Lens thickness & fitted back vertex distance

[ak47]

I do not understand why we don't have to consider Rx lens thickness when measuring fitted back vertex distance. Especially in a high plus lens the thickness is significant and the "back" of the lens is much closer to the cornea then the demo lens.

Can someone smarter than me please explain?

[Robert_S]

I've thought this myself. Glazing lenses into a frame can also affect the wrap and panto. Therefore, on high prescriptions and in certain frames, how accurate are we really being when providing POW measurements?

[drk]

Are you saying, AK, that we use some vertex-o-meter device, but we're measuring a demo lens, vs. a flat-backed high plus or curvy-backed high minus? I've noted that, too.

Heck, I've noticed how sloppy vertexometry is, in general, from phoroptor to vertexometer to adjustment to schnozz-skiing.

[lensmanmd]

Following

[AngeHamm]

Where is the effective "front" of the vertex distance? It seems like it would be the front of the lens, the back of the lens, or the center of the lens. If it's one of the latter two, high hyperopes would certainly be affected. I need some lens guys to chime in here.

[AngeHamm]

Are you saying, AK, that we use some vertex-o-meter device, but we're measuring a demo lens, vs. a flat-backed high plus or curvy-backed high minus? I've noted that, too.

The high minuses shouldn't really make a difference, as their center thicknesses will be similar.

[Robert Martellaro]

I do not understand why we don't have to consider Rx lens thickness when measuring fitted back vertex distance. Especially in a high plus lens the thickness is significant and the "back" of the lens is much closer to the cornea then the demo lens.

Can someone smarter than me please explain?

The plate height (and sometimes bevel placement) has the greatest influence on the vertex distance (see image below). I see this with iseikonic lenses when an increase in the base curve on a minus lens (to decrease minification) is usually offset by a corresponding increase in the vertex distance (increasing minification). Exceptions are moderate plus full back surface progressive lenses and high plus lenses of all types, where the plate height and back curve approaches zero.

Where is the effective "front" of the vertex distance? It seems like it would be the front of the lens, the back of the lens, or the center of the lens. If it's one of the latter two, high hyperopes would certainly be affected. I need some lens guys to chime in here.

Because the effective power of an ophthalmic lens is the back vertex power, the vertex distance is measured from the cornea to the back of the spectacle lens.

Hope this helps,

Robert Martellaro

[drk]

The high minuses shouldn't really make a difference, as their center thicknesses will be similar.

Ah so

[AngeHamm]

I knew Robert would come through.

[ak47]

But, my question is why are we measuring to the eyewire and not accounting for the thickness of the spectacle lens?

Because the effective power of an ophthalmic lens is the back vertex power, the vertex distance is measured from the cornea to the back of the spectacle lens.

Hope this helps,

Robert Martellaro

[optical24/7]

I do not understand why we don't have to consider Rx lens thickness when measuring fitted back vertex distance. Especially in a high plus lens the thickness is significant and the "back" of the lens is much closer to the cornea then the demo lens.

Can someone smarter than me please explain?

Because the effective power of an ophthalmic lens is the back vertex power, the vertex distance is measured from the cornea to the back of the spectacle lens.

Hope this helps,

Robert Martellaro

As Robert stated, the back vertex is where the power of a lens is measured. AK, if you are referring to lens thickness needing to be accounted for, it's compensated at the manufacturer/surfacing lab so the final power is correct at the back vertex.

[ak47]

Okay...so what I am hearing is that I am not crazy, back vertex really means from the back of the Rx lens, but the lab accounts for the actual lens thickness so we would be "doubling down" as fitters if we include the Rx thickness in the so-called "back" vertex distance we give to the lab?

As Robert stated, the back vertex is where the power of a lens is measured. AK, if you are referring to lens thickness needing to be accounted for, it's compensated at the manufacturer/surfacing lab so the final power is correct at the back vertex.

[drk]

In my mind, the labs determination of BVP has nothing to do with vertex distance. I think that's a red herring.

I think, ak, that the difference in vertex between a thick plus lens (like Robert's picture above) and the plastic plano lens (or eyewire) is, unfortunately, going to the slop bucket.

I don't think that's too great a crime, because, believe me, it's cumbersome to refract at a super-precise (like, +/- 1-2 mm) distance.

Yes, the errors could be additive, but maybe they'll cancel out.

I do know, though, that heavy plus lenses always droop into a longer position, so a bias to underplussing when prescribing is not a bad thing (I do that by biasing to the minus on non-presbyopic myopes, too). In fact, a lot of docs "cut the plus", so a little slip won't hurt at all.

Maybe I'm off-base, here.

[optical24/7]

Okay...so what I am hearing is that I am not crazy, back vertex really means from the back of the Rx lens, but the lab accounts for the actual lens thickness so we would be "doubling down" as fitters if we include the Rx thickness in the so-called "back" vertex distance we give to the lab?

Exactly. Back in my old cataract lens surfacing days, we had charts for how much to compensate the back curve to account for the induced magnification of thick plus lenses and any back vertex differentials from refracted at to wear position. Today, surfacing lab software is way more precise.

[lensgrinder]

Assuming a demo lens with 6.00 D back curve the sag of the back surface will equal 3.63. Let's assume we place a +5.00 D lens in the same frame with a 7.50 D front surface, the back surface sag will equal 1.85. 3.63 - 1.85 = 1.78 mm. As Robert states the bevel will make a difference as well. Regardless 2 mm movement on a +5.00 D lens will not make a difference (~0.05 D).
The majority of labs are not compensating lenses for fitted vertex distance since 99.9%(possibly 100%)of prescriptions do not have a refracted vertex. The compensations are based on wrap and pantoscoptic angle. The vertex distance is used for other purposes.