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# How to interpret diopters when it comes to monovision?

I have cataracts in both eyes.  I am strongly considering monovision.   Based on what I've read, it would seem that many doctors consider 1.5 diopters as the sweet spot for the near eye.  And I’ve read a few who believe that most patients can do well with up to 2.0 diopters for the near eye.

An online calculator translated diopters to inches, per below.  Do these seem accurate?

3.0 diopters:  13 inches
2.0 diopters:  20 inches
1.5 diopters:  26 inches

Also, 26 inches seems pretty far away when it comes to cell phone usage.  With 1.5 diopters, would most people need to use glasses to use the larger smart phones?

Last, can you help me understand 0 diopters?   If the post-op vision for the distance eye is 0 diopters, I understand that near vision will be blurred, if looking out of that eye alone.  At what distance does the vision become clear?

Thank you so much.
1 Responses
MEDICAL PROFESSIONAL
Optically the focal length of a lens system can be calculated exactly.  However in the eye and with target post operative uncorrective refractive errors it doesn't work that way.   The most exact formula only allow an error range of plus/minus 0.50 diopter. Larger with very long or short eyes.  The focal point and depth of focus will vary depending on the size of the pupil and where the IOL comes to rest. Read my section on what people need to know before cataract surgery. It's posted on my home page.
Thank you Dr. Hagan.  I re-read your article.  I think I understand your caveats about diopters, but it also seems that many studies use the term.

Can you provide guidance as to how to speak with the surgeon about vision needs?  Thank you.
I have a measure of my vision in contacts.  Rx:  Right: -11.5, nontoric.  Left, -4.00, cyl -1.75, axis 40.  Left eye was 20/30 for distance and right was 20/70.  Reading was 20/50 in both eyes.

Subsequently, after weeks off of contacts, my astigmatism dropped by about 0.4 in each eye; so this RX overstates the current astigmatism.  Also, before the cataracts, my right eye myopia correction was -5.75 D.

I found the contacts worked fine for getting around town, reading my phone in a larger font, light computer work, cooking, going shopping, social events, and working out.  The distance was not sufficient for driving.  I could read small fonts on paper for just a little bit, but larger fonts were fine.

I am 61.  My left eye is dominant.
Were your contacts set for "full monovision"   Is your RE amblyopic and has never seen 20/20 with glasses or contacts? At one time did you have 20/20 vision in both eyes? Have you worn bifocal glasses successfully?   Do you have a glasses RX?   Is your retina okay?
The contacts weren’t set for full monovision and I’m not sure what the actual diopter difference is between the eyes.  I did try a number of different powers and adapted easily to them.  Right eye no amblyopia and both eyes were correctable to 20/20 before the cataracts.  I’ve worn progressive glasses for many years, with ease.  The retina has had mild lattice for many years but is OK.

Glasses rx:  RE -15.25, cyl 1.25, axis 175; LE -3.5 cyl 2.25, axis 40.  This rx was taken before the time off of contacts so the astigmatism would be lower now.
Okay but understand these are generalizations but given how myopic you are and the fact that you are use to glasses and contacts and your vision is so poor you should be extremely pleased with a whole variety of post operative uncorrected refractive errors.

Myopic patients are use to being able to see up close without glasses and most want that after surgery.   I have seen one or two people, against my advice,  shoot for 0.00 in both eyes.   "distance vision" is greater than 20 ft or 6 m.   Then needed glasses for reading,  different glasses for computer, different for TV and were very sorry with their choice.

For most people the dominant eye is given the better uncorrected distance vision and the non-dominant eye for reading.  Full monofocal which might look like 0.00 LE and -2.50 RE  doesn't usually work well unless the person has worn full monofocal contacts and likes it.  NOTE: you are going to have to have toric IOLs or cornea surface laser to get rid of the cornea astigmatism if you want spherical (no cylinder) post op uncorrected error.

Mini-monofocial near bias has worked well for many of my patients.   Dominant eye target -1.00  non-dominant eye target -2.25 or -2.50  The glasses RX would look like this RE  -1.00  LE -2.25    reading add +3.00 in no line progressive multifocal style.

Long eyes like yours with astimatism are hard to hit desired post op uncorrected refractive error because the formula are not as accurate and the error range is likely  +/- 0.75 diopters.

Thank you so much, Dr. Hagan.  That's very helpful.
Best of luck. After surgery come back and give some follow up. it will be helpful to many readers in the future.
Thank you, Dr. Hagan, I will.

Also, can you tell me from my glasses RX if the astigmatism measured then is "with the rule" or "against the rule", for each eye?  Thank you!
Only if you tell me whether your glasses RX is written in PLUS cylinder (+) or MINUS cylinder (-)
The prescription was written by an optometrist, so I assume it is written in Minus Cylinder.   And I see I missed typing in the minus signs.  Here’s the corrected RX:

Right sphere: -15.25
Right cyl:  -1.25
Right axis:  175
Left sphere: -3.5
Left cyl: -2.25
Left axis: 40

Right eye has with the rule astigmatism   Left eye has 'oblique  astigmatism.     With rule has the steepest axis at 90 degree or 12 to 6 on clock.  Against the rule has steepest axis 180 degree or 3 to 9.

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