Correct way to calculate mean snellen visual acuity?

[Modder]

---

Members do not see this ad.

How does one calculate the mean for snellen VA (visual acuity), apparently one needs to calculate a geometric mean NOT arithmetic mean since the snellen acuity follows a geometric progression. Apparently it's wrong to just average out the denominators or decimal equivalents for snellen VAs. I'm working on a project and I need to calculate mean VAs pre & post intervention.

Thanks for helping out!

 

[orbitsurgMD]

LogMAR?

 

[Modder]

LogMAR?

Thanks! I'll look into that.

I also came across this paper, it's a little dated, but I imagine the physics/math behind VA calculations hasn't change either!

http://www.docholladay.com/publications/propermethodforcalculating.pdf

 

[RestoreSight]

Logmar. The issue is calculating CF, HM and LP values. The paper you cited is the standard method. You can also find calculators online that convert Snellen to Logmar values for you.

 

[Gtgdrvlicneedansws]

I believe that there must be a formula that encompasses both the spherical and cylindrical diopters per eye, then averages the two numbers (or keep them separate), eg. OD: [sph diopters + (cyl opters)/2] opters VA equiv (total on chart)
OS: [sph diopters + (cyl diopters/2] opters VA equiv (total on chart)

Lets say we have OD: -1.75 -2.00 x 150°
And. SD: -1.75 -1.50 x 040°
Would be OD: -3.75 --> 20/500 OAV
SD: -2.50 --> 20/250 SAV
= Avg of 20/375 combined
If one ignores a strong astigmatism, the Visual Acuity is likely too generous!
I need to convert my scrip (the example) for my driver's license app. Using a chart I combined my astigmatism diopters/2 and added them, since the direction of astigmatism is additive like the +2.00 is additive to the close Rx which is astigmatic with a slight + x mild to mod - and I have progressives, so the additivity goes gradually from +0.00 for distance to +2.00 for close up, i.e. reading, hence my Rxs at the bottoms are:OD: +0.25 -2.00 x 150° and
SD: +0.25 -150 x 40°
The numbers are all additive. Do I have rationale?

 

[.hematoma.]

If this is for research, I believe logMAR is usually the rule of thumb

 

[PBEA]

I believe that there must be a formula that encompasses both the spherical and cylindrical diopters per eye, then averages the two numbers (or keep them separate), eg. OD: [sph diopters + (cyl opters)/2] opters VA equiv (total on chart)
OS: [sph diopters + (cyl diopters/2] opters VA equiv (total on chart)

Lets say we have OD: -1.75 -2.00 x 150°
And. SD: -1.75 -1.50 x 040°
Would be OD: -3.75 --> 20/500 OAV
SD: -2.50 --> 20/250 SAV
= Avg of 20/375 combined
If one ignores a strong astigmatism, the Visual Acuity is likely too generous!
I need to convert my scrip (the example) for my driver's license app. Using a chart I combined my astigmatism diopters/2 and added them, since the direction of astigmatism is additive like the +2.00 is additive to the close Rx which is astigmatic with a slight + x mild to mod - and I have progressives, so the additivity goes gradually from +0.00 for distance to +2.00 for close up, i.e. reading, hence my Rxs at the bottoms are:OD: +0.25 -2.00 x 150° and
SD: +0.25 -150 x 40°
The numbers are all additive. Do I have rationale?

this is nonsense, sorry but you don't appear to have even a basic understanding of this material. Discussing it would be pointless.

 

[MstaKing10]

I believe that there must be a formula that encompasses both the spherical and cylindrical diopters per eye, then averages the two numbers (or keep them separate), eg. OD: [sph diopters + (cyl opters)/2] opters VA equiv (total on chart)
OS: [sph diopters + (cyl diopters/2] opters VA equiv (total on chart)

Lets say we have OD: -1.75 -2.00 x 150°
And. SD: -1.75 -1.50 x 040°
Would be OD: -3.75 --> 20/500 OAV
SD: -2.50 --> 20/250 SAV
= Avg of 20/375 combined
If one ignores a strong astigmatism, the Visual Acuity is likely too generous!
I need to convert my scrip (the example) for my driver's license app. Using a chart I combined my astigmatism diopters/2 and added them, since the direction of astigmatism is additive like the +2.00 is additive to the close Rx which is astigmatic with a slight + x mild to mod - and I have progressives, so the additivity goes gradually from +0.00 for distance to +2.00 for close up, i.e. reading, hence my Rxs at the bottoms are:OD: +0.25 -2.00 x 150° and
SD: +0.25 -150 x 40°
The numbers are all additive. Do I have rationale?

Holy smokes! There’s no rationale here but I’m impressed with the way you moved the numbers all over the place to make it sensible to you. Read up on optics then give this one another try