questions about contact lense vertex distance - Eye & Vision Message Board

myope33 · 11-12-2005, 09:36 PM

I wish to post my pescription again with some questions:

I was manually refracted at -5.5(left) with -.5 astigmastim and -5(right) early this year. I believe im a -5(left) and -4.5(right) with -.5 astigmastim in each eye. My nearpoint test reveals 20cm and 22cm respectivately further backing up my belief that I improved a half diopter in each eye in the last 8 months.

Why is there so much controversity reguarding vertex distance for contacts?

Then explain how I am seeing 20/50 with -3.25 contacts but 20/100 with -3.25 glasses. Explain why im seeing very nearly as well with -3.25 contacts as -4.25 glasses. Explain why im seeing with -4 contacts as well as -5 glasses, both giving me my 20/30 BCVA? Tilting my -5 glasses to give it more minus power makes things from near more blurry and eventurally more blurry from far.

I am not saying I know more then those optometrist experts, I actually want to know why every contact I wore and tried was giving me a proportional vertex distance of 1.25 times more than glasses. My -2.25 contacts were giving me -2.75 correction, my -2.75 contacts -3.5 correction(I see clearer with those than -3.25 glasses) -3.25 contacts giving me very nearly the same vision as -4.25 glasses. I wonder why this is the case if some people disagree?

I put glasses over my -3.25 contacts and I found that I only needed another -1 diopter in glasses to give me my 20/30 BCVA. I tried -1.5 diopters and there was no difference(but blurrier from near) If someone wants to claim -3.25 contacts is equal to -3.25 glasses and I need another -1 to see my 20/30 BCVA then he would consider me a -4.25 yet I see 20/40(20/50 in left) with -4.25 glasses and I need -5 glasses to achieve my BCVA(20/30 in left)

myope33 · 11-14-2005, 11:27 PM

I have asked this in other forums too and seems that no one knows

very confusing! someone did say results are subjective, however vertex distance is a fact

twoeyez · 11-15-2005, 06:42 AM

myope33,
You seem to have extream consern about "vertex distance".
It would be helpful for you to review some basic in geometrical optics :

Vertex Distance

The distance from the back surface of the glasses lens to the front surface of the eye (the vertex distance) can affect the effective power of the lens, especially in higher powered prescriptions.

The vertex distance changes the effect of plus lenses and minus lenses in opposite directions, as shown below:

1) Increasing the vertex distance of plus lens will increase the effective power of the lens.

2) Decreasing the vertex distance of a plus lens will decrease the effective power of the lens.

3) Increasing the vertex distance of a minus lens will decrease the effective power of the lens.

4)Decreasing the vertex distance of a minus lens will increase the effective power of the lens.

In order for the glasses prescription to have exactly the same effective power as the refraction, the vertex distance of the phoroptor or trial frame must match the vertex distance of the lenses in the frames that the patient will wear. A difference in the two vertex distances only becomes significant if the diopter power of the prescription exceeds 6 diopters.

Vertex Compensation Formula

The formula for the needed compensation per millimeter of displacement, per diopter of lens power, is as follows:

diopters squared, divided by 1000

The answer is multiplied by the millimeters of displacement. The result is added or subtracted from the diopter power according to the following set of conditions:

1) Plus lens moving closer - add to increase the diopter power

2) Plus lens moving farther away - subract to reduce the diopter power

3) Minus lens moving closer - subtract to reduce the diopter power

4) Minus lens moving farther away - add to increase the diopter power

Spherical lens example:

Consider a -12.00 Sph Rx that was refracted at 13mm. The lens in the patient's new glasses will sit 10mm away from the patient's eye.

12 squared = 144, 144/1000 = .14

The movement is 3mm closer to the patient's eye, with a minus lens.

3 x .14 = .42, so .5 D is subtracted from -12.00 to reduce the lens power to -11.50 D.

Is it just possible that to much "insight" is not really needed for good "sight ?

Eyes

myope33 · 11-16-2005, 03:55 AM

12 squared = 144, 144/1000 = .14

Ok I am -5

5 squared= 25/1000= .025
If my glasses sit 20mm away, thats .5 diopters reduced for contacts.

This is where I think I made an error in the math. I can see as well with -4 contacts as -5 glasses.

I also have a pair of -4.25 glasses.

4.25 squared =18/1000 is .018 times 20 is .36 diopters.

Yet -3.25 diopter lenses give me the correction of -4 glasses and -4 contacts
give me as much as -5 glasses.

Lets try the formula for crazy amounts of myopia.

50 squared= 2500/1000=2.5 diopters per milimeter!

If his glasses sit 10mm, he would need -25 contacts.
If his glasses sit 15mm, he would need -12.5 contacts.
If his glasses sit 20mm, he would need -0 contacts.

take a -25 myope:

25 squared=625/1000=.625

If his glasses sit 10mm, he would need -18.75 contacts.
If his glasses sit 15mm, he would need -15.5 contacts.
If his glasses sit 20mm, he would need -12.5 contacts.

If we go by that formula, a -50 and a -25 myope concievabily could be wearing the same minus power contacts! My formula of 1.25 has worked for me on all the contact samples ive tried.

-3 contacts times 1.25 equals -3.75 glasses
-3.25 contacts times 1.25 equals about -4 glasses
-4 contacts times 1.25 equals -5 glasses

contacts stronger than -4 would make me a hyperope.

twoeyez · 11-16-2005, 05:59 AM

myope 33 :

I'm glad that you have thought thru, in such detail, some of you examples on vertex distance. You have some real concerns about the "WHY" of how vertex distance is an important concept for Rx strangth in both contacts and glasses. Maybe, to supplement your "day job",you might want to consider teaching "Geometrical Opticts"

Is it just possible that to much "insight" is not really needed for good "sight ?

Eyes