Surfacing High Plus RX on Low Base Curves

[laytonic]

Dear all,

I am in need of some surfacing help when it comes to high plus prescriptions on low base curves. I am used to doing a +7.50 sph in poly on an 8 BC lens, but when it comes to this presciption in a progressive lens, I run into some difficulty.

The highest curve I can find with some vendors is 7 or sometimes 6 and if calculate this base curve with the given RX, my innovations system calculates a (-) tool. Some labs are different with tool selections, mine happens to be all (+) tools, meaning not (-) sign in front.

Im using an SL2 generator from Gerber Coburn and I'm able to cut my own laps if need be, but this machine will not cut (-) tools. Is there a way around this?

And what about these vendors stopping at 7 or 6 for the highest BC available? Has the industry moved to digital lenses that vendors have discontinued manufacturing high BC's?

If anyone can help please let me know how I can surface a +7.50 RX on a 6 or 7 base lense with optical success!

Thanks,
Layton

[WFruit]

Most progressives I know of go up to an 8 base, or in some cases 7, depending on vendor and index (I don't know of any that stop at 6). What lens are you having trouble with?

As for digital/Free Form lenses, their advent has actually forced lens vendors to have a much larger range of base curves (albeit single vision), in some cases going up to even a 12 base.

I'm not familar enough with the SL2 to help you with that, but I am surprised you can't cut minus tools on it. I know you can on some of the other gerber generators (SGX, SG8 come to mind).

[mike.elmes]

Why not use a lens in a higher refractive index? That will keep the laps in the minus range.

[mshimp]

A +7.50 on an eight base should be fine cutting a lap in the normal range.

[HarryChiling]

The SL2 cuts a +7.00 to a -20.00 so you should be able to cut a a lap and lens int he plus back curve range and it's corresponding minus tool. I remember the SGX+ having issues with the base and cross being switched which would cause errors and not allow you to cut the lens or corresponding lap, I remember swicthing some values around and it worked. If you put up the data from the generators screen I might be able to help.

[LENNY]

I dont think you can surface that flat with OPTICAL success!
But depending on the index of refraction you can get flater than the final power on the base.

[Darryl Meister]

Yes, you're going to get horrible optics from a +7.50 D sphere used on a base curve designed for a much lower prescription range, even if the patient doesn't experience "lash crash," excessive geometric distortion, or noticeable backside reflections from such a flat lens. But, as some of the others have already noted, a high-index material will reduce the likelihood of producing a biconvex or saddleback rear surface.

Best regards,
Darryl

[Geirskogul]

Lash Crash is a great name for a band, and a name for something that I didn't know had a name before. I will use it from now on.

[scriptfiller]

You can get an Image 1.67 in a 7 base. I ran the calculation on a +7.50 sph. and it derived a lap tool of 2.00.

[Speed]

if the lashes are touching the lenses, they are too long anyway.
just trim them back until they are no longer touching, like a good optician does.

[hxlens]

Why not use a lens in a higher refractive index? That will keep the laps in the minus range.

i agree, try a higher index

[gunner05]

if the lashes are touching the lenses, they are too long anyway.
just trim them back until they are no longer touching, like a good optician does.

I always offer the use of my pocket knife. No one has ever taken me up on it. :(