Thin as possible or at least as thin as the previous pair

[Neophyte Optician]

I have a pair of -4.75 OU glasses that were made out of Pentax 1.67 aspheric lenses that are the flattest and thinnest -4.75 lenses that I've seen. Even if I were to use the patient's old frames, I still wouldn't know how to make them as thin and flat. How would I make the new lenses as flat at the edge? Or, maybe my question should be how do I make the back surface curve as flat as possible? The back surface literally looks like there is no appreciable curve. It's as flat as a tabletop. Would I need to design a bi-concave lens in order to get the flattest lens profile? How would I even do that, or can I even do that? As you can tell, I'm all over the place. Your help would be greatly appreciated. Thanks.

[lensmanmd]

I have a pair of -4.75 OU glasses that were made out of Pentax 1.67 aspheric lenses that are the flattest and thinnest -4.75 lenses that I've seen. Even if I were to use the patient's old frames, I still wouldn't know how to make them as thin and flat. How would I make the new lenses as flat at the edge? Or, maybe my question should be how do I make the back surface curve as flat as possible? The back surface literally looks like there is no appreciable curve. It's as flat as a tabletop. Would I need to design a bi-concave lens in order to get the flattest lens profile? How would I even do that, or can I even do that? As you can tell, I'm all over the place. Your help would be greatly appreciated. Thanks.

Since you didn’t mention if this was SV, I will assume that. Most likely, this was a FSV straight from an envelope at 1.2 CT on something approaching a .50 BC. Good luck replicating that by surfacing that without warping it your hard coater at that thickness.
You could pull a .5 BC MR10, or something close, then dip coat it to come close. What eyesize are you considering?

[Don Gilman]

Since you didn’t mention if this was SV, I will assume that. Most likely, this was a FSV straight from an envelope at 1.2 CT on something approaching a .50 BC. Good luck replicating that by surfacing that without warping it your hard coater at that thickness.
You could pull a .5 BC MR10, or something close, then dip coat it to come close. What eyesize are you considering?

Probably a 62 eye goggle and a 58 pd

[Tallboy]

Seiko full field aspheric 1.67 is very thin, should do the trick.

[lensmanmd]

You can also edge it with a 33% bevel. It should be flush to the front (zyl/acetate) and reduce the perceived thickness behind.

[lensmanmd]

Probably a 62 eye goggle and a 58 pd

You are killing me Don!

[vv0vv]

Probably a 62 eye goggle and a 58 pd

If his Frame PD is larger than yours and considering both of your pds are equal it will always turn out thicker on the lens edge unless you get a lower CT or like you said bi-convex lenses (which i feel is kind of overkill and in my opinion "not so good in the cosmetics department") but up to a point.

[lensmanmd]

I wouldn’t wish a bi-concave for this RX on anyone. Fishbowl anyone?

[Neophyte Optician]

Since you didn’t mention if this was SV, I will assume that. Most likely, this was a FSV straight from an envelope at 1.2 CT on something approaching a .50 BC. Good luck replicating that by surfacing that without warping it your hard coater at that thickness.
You could pull a .5 BC MR10, or something close, then dip coat it to come close. What eyesize are you considering?

52 eye size

[Neophyte Optician]

Does changing the ratio of the front curve to the back curve change the minification effect of a minus lens RX? For example, you could make a variety of -4.75 lenses using different combinations of front and back surface curves. However, would each different combination change how things look to the patient as well as how the patient looks to an observer? What sort of combination, if any, would reduce the minification effect?

[lensmanmd]

Robert? Your words of wisdom?

Rule of thumb. Steeper the back curve, more minification on minus. Flatter the back curve/thicker the lens. More magnification.

Your original question was about thickness. Was it not? If you are fitting a 52, what was the original size? -4.75 has some wiggle room with ET, but not much. 2mm difference in eyesize will have some differences, based on PD and OC.

[optical24/7]

Does changing the ratio of the front curve to the back curve change the minification effect of a minus lens RX? For example, you could make a variety of -4.75 lenses using different combinations of front and back surface curves. However, would each different combination change how things look to the patient as well as how the patient looks to an observer? What sort of combination, if any, would reduce the minification effect?

With your given Rx, vertex distance is going to be the biggest factor in magnification/minification. As lensman stated, steeper curves = more minification, the lens surface will be further from the eye than a flatter curve. You can make a -4.75 on multiple base curves. Best Form lens design would call for approximately a +2.50 front base curve to avoid peripheral and other lens aberrations. Using an aspheric design allows you to use a much flatter base curve, thus moving the lens surface closer to the eye ( creating less minification).

Again, as lensman stated, a stock 1.67 aspheric would probably be the best choice for this Rx if you were looking for as little of minification as possible. ( Stock lenses can be molded thinner than you can usually grind them and use an aspheric design to flatten the B.C.). Be sure to select a frame with as close of vertex as possible to keep the minification as low as possible.

( And yes, please don't try putting this Rx on a bi-concave. Epic failure is assured).

[Oscar]

With your given Rx, vertex distance is going to be the biggest factor in magnification/minification. As lensman stated, steeper curves = more minification, the lens surface will be further from the eye than a flatter curve. You can make a -4.75 on multiple base curves. Best Form lens design would call for approximately a +2.50 front base curve to avoid peripheral and other lens aberrations. Using an aspheric design allows you to use a much flatter base curve, thus moving the lens surface closer to the eye ( creating less minification).

Again, as lensman stated, a stock 1.67 aspheric would probably be the best choice for this Rx if you were looking for as little of minification as possible. ( Stock lenses can be molded thinner than you can usually grind them and use an aspheric design to flatten the B.C.). Be sure to select a frame with as close of vertex as possible to keep the minification as low as possible.

( And yes, please don't try putting this Rx on a bi-concave. Epic failure is assured).

how are stock lenses created to achieve this which surfacing cannot?

[vv0vv]

how are stock lenses created to achieve this which surfacing cannot?

The thing with good stock aspheric lenses for minus prescriptions is that they already have the best edge thickness for its center thickness and curvature even without surfacing. A larger diameter minus lens with the same center thickness and curvature will have the same edge thickness as a smaller diameter blank edged into the same frame with the same patient PD and decentration.

A plus lens center thickness is affected by blank size and curvature. A smaller blank and specially surfaced lens for that frame will yield the best center thickness for that frame if you tell the lab to order thin as possible or knife edge (assuming it’s not a nylon or metal rimless groove).

The aspheric lens will bring the eye closer to the lens which will also reduce minification of the eye for minus lenses and magnification for plus lenses.

[Oscar]

( Stock lenses can be molded thinner than you can usually grind them....

sorry i was referring to this line.

[lensmanmd]

Molded lenses do not suffer from heat/flex induced aberrations introduced during the surfacing process. Even under the strictest of controls, thinner lenses will tend to flex, even with soft polish processes.

[vv0vv]

Stock lenses are molded thinnest for its prescription/curve/center thickness assuming its a good aspheric brand.

When you surface the best it will reach is as thin or slightly thicker than the stock, and assuming its a compensated rx "free-form lenses" will probably be thicker too

[Oscar]

Stock lenses are molded thinnest for its prescription/curve/center thickness assuming its a good aspheric brand.

When you surface the best it will reach is as thin or slightly thicker than the stock, and assuming its a compensated rx "free-form lenses" will probably be thicker too

interesting.. curious to know how the molding process is done. so instead of carving curves out of a semi-finished blank. the material is poured into a mold of the rx?!!! how is the optical quality of that?

[vv0vv]

Woops double posted in error

[lensmanmd]

interesting.. curious to know how the molding process is done. so instead of carving curves out of a semi-finished blank. the material is poured into a mold of the rx?!!! how is the optical quality of that?

Yup. About sums it up. Optical properties are very good, as long as the mold is in good shape.

[Don Gilman]

You are killing me Don!

With kindness

[Neophyte Optician]

With your given Rx, vertex distance is going to be the biggest factor in magnification/minification. As lensman stated, steeper curves = more minification, the lens surface will be further from the eye than a flatter curve. You can make a -4.75 on multiple base curves. Best Form lens design would call for approximately a +2.50 front base curve to avoid peripheral and other lens aberrations. Using an aspheric design allows you to use a much flatter base curve, thus moving the lens surface closer to the eye ( creating less minification).

Again, as lensman stated, a stock 1.67 aspheric would probably be the best choice for this Rx if you were looking for as little of minification as possible. ( Stock lenses can be molded thinner than you can usually grind them and use an aspheric design to flatten the B.C.). Be sure to select a frame with as close of vertex as possible to keep the minification as low as possible.

( And yes, please don't try putting this Rx on a bi-concave. Epic failure is assured).

I was reading an article that stated the following:

The steeper the front/base curve and the greater the lens thickness, the greater the magnifying effect. The combined effect of front curve and thickness is called a lens' shape factor.

So does that mean a +6 BC will lead to more magnification effect than a +2.50 BC? Or, in other words would the +6 lead to less minification effect along with the shorter vertex distance? If so, would it be a bad idea to request a +6BC?

[Tallboy]

If all you care about is the effect of the size of your eye yo other people (and you aren't one of my small eyed clients who wear high base plus lens eyeglasses as makeup) flatter, closer to the eye will always give you less minification.

[lensmanmd]

I was reading an article that stated the following:

The steeper the front/base curve and the greater the lens thickness, the greater the magnifying effect. The combined effect of front curve and thickness is called a lens' shape factor.

So does that mean a +6 BC will lead to more magnification effect than a +2.50 BC? Or, in other words would the +6 lead to less minification effect along with the shorter vertex distance? If so, would it be a bad idea to request a +6BC?

For a -4.75, absolutely not. I would not touch an order like that. For a +4.75, I would consider it, but at the cost of peripheral aberrations.

[optical24/7]

I was reading an article that stated the following:

The steeper the front/base curve and the greater the lens thickness, the greater the magnifying effect. The combined effect of front curve and thickness is called a lens' shape factor.

So does that mean a +6 BC will lead to more magnification effect than a +2.50 BC? Or, in other words would the +6 lead to less minification effect along with the shorter vertex distance? If so, would it be a bad idea to request a +6BC?

Think of designing iseikonic lenses. You manipulate image size differences by altering base curves, lens thicknesses, vertex distances and to a much lesser degree, lens material.

Vertex distance is, by far, the largest contributing factor in altering magnification/minification. As example, with your -4.75 lets look at how each contributes to image size differences;

Lets start with; -4.75 on a 2.50D base with 1.2 c.t a vertex distance of 13.5 in 1.67 material. We get approx a -5.9% minification.

Alter only the base curve to a 4.50 will give us a -5.7% minification.
Leave the original formula the same again, but alter the c.t. to 3mm will give us a -5.6% minification.
Leave the original formula the same again, but alter the material to CR39 will not alter the minification from -5.9%

But alter the vertex distance only from the original formula from 13.5 to 11.5mm will reduce the minification down to -5.0%, almost a full percent doing that alone.

Now, you could put all those changes into your lens design ( use a 4.50 base, 11.5mm vertex and c.t. of 3mm) and reduce it to -4.4% minification. But, your patient would have much more off axis distortion from using a non-best form base curve and have thicker (and heavier) lenses, defeating the purpose of using 1.67 material in the 1st place.

So once again, vertex distance is going to be the biggest factor you can manipulate to reduce minification.