Prism trouble shooting question

**drk** · 08-01-2023, 08:07 AM

So your facts show that really, the double increase in chroma (assuming that's the problem) is from double the prism. Lens material isn't the factor.

BUT, you can use a low index material to get a little of the constringence back.

**Kwill212** · 08-01-2023, 10:11 AM

Originally Posted by drk

Ooo. Good first reply.

Don't make it your last!

Here's my stupid easy take.

poly and 1.67 have the same optics and general usefulness. It's "poly" and "ultra-poly". That's the way I roll.

(I now avoid 1.74)

trivex is a mid-index tweener between basic old CR-39 and poly/ultra-poly used for special circumstances. (And 1.6 is polycarb that went to the gym and firmed up it's abs a little, but it's still a high index like poly.)

I disagree with pretty much all of this. Looks like a bad AI bot wrote this comment.

**drk** · 08-01-2023, 10:59 AM

Well, then don't just dog me...give me your take.

(After you get a coffee. You're cranky again.)

**ChrisBowers** · 08-07-2023, 07:43 AM

Between Cr-39™ and 1.67 we are only talking 3/10 mm difference thickness. Dump the drilled rimless whare the lab has adopted the strap thickness for tensile strength and use a metal or Zyl {age} mount where we can reduce edge thickness to a minimum. Lower the index of refraction as best you can. What your experiencing is light being broken down into spectral components where blue light and red light are focusing too far apart.
Chris

**chamilun** · 08-07-2023, 02:11 PM

drk. Your posts sound like they were written by AI....

now THATS insulting!

;)

**AngeHamm** · 08-07-2023, 02:39 PM

Originally Posted by Elvis Is Alive

No, I am aware of the supreme brittleness of 1.74.

It has more to do with me. Personally I don't like fit a drill mount in very many RX's above 4 diopters unless it is a small eyesize or the decentration is quite small.

I'll have a long conversation with someone if they insist on a drill with high plus/minus.

My -9.50 Silhouettes in 1.74 look amazing. Just saying.

**ChrisBowers** · 08-07-2023, 04:11 PM

my bad I meant lower index

**drk** · 08-08-2023, 08:14 AM

Originally Posted by AngeHamm

My -9.50 Silhouettes in 1.74 look amazing. Just saying.

Amazingly thick.

**Elvis Is Alive** · 08-08-2023, 10:03 AM

Originally Posted by AngeHamm

My -9.50 Silhouettes in 1.74 look amazing. Just saying.

If you like it, that's all that matters I guess...

**NAICITPO** · 08-08-2023, 01:41 PM

Duplicate

**NAICITPO** · 08-08-2023, 01:42 PM

Originally Posted by luka

Excuse me as it's my first reply. If reducing the prism is absolutely not an option based on the optom, the trivex would be the way to go to reduce the transverse chromatic aberration.

Based on the article Robert linked, for a plano polycarb lens with 2Δ, we have a colour spread of about 0.067D. Going to a 4Δ lens in 1.67 we have about double, or 0.125D, which is the problem. 1.6 won't solve the problem (0.111D @ 4Δ) but trivex gives us only 0.093D, only 1.5x more than in the poly lens. So assuming everything is pretty much equal between the old glasses and the new except for the prism, trivex would be the choice to go for if you're doing a remake.

Cosmetically it'll be thicker yes. Sometimes I discuss with patients and get the lab to narrow drill mount lenses by a mil or two which could save you some of the nasal thickness. I would agree with Kwill about ordering the 70.5 PD.

Agree with DRK great post, please post more!

For those that don't know where the 70.5 is coming from:

PD's from pupilomters are 33.5/34.5. According to System for Opthalmic Dispensing you take the amount of prescribed prism and multiply it by 0.3. 4 prism diopters x 0.3 = 1.2. For BI you add the 1.2 onto each individual PD, for BO you subtract.

R = 33.5 +1.2 = 34.7
L = 34.5 +1.2 = 35.7

Total of 70.4.

**lensmanmd** · 08-08-2023, 04:26 PM

Originally Posted by drk

Well, then don't just dog me...give me your take.

(After you get a coffee. You're cranky again.)

Yep, it’s like Kwill is trying to channel Chris in a grating way. To each their own.

Thumper comes to mind, and I’m not heeding his advice right now.

And you wonder why I’m not as active here as I used to be. Too much personal opinions, not enough advice.

I’ll return to my dark space now.

**chamilun** · 08-09-2023, 11:11 AM

Please dont let other posters affect your participation. I rarely visit the site but has a lot of great info.

Originally Posted by lensmanmd

Yep, it’s like Kwill is trying to channel Chris in a grating way. To each their own.

Thumper comes to mind, and I’m not heeding his advice right now.

And you wonder why I’m not as active here as I used to be. Too much personal opinions, not enough advice.

I’ll return to my dark space now.

**Robert Martellaro** · 08-09-2023, 02:52 PM

Originally Posted by AngeHamm

My -9.50 Silhouettes in 1.74 look amazing. Just saying.

I've done that but at least one eventually cracked. I wouldn't warrant it in a notch or drill.

Originally Posted by ChrisBowers

my bad I meant lower index

Or better still, higher Abbe value.

Originally Posted by NAICITPO

For those that don't know where the 70.5 is coming from

Light is deviated 1cm at a distance of one meter. The distance from the center of rotation of the eye to the cornea, about 13.5mm, plus the vertex distance, about 13.5mm, averages 27mm (the stop distance), hence a deviation of about .27mm per diopter of prism. Adjust the fitting point for all meridians of prescribed prism.

Best regards,

Robert MArtellaro