What Is Oculus Sinister?
OS or Oculus Sinister is the Latin term that translates to “left eye”. An eyeglasses prescription usually has three important medical abbreviations: OS (Oculus Sinister), OD (Oculus Dexter), and OU (Oculus Uterque). The Latin term oculus translates to ‘eye’ in English, and sinister translates to ‘left’.
- OS or Oculus Sinister is the Latin term that translates to “left eye”, often used to denote the left eye’s glasses prescription.
- If the OS numerical value has a minus sign (e.g. -2.00) then the person is nearsighted in the left eye.
- If the OS numerical value has a plus sign (e.g. +2.00) the person is farsighted in the left eye. If there are three numerical values (e.g. +2.00 -1.00 x180) the person also has astigmatism in the left eye.
Understanding Oculus Sinister
OS is an acronym for Oculus Sinister, used widely by ophthalmology and optometry professionals. Most notably, OS is a term written in the glasses prescription indicating the prescription for the left eye. The OS value in a spectacle prescription is the amount of lens power, measured in diopters, that is the correction for nearsightedness or farsightedness.
Generally, the strength of eyeglasses is measured in terms of diopters (D). The dioptric power is the refractive power, which is the extent that a lens refracts light. A lens that has 5.00 D, or five diopters of refractive power, bends light more than a 1.00 D lens, or one diopter of refractive power. The amount of diopters is essentially the amount that a lens bends light in order to create a clear image on the retina. A lens that has a negative dioptric power (-4.00 D) diverges light, whereas a lens with a positive dioptric power (+4.00 D) converges light.
Optically, a diopter is also the inverse of the focal length in meters. For example, if the left eye’s prescription is written as “-2.00 D”, which means two diopters of nearsightedness, the lens in this pair of glasses has a focal length of 1/-2.00, or -0.50 meters. This means that an eye with a prescription of -2.00 D can see clearly at 50 centimeters from the eye without any glasses correction. Wearing a higher diopter power lens allows for sharp vision closer to the eyes.
Farsightedness, also known as hyperopia, is defined as a refractive error where parallel rays of light from a distant object enter the eye and focus behind the retina, instead of directly onto the retina. This problem occurs when the eye is too short in length. Hyperopia, or farsightedness, is corrected with a plus-powered convex lens in glasses. Farsightedness makes a person have trouble viewing close-up objects, such as phones or books. Most children are farsighted because their eyes are still growing and their eyeballs are short. However, children with farsightedness may not have blurry vision because they have strong focusing systems. Symptoms of farsightedness may manifest as headaches or eye strain after a prolonged time reading. Farsighted children often have problems in school.
Nearsightedness, also known as myopia, is the most common refractive disorder in which parallel rays of light from a distant object enter the eye and focus in front of the retina, instead of directly onto the retina. This problem occurs when the eye is too long in length. Myopia, or nearsightedness, is corrected with a minus-powered concave lens in glasses.
Nearsightedness results in blurred distance vision, such as trouble seeing the television subtitles or exit signs on the highway. Symptoms of myopia may manifest as squinting to see clearly, blurred vision exacerbated at night, or eye strain after a prolonged time doing distance tasks such as driving. A high amount of nearsightedness can be detrimental to the eye’s health and has an increased risk of eye diseases such as glaucoma, retinal detachment, and retinal thinning.
The word “presbyopia” stems from the Greek medical terminology of presbys, meaning elderly, and opis, meaning eye. Presbyopia is defined as the loss of focusing ability which occurs normally with age. At birth, the lens inside of the eye is flexible and is able to move forward and backwards effortlessly to focus on objects up close. As humans age, the lens loses its elasticity, and it becomes thicker and denser over time. Eventually the lens has minimal focusing power as it is unable to change its shape. Presbyopia results in the decreased ability to see clearly at near.
Presbyopia typically starts at forty years of age and stabilizes at sixty years of age. Symptoms of presbyopia can manifest as eye strain or headaches after reading, having to hold items further away to see them, or trouble reading in dim lighting. If an eye doctor determines that someone has presbyopia, they will include an ‘ADD’ magnifying value in the glasses prescription to assist with reading. The magnifying power will be applied to the bottom of the multifocal lenses. Multifocal lenses such as progressive lenses or bifocals are often prescribed for patients with presbyopia. These lenses have two or more powers, with the top of the lens used for distance viewing and the bottom of the lens used for reading.
Astigmatism is defined as an irregularly shaped cornea. This irregularly shaped cornea will bend the light that enters the eye differently so that light cannot focus onto a single point. Because the corneal curvature is not perfectly round with astigmatism, there are different refractive powers in different meridians, and the prescription in one meridian is different than the other. This requires the glasses prescription to have two powers and a direction of astigmatism, such as “-1.00 +1.00 x010”. Astigmatism will cause blur with both near and distant objects, as well as image distortion.
Symptoms of astigmatism may manifest as ghost images or double vision with one eye closed, eye strain, headaches, or squinting. Astigmatism is a common refractive error, and approximately 40% of Americans have this condition. It is a benign condition that is corrected with glasses, similar to nearsightedness and farsightedness.