RGP vs glasses Rx - Large difference in power

Because of increasing astigmatism my doctor switched me to RGP lenses.
Could you explain why there is such a great difference in power b/w my
glasses Rx and RGP rx?

Glasses:
sph cyl ax
OD -4.00 -1.00 15
OS -3.00 -1.00 5

RGP:
power diam. b.c.
OD -3.00 9.0 7.75
OS -1.50 9.0 7.75
(the doctor did not give me the print-out with my 'k' readings)

Thank you,

Victor

 

I'm not a doctor, but . . . to my understanding . . . this is because of
"vertex distances."

The vertex distance is the distance between the front surface of /your/
cornea and the back surface of the corrective lens. When a lens for
/nearsightedness/ (what you have) is moved /closer/ to the eye, it becomes
"stronger;" therefore, when you fit a contact lens, instead of glasses, you
need a lower power lens.

If you were farsighted, the power of the contact lenses would be /higher/ by
roughly the same amount.

Does that make sense?

Neil

 

Thanks all for your replies.

Robert Martellaro wrote:

No, the lenses are regular daily wear. (no monovision)

I'm just curious if it is the normal practice to fit rgps with -1.0 or
-2.0 lower power than the glasses rx.

 

The final power of the cl is determined by the fitting
relationship of the k readings and the bc of the lens
(as M.T. alluded to).

After vertex is taken into account, we use SAMFAP (steeper add minus,
flatter add plus)- basically means if the base curve of the cl
is flatter than the flatter k, plus pwr (or less minus)is
added and if the bc is steeper, more minus is added to the final
pwr.

frank