Unusual Lens Form

[go_hercules]

I was trying to identify several pairs of spectacle lenses and came across an unusual one. The supposed prescription of the lens is 1.25-1.75@175 with a 1.5 FT add. I don't know if the front curve was spherical or aspheric, so I clocked it and what I found was odd. The front base curve at 180 degrees was 6. I checked the segment and it clocked at 7.5. So far so good. So I went on to clock the back surface. At 180, it clocked at 4.75. 6 - 4.75 = 1.25 like the prescription. Then I clocked the back surface at 90 and found a curve of 8. That would mean the power at 90 would be -2 rather than the expected -.50 (1.25 - 1.75). So I clocked the front curve again and indeed got 6 at 180, but then I clocked the front at different angles and actually got around 7 to 7.5 at 90 degrees. So, 7.5 - 8 = -.50 >>> this is indeed the power expected. So the prescription is correct, but why oh why would the front surface be so toric? Even a flatter aspheric lens should at least be equal curvature all around, and not so toric. I can't imagine the lens (CR39) would have warped that bad. Any ideas here? Thanks.

[Lelarep]

Is it some kind of free form compensation design? From what I know, if you try to use traditional methods on those, you'll get all kinds of strange results since they don't follow the usual rules.

[go_hercules]

This lens was purchased online pretty cheaply so I am pretty sure it is not anything sophisticated like a free form lens. I would guess just spherical, maybe an aspheric at best.

[rbaker]

Did you remove the lenses from the frame prior to clocking them? What is the F.C.T. of the lenses?

[Lelarep]

Oh, online CR-39... I'd just assume its some kind of screw-up and forget about it.

[go_hercules]

Dick, I took the lens out of the frame after your post and rechecked it. Also checked the mating lens while I was at it, and it has a purely spherical front curve. No difference, but good suggestion for sure. What is F.C.T. ?? Thanks.

[rbaker]

F.C.T. = Finished Center Thickness.

[go_hercules]

Finished center thickness is about 3.5mm.

[lensgrinder]

The front base curve at 180 degrees was 6

The lens clock does not equate to the actual surface power of the curves. The lens clock is calibrated to 1.53. Your actual surface power for a CR-39 lens with a 6.00 D front is 5.64 D

Then I clocked the back surface at 90 and found a curve of 8

The surface power of 8.00 D is 7.51 D

This would put your cylinder at:
5.64 - 7.51 = -1.87

actually got around 7 to 7.5 at 90 degrees

Actual surface power of 7.00 D and 7.50 D are 6.58 and 7.05 D

This would put your cylinder at:
5.64 - 6.58 = -0.94
5.64 - 7.05 = -1.41

I am curious as to why you are getting three different readings at 90? Your differences have spanned 1.00 D.

[go_hercules]

Thanks for the reply. I understand what you are saying about the 1.53 index the clock is calibrated to vs. the lower index of CR39 and that definitely would sway my results. BUT the anomaly I am seeing is what you mentioned: different readings at 90 degrees. Since the front curve at 180 was 6, I expected the front curve at 90 to be 6. But it was 7 to 7.5 (I had measured quickly - after your post I remeasured more carefully and would say it is 7.5). So I am getting consistent readings on the front but they are NOT indicating a spherical lens, more of a toric. The back is also toric like expected of a minus cyl.. The total powers in each meridian work out to the final prescription okay. It is looking more and more to me like the lens warped after manufacture. Is that feasible??

[lensgrinder]

Thanks for the reply. I understand what you are saying about the 1.53 index the clock is calibrated to vs. the lower index of CR39 and that definitely would sway my results. BUT the anomaly I am seeing is what you mentioned: different readings at 90 degrees. Since the front curve at 180 was 6, I expected the front curve at 90 to be 6. But it was 7 to 7.5 (I had measured quickly - after your post I remeasured more carefully and would say it is 7.5). So I am getting consistent readings on the front but they are NOT indicating a spherical lens, more of a toric. The back is also toric like expected of a minus cyl.. The total powers in each meridian work out to the final prescription okay. It is looking more and more to me like the lens warped after manufacture. Is that feasible??

I mis-read where you stated it was the front surface, sorry.
It must be warped, because I do not know of a toric CR-39 FT lens. Just curious, when you take you BC @ 90, the leg of the clock is just above the segment, which is about 15-20mm from the center of the clock. Are you reading the power 15-20 mm above the seg?

[go_hercules]

The lens is a small oval shaped lens and there is just enough room above the seg to place the 3 points of the clock. That is where I am taking the 90 degree reading. I also took the 90 degree reading just outside of, or beside, the seg where there was room for the clock. In both cases I got the same 90 degree reading. Like I mentioned, the prescription actually comes out right so the lens was apparently made correctly. But it is strange that it appears to have warped right along the axis, since the BC is correct at 180 (axis is 175).

[Robert Martellaro]

This lens was purchased online pretty cheaply...

The ANSI Z80.1 mechanical tolerance for warpage is a maximum of 1 D. The warpage might have come right out the box, or ocurred during surfacing or finishing.

It's not easy to make a customized ophthalmic lens that meets or exceeds Z80.1 specifications, to find someone who has the know-how to optimally interpret an Rx, and then design a custom lens to match the individual eyeglass wearer's visual requirements. Although a higher price for ophthalmic services doesn't guarantee a first-rate pair of eyeglasses, low prices without a face-to-face fitting by a certified optician is a guarantee of low quality, typically with grievous errors that negatively effects visual acuity and/or comfort, along with poor physical comfort.

[go_hercules]

Hi Robert, you mentioned the warpage might have come right out of the box. If that were the case, would the back curves be ground to match the irregular front curve (or in this case front curves plural)? I ask because the total powers in this lens are correct in each meridian. So if the front started out weird, the back would have to had been ground weird to match. I appreciate the help. I know many figure what the heck, just move on. But I get intrigued by such stuff and really want to figure it out.

[Robert Martellaro]

Hi Robert, you mentioned the warpage might have come right out of the box. If that were the case, would the back curves be ground to match the irregular front curve (or in this case front curves plural)? I ask because the total powers in this lens are correct in each meridian. So if the front started out weird, the back would have to had been ground weird to match. I appreciate the help. I know many figure what the heck, just move on. But I get intrigued by such stuff and really want to figure it out.

Warpage out of the box for finished lenses only (SV). There was a thread awhile back where the optician surmised that a lens was ground with a toric curve on both sides as one way to explain a certain anomaly in the lens, but I can't remember the specifics. Regardless, if you had a warped front curve on a semi-finished lens it would have go into the garbage.

[go_hercules]

Well this is a FT bifocal with minus cylinder, so I guess I will have to be satisfied that it was ground correctly from a good blank, and must have warped at some point. Thanks a lot for the help.