Choosing between Abbe numbers or eyeball miniaturization

[miwe13]

For a -6.00 bilateral lenses would there by a great benefit going from Essilor 1.74 to 1.6 index lenses as far chromatic abberation is considered. I believe the jump in Abbe numbers is from 32 to 41 respectively. Of course the down side is the miniaturization effect, particularly around the edges of the lenses, will be far more pronounced on the 1.6 index lenses versus 1.74. Which would you say is the better trade off. I realize the preference would be individual dependent, but how significant are these Chromatic abberation versus miniaturization. Which would be better to live with in the long-term?

[Uilleann]

Don't fit the patient in a 58+ eye rectangle or aviator, and you should have little/no issues with 1.74 / 1.71. Small, round, and keep the frame PD and Pt PD as close as possible. Simple.

[Quince]

I would focus on a well fit frame as well. 1.6 is definitely my preferred index for higher Rx if I can get away with it. I don't usually bother with 1.74 until I'm over -7 or -8 total power.

[miwe13]

Don't fit the patient in a 58+ eye rectangle or aviator, and you should have little/no issues with 1.74 / 1.71. Small, round, and keep the frame PD and Pt PD as close as possible. Simple.

The frames will be round and medium sized think Gregory Peck in 'To Kill a Mockingbird'. Patient preference I have to work with.

I would focus on a well fit frame as well. 1.6 is definitely my preferred index for higher Rx if I can get away with it. I don't usually bother with 1.74 until I'm over -7 or -8 total power.

I suppose this is because of the Abberation issue or cost? I am ignoring costs here completely. Only basing the decision on the trade off I mentioned above. Would you say the miniaturizing effect isn't that significant for a 1.6 compared to 1.74?

[drk]

I think the miniturization effect is tough to control, unlike high plus.

[optical24/7]

I think the miniturization effect is tough to control, unlike high plus.

Correct. Mini/magnification are most effected by vertex distance and lens thickness. A plus lens can vary greatly in magnification if made thicker ( same material or because of using a lower index material) and vertex distance changes. A minus lens though, given the same thickness and vertex distance will have virtually no difference between the two when made of differing indexes. In other words you'll have no miniaturization difference between 1.60 and 1.74 if you use the same thickness and base curve on the above Rx (say 2 base w/1.5 ct's.)

[Uilleann]

The frames will be round and medium sized think Gregory Peck in 'To Kill a Mockingbird'. Patient preference I have to work with.

Oof. Well, your work is laid out for you then. Not much else to do. When fashion taste trumps optics & best form, that's what you gotta do.

[Tallboy]

Essilor 1.74 is crap.

Zeiss individual 1.74 or Seiko or I'll just pass on the resin as a whole these days. Since they stopped making the stock Crizal 1.74 blanks its in house only and I find their consistancy to be lacking. The Zeiss product is on a whole other level of quality. That's coming from a guy who rarely uses zeiss lenses.

2016 was the year of 1.74 Crizal coating defects for me, I think like 70% of the ones I dispensed came back with peeling.

[Uilleann]

Huh. Interesting.

20 years dispensing the stuff, and I could count the lab AR/HC defects of Essilor's 1.74 on one hand - with fingers left over. Zeiss for us was a complete disaster. Pretty to look at, but the second you touched it, their Purecoat AR would scratch and fall off. Optically, neither are going to match crown glass. High index resins are what they are. There's a time and a place, and for the vast majority of pt's I've fit in 1.74, they love the result.

[Tallboy]

U Im talking about the newest iteration of the individual with their new duravision AR, the ones only surfaced in japan or germany. I agree purecoat wasnt very good, and maybe I'm dealing with some new issues from a new easilor coating lab but I am pretty sure its always outsourced to Omega. Anyway if you ever get the chance to put a patient into the duravision 1.74 individual sv or pal try it - you will be amazed.

[Fread]

As rightfully stated above, miniaturization effect won't be greatly decreased with 1.74. Also, manufacturers often like to say there is no point to worry about 1.74's Abbe number (and even Poly...).
Personnally, I think 1.74 for a -6.00 is kinda overpowered. You could also try 1.67...? (as far as I know, 1.67 results in a little bit more chromatic aberration than 1.74, however it has a better light transmission ratio)
But as soon as the frame is well fit, 1.6 (maybe aspheric ?) will give a nice result in my opinion.

I understood you are mainly concerned about optical quality, but let me also give you this (comes from Essilor) as weight and thickness could also matter ;)



(full resolution here : http://tof.canardpc.com/view/5eb8418...9ad07caffb.jpg)

[merrymaker]

2016 was the year of 1.74 Crizal coating defects for me, I think like 70% of the ones I dispensed came back with peeling.

This X10. As in at least 10 patients with failed Avance on 1.74. Had to convince several that indeed they really did have a good coating on their lenses....

[Paul Smith LDO]

My first choice would be a digital back surfaced Hi Vex and a second option would be a digital back surfaced 1.60.

[Uilleann]

In a -6.00 OU, you'll want to gauge your client's hatred of visible lens thickness (and unavoidable frame distortion if large, rectangular/square, and zyl), against the fear of the horrible devil Abbe! ;) You may well find they value the added thinness of a 1.74, 1.071, or even 1.67 above anything else. Have you had that discussion with them?

[AngeHamm]

Never underestimate the importance of patient education and managing their expectations. The "eye miniaturization" effect is, at its heart, unavoidable physics; it's the inevitable result of their lenses doing their job. Frame choice above all will reduce it, but nothing will eliminate it from all angles in all circumstances.

[ThatOneGuy]

U Im talking about the newest iteration of the individual with their new duravision AR, the ones only surfaced in japan or germany. I agree purecoat wasnt very good, and maybe I'm dealing with some new issues from a new easilor coating lab but I am pretty sure its always outsourced to Omega. Anyway if you ever get the chance to put a patient into the duravision 1.74 individual sv or pal try it - you will be amazed.

Let's be abundantly clear. Purecoat was the most disgraceful product to ever wear the Zeiss name.

[AngeHamm]

Let's be abundantly clear. Purecoat was the most disgraceful product to ever wear the Zeiss name.

If you say so. I have two pairs of glasses with Purecoat AR and I love it. But then again I baby my glasses.

[MakeOptics]

Hi-index tends to be aspherized to flatten the front of the blank which causes the minimization to be exaggerated. If you can get a higher index in a spherical blank and push towards the peripheral of best form towards the higher base side of things that would help. Of course it's much easier to order a digital lenticular in a lower index from a lab like FEA that would give you all the benefits of the lower index options and reduce thickness as well.

[Tallboy]

Let's be abundantly clear. Purecoat was the most disgraceful product to ever wear the Zeiss name.

And lets be abundantly clear that I am not talking about nor have I reccomended zeiss lenses with purecoat? I am not beholden to any single brand in my shop. I use whatever lens/coating/ frame "works" for that patient and their money. The zeiss 1.74 individual with duravision AR is the best, by far, of any lens for high myopes I have ever used. My clients adamantly agree.

[drk]

M(asked) O(ne)

Are minus aspherics flatter centrally? I didn't think so.

[MakeOptics]

M(asked) O(ne)

Are minus aspherics flatter centrally? I didn't think so.

Nope, but when viewing someone through their glasses we're not peeking through a pinhole. Paraxial affect should be considered, if the Rx cant be adjusted the front curve, thickness, and position become the only variables. Steeper front with aspherized back means mag control and best form. Digital lentic reduces edge thickness, whats not to love.

[Robert Martellaro]

Hi-index tends to be aspherized to flatten the front of the blank which causes the minimization to be exaggerated.

Maybe I'm not reading this correctly, so let me try a different explanation.

Best form with a -6.00 D 1.74 refractive index lens using a spherical surface design would call for a +6 base curve. We wouldn't do that because the lens would be thicker, heavier, with an increase in image minification due to increased plate height/vertex distance, as well as decreasing the size of the eye size cosmetically (the latter only very slightly). Instead, the base curve is flattened to about +2.00 or less, which would normally result in a significant increase in marginal astigmatism and power error, but can be corrected by the use of an appropriate aspheric surface design.

Best regards,

Robert Martellaro

[Pete Hanlin]

Personally, I would dispense a 1.67 Essilor SV 360 (merely because I'm most familiar with the Essilor line of products, since I work here).

Points considered:
1. At -6.00 the thickness difference between 1.74 and 1.67 will be minimal (and from a processing standpoint I like 1.67 better).
2. Abbe value difference of 31 vs. 41 is insubstantial (chromatic aberration = prism/abbe or power/abbe, the differences just aren't that great unless the patient spends a lot of time looking through the far periphery of the lens).
3. As mentioned, a lens that is digitally surfaced backside may help with minimization if you order something with asphericity on the back. In plus lenses, magnification can be somewhat controlled by making the front surface aspheric. In minus lenses, it's the back surface that controls thickness and can benefit from asphericity.

So basically, any back surface aspheric 1.67 product should do quite nicely (back when I was dispensing, Optima made a wonderful stock product for this kind of situation).

[optical24/7]

Good seeing you here again Pete!

[MakeOptics]

Maybe I'm not reading this correctly, so let me try a different explanation.

Best form with a -6.00 D 1.74 refractive index lens using a spherical surface design would call for a +6 base curve. We wouldn't do that because the lens would be thicker, heavier, with an increase in image minification due to increased plate height/vertex distance, as well as decreasing the size of the eye size cosmetically (the latter only very slightly). Instead, the base curve is flattened to about +2.00 or less, which would normally result in a significant increase in marginal astigmatism and power error, but can be corrected by the use of an appropriate aspheric surface design.

Best regards,

Robert Martellaro

-6.00 in 1.74 should go on a spherical 2.50 in a point focal design "punktal". My suggestion if a blank exists is push a little higher, 1.00 diopter is as far as i would push. So a 3.50 base, aspherize the back to compensate the error.