The closest point is the point closest to the eye at which one still can see sharply, using the maximum available accomodation.
Because the hypermetrope uses a part of the accomodation to see far away, he has less available to see closeby; therefore, with the same accomodation the hypermetropic eye has a closest point that is farther away.
An emmetropic eye with 10.0D of accomodation has the closest point at 10 cm (100/10= 10 cm) while the hypermetropic eye of 4.0 D will have it at 16.6 cm (100/(10-4)= 16.6 cm).
A hypermetrope of 4.0D, with 10.0 D of accomodative amplitude, is in the same conditions of the emmetrope with 6 diopters of accomodation because the closest point is more distant from the eye. The measurement of refaction may be useful to determine the hypermetropia to estimate the remaing latent part.
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One).
Related article: The remote point (Hypermetropia)
Archive for January, 2008
The closest point (Hypermetropia)
Tuesday, January 29th, 2008Aphakia (5/5)
Saturday, January 26th, 2008
Let’s remember that there exists a compensation between the effects of aberration of the cornea and that of the chrystal (if one undercompensates while the other overcompensates) but the individual difference is such that it makes no sense to determine an average value. Anyway, the influence of spheric aberration of the chrystal is proven by experiments that have demonstrated how, with varying accomodation states, one goes from undercorrection to overcorrection of the retinal image. There exists also a phenomenon that originates from the prismatic effect caused by the rim of the lens. This effect increases with the angle of the prism, i.e., the lens strength. This phenomenon is called “Jack in the box” and also “Roving ring scotoma”.
The dimensions of this blindness “ring” surrounding the visual field depends on several factors: strength of the lens, its dimensions, pupil size, distance from the cornea, the configuration and thickness of the lens.
One of the inconveniences of the above described scotoma is that it tends to move independently from the eye’s rotation. If the bulb rotates laterally by 30 degrees, the scotoma rotates inwards 20 degrees and comes close to the fovea. When the patient sees an object, he sees a dark spot coming closer to him. Without thinking about the consequences he moves his eye in order to focus on the object. Aspherical lenses, also called “panoramic”, have been developed but with modest success.
A (partial) solution can be found correcting the aphakia with contact lenses.
From Optometria e Oftalmologia by Prof. Sergio Villani
The prescription of corrective lenses for special purposes (1/2)
Wednesday, January 23rd, 2008
Often special purpose glasses are requiested, normally for work at intermediate distances. Usually the patient is presbytic and finds that the correction of farsightedness and nearsightedness does not allow him to see at intermediate distances. Thus, the glasses for reading music, for playing at cards, to work at the video, to paint, are prescribed for use between 50 cm and 1 meter. The prescription is thus close to the limit of nearsightedness correction. We already gave some examples but we believe that insisting doesn’t hurt.
Therefore:
Available accomodation: 1,00 D,
Distance to work: 67cm: required accomodation 1,50D,
Available usable accomodation (2/3): 0,66 D,
Additionally required lenses = + 0,75 or 1,00 D sf.
The correction must then be ergonomically verified while the patient holds the cards or reads the music, works at the video or paints, or when he is manipulating a sample of his work. There are also trifocal lenses for normal prescription. We have to remind though that the intermediate segment has a predetermined range.
An alternative to bifocal and trifocal lenses are variable, also called progressive, lenses. They have a variable strength from the center downwards, the part that the patient uses while reading. These lenses which, in the latest realizations, are the most advanced even though optically limited are being used by young presbytes.
The above is for presbyte patients. In the industry there is also a need for special glasses for work even at 10 cm with workers of at average 24 years of age.
In these cases we have to consider two factors:
a) The required close-by accomodation;
b) The instability for work at close ranges.
Related article: The prescription of corrective lenses: Longsightedness
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two)
Aphakia (4/5)
Sunday, January 20th, 2008
The aphakic eye is unable to focus on objects at different distances and vision is worsened also by the fact that a so strong ametropia reduces field depth; therefore, in theory, one should use a corrective lens for every distance; in practice only 3 types of corrections are required: one for far away, one for medium distance of 80-90 cm and one for reading. If the aphakia is binocular one can obtain an excellent correction also with glasses and, even better, with aspheric lenses. Many aphakics, in order to increase the space in which their vision is sharp, close their eyelids in order to reduce the pupilar area and thus reduce the disc of confusion formed on the retina and increase field depth.
From Optometria e Oftalmologia by Prof. Sergio Villani
Related article: Afakia (3/5)
The remote point (Hypermetropia)
Thursday, January 17th, 2008
This point is the conjugated focus on the retina with completely relaxed accomodation.
In the case of emmetropia the remote point is infinite because the optical system focuses on the retina.
With hypermetropia the retina is ahead of the focus; only with radiations of the right convergence is it possible to focus on the retina.
If we try to aim a light on the retina of the hypermetropic eye, its rays exit from the eye through the cornea in different ways; lengthening these rays will give us the virtual remote point. The more severe the hypermetropia, the more divergent are the rays that exit from the eye and the more the virtual remote point is located further in the eye. The inverse of the distance in meters between the remote point and the principal plane of the eye, gives us the measure in diopters of the hypermetropia; example: for a hypermetropic eye of 2-4-6 D the remote point is 100/2=50, 100/4=25, 100/6=16.666 cm.
If the hypermetropic level is expressed by the lens that corrects the defect, the distance between the lens and the principal plane must be added to determine the remote point, i.e., if a hypermetropia is corrected by a lens of +3.00 D, at a distance of 15 mm from the principal plane, the rela remote point is at 100/3+1.5=-31.83 cm.
The remote point of the hypermetropic eye is always negative and can be determined only with optometric measurements; its value is only apparent due to the difficulty relaxing the accomodation completely.
By definition the remote point is represented by the maximum distance at which one sees sharply and with relaxed accomodation; in the not-compensated hypermetropia there cannot be a distance at which the sight is sharp because it is hazy at all distances. When the hypermetropia is completely corrected the remote point is at infinite.
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One).
Related article: Variations in the degree of Hypermetropia
Aphakia (3/5)
Monday, January 14th, 2008
When the afakia is monocular, the strong imbalance will not permit a normal binocular vision; the cause being the amplication of the retinal image by the corrective lens which is at a distance of the eye. In fact, contact lenses can almost totally eliminate the amplification and return a normal single binocular vision.
This effect is possible because the flaw of the aphakic eye is of a refractive nature. Removing the chrystal, the measurements that first were taken from its principal point to the retina, will now be taken from the corneal apex to the retina; this new reference point creates a lengthening of the ocular dioptric system (of the axial myopic type). For the axial ametropia this lengthening is negative to the point that if one could eliminate this effect, the hypermetropia would be 6 diopters more than it really is.
If the monolateral aphakia is not corrected, the eye without the chrystal tends to move to the outside. This exotropia starts to show itself 8-12 months after the loss of the chrystal and is one of the causes that make it impossible to correct an aphakic eye belatedly.
From Optometria e Oftalmologia by Prof. Sergio Villani
Related article: Afakia (2/5)
The prescription of corrective lenses: Longsightedness
Friday, January 11th, 2008
In cases of presbyopia with slight ametropia, normally both pair of glasses are required. Statistics show that this is true for 30% of the patients. We have already seen that, in the majority of cases, lenses have to be to be prescribed only for close-by work given that the visus for long-range is often almost normal and the difficulties manifest themselves with close-by activities, true for presbites of up to 55 years of age.
Normally the correction for farsightedness is easy to find, to which one adds the accomodation to arrive thus at the correction for nearsightedness.
Presbytes who can see well afar, have difficulties when only lenses for nearsightedness given that they see far-away objects hazily.
Often they object when told to take off their glasses to look afar. This “afar” is very relative because it starts already at 50-60 cm.
In these cases bifocals must be prescribed which are very useful when both types of lenses are required, on the condition that the patient’s temperament is not inclined to symptoms of intollerance.
Prescribing bifocals, when the patient requires mainly reading glasses, one must remember that they will be used mainly for close-by work and therefore the segment for nearsightedness must be placed sufficiently high, slightly under the pupil, because that way the patient will not encounter difficulties when looking through the higher segment. The natural position is to bend the head down and that can only be achieved if the segment for nearsightedness is placed sufficiently high in the lens. Glasses with the nearsightedness segment placed low are good for walking, to go to the theater, etc.; in general, if the nearsightedness requires a correction, it is always better to prescribe bifocals if the nearsightedness segment is placed 4-6 mm under the pupil.
At this time, for “simple” presbytes half-high glasses are used in a half-high frame which leave the upper part free for the longsighted vision.
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two)
Aphakia (2/5)
Tuesday, January 8th, 2008
The absence of the chrystal can be recognized by the depth and the conical aspect of the front camera, the trembing of the iris which, not supported by the chrystal, varies with the movement of the eye: or also by holding a lamp in front of the eye one can notice the lask of reflexes (Purkinje images) that would have been caused by the front and upper surfaces of the chrystal. The trauma suffered by the cornea after surgery a certain astigmatism remains which, in the healing progress, is reduced by ca. .5 - 1.5 diopters.
In this transition period (lasting approx. 20 days) one must wait for the completion of the healing process before taking corrective action, in the mean time a temporary correction may be given to the patient that gives him his independence. Ignoring the astigmatism, the aphakic eye distinguishes itself by its high degree of hypermetropia (i.e., if the eye before the intervention did not have a myopia > 15.0 D) caused by the absence of the chrystal which when in place has a power of ca. 12 diopters); the spherical refractive flaw of the eye results to be on average 12 - 14 diopters when there are no other complications, the well corrected aphakic eye may be up to 10/10.
From Optometria e Oftalmologia by Prof. Sergio Villani.
Related article: Afakia art. 1/5
Variations in the degree of Hypermetropia
Saturday, January 5th, 2008
In a child the eye tends to develop in parallel with the development of the body; thus we can say that, in general, from first infancy to maturity the total hypermetropia tends to diminish.
It remains then stationary till the age of 50-55 when the hypermetropia can increase again obtaining what we have called acquired hypermetropia.
However, while the total hypermetropia remains the same between 12-13 and 40-50 years, the manifest hypermetropia may vary significantly augmenting from year to year while the accomodation diminishes.
From Optometria e Oftalmologia by Prof. Sergio Villani.
Related article: The real degree of Hypermetropia
Afakia (1/5)
Wednesday, January 2nd, 2008
An eye is said to be aphakic when the chrystal is missing. Normally the term is used whenever the chrystal is not in its normal place, behind the pupil and therefore optically does not take part in the formation of the image on the retina.
One can therefore also talk about aphakia when the chrystal is inside the eye, but its optical function is lost; the cause may be the luxation of its natural seat (Zinn area). Even when recognizing that there are many causes for the missing chrystal (congenic apakia, trauma, etc.), is is almost always caused by surgery; these interventions are done, in the majority of cases, to remove a cataract or rarely to correct a major ammetropy (myopia).
From Optometria e Oftalmologia by Prof. Sergio Villani.