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I don't understand why optometrists dilate your eyes, which is a very annoying thing, when eye power can be measured by just a ruler at your home as suggested by this, which just uses a 100/(distance after which your blur horizon begins), which logically is the correct method, as suggested by physics laws governing lenses. Also, is it a myth that they over-prescribe the eyeglasses?

Thanks in advance.

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The value of dilation of the pupil far outweighs the inconvenience.In fact, I'd offer that if an exam is done without it, you potentially lose the following:

--full access to all the structure of the retina, macula, and associated blood vessels. Dilation opens up the aperture of the pupil enough for the examiner to see all there is to see. I could fill up two pages on what's "discoverable" on an eye exam.

--depending on age, dilation also paralyzes the ciliary muscle of the lens which is responsible for near-vision (sic: accommodation). The older you are, and the more you stare at a computer, the more that muscle stays tonically-contracted. When a distance vision exam is done on a tonically-contracted (accomodated) eye, the refraction may show a near-sighted condition that really doesn't exist.

Keep this in-mind: distance vision is a function of the cornea-macula distance. The lens is at its most flat state, adding next to nothing to visual acuity. When you get inside 20-inches, the lens changes configuration to increase the refractive index to keep object in focus. As people age, the ciliary muscle weakens and the lens becomes stiffer, which results in a gradual and progressive loss of ability to see up-close. Hence, the need for reading glasses. The two processes--near-sightedness and the inability to see up close as you age (Presbyopia) are two different processes.

To get back to "dilating" the eye, you can certainly bully and optometrist into not dilating your eyes, but you lose value and increase risk when you try to side-step that part of the exam. R.S. Carnes, M.D.

References: For physiology of the eye, the standard first-year medical school text is good enough: Hall and Guyton, “Medical Physiology.” For use of cycloplegics and fundascopic exams of the retina, almost any clinical practice manual, but “Harrisons Principals of Internal Medicine” details the range of diseases and conditions discoverable on fundascopic exam.

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    Welcome to Medical Sciences.SE! This sounds like a good answer, but we have a policy that requires all answers to provide supporting references regardless of your professional credentials. You don't have to meet the standards of a journal article, but citations supporting your main assertions of fact are required. ​Unreferenced claims can lead to answers being deleted
    – Carey Gregory
    Jun 26 at 0:52
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    The answers provided are established first principals of medicine and physiology and clinical practice. Both have been firmly established since the late 19th Century. Contemporary medical research publications assume knowledge of these facts and are mot helpful. Jun 27 at 1:22
  • Looks like my “references” never made into an edited post. For physiology of the eye, the standard first-year medical school text is good enough: Hall and Guyton, “Medical Physiology.” For use of cycloplegics and fundascopic exams of the retina, almost any clinical practice manual, but “Harrisons Principals of Internal Medicine” details the range of diseases and conditions discoverable on fundascopic exam. Beyond that, tapping all this on my iphone is more exhausting than passing over a question for something more critical to life and limb. Jun 27 at 1:45
  • I understand. Thank you for your answer.
    – Carey Gregory
    Jun 27 at 4:09

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