to distance of comfort for eyes convergence and, in addition, each eye independently feels far distance - IOL is flat – healthy to the eyes (no risk of myopia development)
MoGo-Oculars.v1 , +12D, 19PD Base In, “transfer” object 3" from the lenses to image of 10" distance (that is in comfort zone for eyes convergence). The MOGO-Oculars.v2, +12D, 23PD Base In, that are currently in development, will transfer the image to 13" convergence distance.