Introduction to Optics (Video)

This video covers the basic optics you need to know to understand basic glasses prescriptions.

When I began my ophthalmology residency, I had no idea how to read or write a glasses prescription. In this video, I try to explain the basic topics I wish I had known. This includes lens theory, basic spherical problems (near and far-sightedness), astigmatism, cylinder conversion, and presbyopia (bifocals). While this lecture is “basic” and assumes you have no prior glasses experience … this is still important stuff. I hope you find this useful!

Screen Captures from this Video:
(comments below)

Glasses prescription components
The goal of this video is to completely understand what a glasses prescription entails.
Lens converge diverge
A lens can be thought of as two prisms … that either converge or diverge light.
Loose Lens Rack
A lens rack. The red lenses are “minus” while the black lenses are “plus”
Dioipter Power
Lenses are marked with diopter power.
Diopter Power example
Definition of a diopter is demonstrated, along with lens stacking to create more powerful convergence.
Diopter of the eye
The eye has an overall power of approximately 60 diopters.
far-sighted child
Children tend to be slightly hyperopic as they have small eyes.
Astigmatism example
Astigmatism is when the corneal surface is more “round like a football” and less like a basketball.
With the rule astigmatism
Children have “with the rule” astigmatism, while elderly tend to have “against the rule”
The three measurements we check when using the phoropter to “manifest” a patient
Converting plus to minus
Converting a prescription to minus cylinder
Accomodation of the lens inside the eye
Presbyopia makes it hard to see near objects.
BIfocal Power
Bifocal power. We rarely prescribe greater than 3 diopters as few people read much closer than 1/3rd of a meter (33 centimeters).

Dr. Timothy Root is a practicing ophthalmologist and cataract surgeon in Daytona Beach, Florida. His books, video lectures, and training resources can be found at:


  1. I’m in my last year of med school and still learned a lot from your video. It’s amazing how you can make these topics so simple to understand. I really wish I had more professors like you! Thank you for your time!

  2. Wow, this is how you nail a subject. I just graduated from medical school and this helped a lot.

    You just helped some patients. Thank you soo much!!

  3. thank you very much. i love this site and have learnt alot from here. i want to be ophthalmology surgeon. i’m so happy.
    thank you again.

  4. Dear Dr.

    I truly appreciate your time and diligence making such an informative video for students such as ourselves. It was clear concise and an addendum to our school lectures. I would be interested in discussing with you about making future educational videos for students. Please contact me at your earliest convenience.

    Thank you

  5. thank you Dr.Thimothy, this video & other staff in brilliant website are treasure for ophthlmic residents.. I wish i found this site when i was in my 1st yr.. anyway im not too late, im 2nd yr now;)

  6. excellent website..very good and informative videos for ophthalmology residents and doctor…keep on the good work sir…u are helping us a lot…

  7. thanks sir you are sooo great at teaching ! I understand alot and things go smooth after watching your great videos ! Hope to hear more videos from you sir , you help us students alot . =)

  8. I love the videos, downloaded all the podcasts. Will you be making the rest available as podcasts soon?
    Or make it possible to view them from the website on an apple device?

    Best wishes and keep up the good work.

  9. Yikes: I am teaching our residents here to always write RX’s +2.00 -1.00 x 180 degrees rather than +2.00 -1.00 @ 180 degrees….it is very confusing to place on the Power Cross when doing Prentice’s Rule Induced prism problems if one doesn’t make this distinction.
    I hope this is just a typo. but OD’s and MD’s and Opticians around here will be ever confused unless that is just a local/regional way it is done in your area.
    PS I love your effort and the results on your video’s and rec. them to residents here with the disclaimer about the prescription writing.
    Roger Deshaies MD

  10. Excellent video. Short and concise but covering the important things. Very easy to understand.

    Thank you so much for taking the time to make this and for having available for everyone.

  11. Thank you so much Ophthobook. I have been trying to understand this for some time but the ophth registrars and optometrists explanation were over my head.

  12. Wonderful video explaining all the basics.

    One thing to note though, the reason why Optometrists use minus cylinder form is because as you said you can’t add cylinder to the spherical lens. Minus cylinder makes sense because the factory making the lenses use it because it is how they are actually made. Although conceptually positive cylinder is easier to understand… but leaves room for mistakes when they have to convert positive cylinder to negative cylinder form.

  13. Wow amazing thank you so much!
    this website, videos, yeah well everything is just such huge help in making ophthalmology easy and fun!

  14. I am thirty and my fiancé and I are taking over my father’s three-generation old, small, Georgia practice. This is amazingly helpful, especially to the employees to reduce error and to increase optical intelligence in the office. Thank you!

  15. Sir
    Thank you so much for making such a sincere effort to help students of ophthalmology the world over. This is a brilliant website- practical, useful, and therefore extremely popular among all my college-mates.
    We’ve stopped worrying about understanding the concepts of optics and dark room procedures (a feat in itself, considering it’s one of the toughest subjects, conceptually) because we know that we can rely on your website.
    Thank you once again!

  16. U r doing a brilliant job…I have never felt any lectures so interesting and entertaining as that of yours…. plzzzzzzz upload eye optics video as my exams r near…I hope u will update it soon…plzzzzzzzzzzzzz:-)

  17. I will be taking a Royal college of London post-grad exam this Jan and this video has helped me a lot in understanding the optics section of the syllabus. Thank you for making it so easy. I am still having trouble grasping the concept and formulas of “the magnifying glass/loupe” and “anterior and posterior vertex lengths and powers”. Is there any related video or lecture that can help me understand them?

  18. HI,
    Amazingly useful videos.
    HOWEVER I thought you should know: the Vimeo player does not allow skipping ahead or back. Which is really awkward if trying to revisit parts of the recordings. I’ve tried both IE and Chrome in latest versions to no avail.

  19. Thank you for this great video! I’m applying to be a medical assistant at an eye doctors office and this really was a great introduction to some of the basics.

  20. You are great!!
    I had never understand all this complicated concepts and numbers although I had study it so many times
    You did explain it well and only in 20 minutes 🙂
    Thank you so much
    oh by the way, thank God I’m still a student 😀

  21. Hi I am an ophthalmology trainee , trying to ‘brush up’ ensure I understand optics…this really set on the path of understanding optics better . Thank you so much you are an excellent teacher!

  22. I’m astigmatic only for few year and is a ‘with the rule’ astigmatism. I knew a child that had ‘against the rule’ astigmatism at six.

  23. Dr root
    Your videos are so helpful
    And also you make it enjoyable to watch
    And your thorough
    So thank you
    I’m about to take my COA test
    And your videos help
    So thanks so much !

  24. am jerusalem from N.F enekorogha, this site is very very interested to me,now I can exam a eye patience. THANKS

  25. dear Dr. your videos are great they are one of a kind in terms of there is no other good introductory resource produced online . optobook videos are great i hope you plan to do some more advanced videos sort of like a optobook 2.0

  26. thanks alot , its realy amazing lecture, please if you can do one lecture about lenses and clinical issues of unifocal lenses and multifocal lenses

  27. studying for my COT JCAHPO having a hard time with the math and physics- formula for converting focal length to Diopters and vice versa-
    I just can’t get it-any advice or tutorial you may have or know of would be great. my collection of books leave me bewildered!


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