Changes in retinal image size after correction.

**msc** · 10-03-2009, 03:54 PM

I have a question regarding the changes in retinal image size when refractive errors are corrected with standard corrective eye wear.

Assuming both eyes have the same fl, and you are correcting a 2 diopter refractive error in one eye to make both eyes emmotropic (can focus at infinity), what will the change be in retinal image size (retinal magnification mismatch) after the correction?

I have read, eye glasses can be designed, so the glass will correct the refractive error, while at the same time, not change the retinal image size. However, this article did not mention if this procedure is commonplace for all refractive corrections, or possibly only for extreme mismatched errors between the eyes??

Any input would be greatly appreciated, TYIA.

**Carbon Pit** · 10-03-2009, 04:25 PM

I think your referring to Iseikonic lenses. Most people tend to use them in extreme cases of spectacle induced anisometropia.

Here are some texts on how to design these lenses:

http://www.opticalheritagemuseum.org...blications.htm

**fagin** · 10-03-2009, 04:28 PM

I think you may be refering to "size" lenses that are used in analysing aneisokonia, but are not relevant in day to day use with spectacle lenses.

In real life spectacle magnification is an issue that is accepted rather than dealt with, as for the most part, it is difficult to manipulate, and the results rarely justify the means !

**fagin** · 10-03-2009, 04:29 PM

ha ha, there you go - someone always gets in there with a quicker reply !!:D

**msc** · 10-03-2009, 04:59 PM

Carbon, thank you for link... actually, i am not interested in making the lenses themselves, I was curious how this condition is "dealt with"... it appears Fagin answered this.... its "accepted" vs. dealt with, except maybe in extreme cases, which i assume would be in the 3+ dioipter mismatch range?

So my question is, for each diopter of correction what is the change in retinal image size in % ?

I am referring to infinity based correction, not close-up or reading glasses... how does one calculate this?

TYIA

**Carbon Pit** · 10-03-2009, 05:25 PM

Diopters alone do not determine retinal image size. Other factors like lens form and thickness also play a role. Whether the refractive error is axial or refractive will also play a role.

In order to build an Iseikonic lens you have to calculate image size. Did you read the articles? They are a wealth of information. Also you may want to purchase Ellen Stoner's Optical Formulas Tutorial. The book is a wealth of information. I lost mine

.

This one should have some of the info you need:

http://www.opticalheritagemuseum.org...zeProblems.pdf

**fagin** · 10-03-2009, 05:40 PM

Phew, can we do the math , later ? Are you lookng for a ball-park figure for the point at which this would cause a problem, or are you after a specific calculation ?

realistically if I have a patient with RE:+1.00DS, LE:+5.00DS, they are unlikely to have any particular problems with a SV lens that they can't deal with after a while. They could suppress LE, if VA RE is sufficient or Vice-versa. I probably wouldn't be falling over myself to get involved in bifocal or progressive lenses as differential prism would be an issue, but that is a side issue her, I guess.

A percentage calculation on retinal size change / per dioptre isn't easy, as Corrected Retinal Image size relies on many more variables, but I guess a sliding percentage scale could be ascertained if someone has the time.

**msc** · 10-03-2009, 07:42 PM

Thank you very much for the informative responses... Carbon, the links were a bounty of information, and I was doing my best to read through many of the pdf files, thank you..... some of it was a bit fuzzy for me, as I am not an optometrist. But much of it was very clear and helpful. I am convinced the answer to my original quesiton is in one of those tables....

Fagin, yes, I am looking for ballpark, or "general" answers here to be sure I have a grasp.

I have designed a stereo viewer with its own set of optics. The question we were trying to get answered is.... for those who wear spectacles, how does the spectacles alter the image sizes. (assuming our optics are of equal fl and deliver equal size images to both eyes). Of course, this depends on the level of refractie error. Since most of the refractive mismatches are within 1 diopter (from the pdf files) and for sake of this discussion, lets assume the errors are refractive only, not axial.

So under this set of parameters, what is the change in image size spectacles will cause for a 1D of correction?

**Carbon Pit** · 10-03-2009, 08:15 PM

Use the formula in the paper I posted. It allows you to manipulate several aspects that control image size. Assume it's on a 6 base with 2mm center thickness. I think of all formulas this one can be the most fun! It's on page 9.:cheers:

**msc** · 10-03-2009, 09:34 PM

Carbon, thank you for the guidance... I loaded the formula in my spreadsheet.... but one question....

Here is a typical example using your values, please advise if I interpreted properly....

1.00 Diopter correction
6.00 Front curve (default +6)
2.0 mm lens thickness
12.0 mm distance from cornea (air space)

2.00 % MT = % Approx Magnification Total

So to correct a persons -1D error, we have a 2.0% increase in image magnification, is this stated correctly?

Now, if I got that right, and I change the Diopter correction to -1D....

(1.00) Diopter correction
6.00 Front curve (default +6)
2.0 mm lens thickness
12.0 mm distance from cornea (air space)

(0.40) % MT = % Approx Magnification Total

Now we have a .4% change in image magnification? I assume the curve value would have to change with this... is there a table that shows these relationships? Also, that seems like a big change in magnification?

HELP? Thanks again ...