More *Free-Form* Confusion!

[Jacqui]

Fezz,
You raise great questions. I'll blow your mind even further. I ordered three digital progs(i probably shouldnt mention which brand) from three different labs. They are considered the top digital prog labs. 2 on east coast and one in mid-west. I ordered the exact same rx, same lens , same measurements and provided same exact frame tracing. They send you a sheet with the verified rx that we should check. They should be the same right?????? Well they werent. All 3 labs provided different verified specs. Why? How do I know which of the 3 are the best for the patient? Is there no standard? Conduct that test yourself and see what you get. All three labs are provided with special software from the same manufacturer. But they also have there own lab software system which i guess works with the lens manufacturer software. My guess is when you mix the lab software with the lens company software its causing a different result?? So now the question becomes which is right? They all cant be right or can they? Are we to accept blindly what is on the piece of paper? Would love to hear feedback

Been trying to say that for some time now and no one listens. Not only are there software differences, but also equipment to take into consideration.

[HarryChiling]

Been trying to say that for some time now and no one listens. Not only are there software differences, but also equipment to take into consideration.

Jacqui,

I've been right one board with you. I even made a suggestion for the company of the software to provide decompensation software to opticians so that they can verify that the Rx is correct according to their specs. To many variables and who's to say that the end result was what the designer's software intended when the job ultimately gets delivered.

[Jacqui]

Jacqui,

I've been right one board with you. I even made a suggestion for the company of the software to provide decompensation software to opticians so that they can verify that the Rx is correct according to their specs. To many variables and who's to say that the end result was what the designer's software intended when the job ultimately gets delivered.

We need some way using conventional equipment to verify these. Not all of us (even freeform labs) can afford Rotlex FFV units to check these lenses.

[chip anderson]

Are Rotflex gadgets accurate? If they can't be checked with a lens bench or something how would one know they were calibrated accurately?

chip

[HarryChiling]

We need some way using conventional equipment to verify these. Not all of us (even freeform labs) can afford Rotlex FFV units to check these lenses.

Actually some of Rotlex's equipment isn't to far out their like many would think. They have equipment that would cost you in the same ball park as an automated lensometer. I don't think though that that is even necessary, checking localized powers like we do now with a standard lensometer can be accurate enough, but to know that the various compensations employed nowadays matches to the software and ultimately designers intended design is going to be paramount. I have heard before the trust us line and I of course will only do business with those that I trust, but at the same time their needs to be a way to verify results. Anyway I think what's on the market now FF wise is all pretty darn good and getter better by the day. More and more competition is breaking into the FF arena and eventually we will start seeing more of the pioneers start to share information about how their designs are comparatively better than others on the market and then we'll find out who's been cheating us with fancy new technology and who's been using fancy new technology to improve their product.

Until then I will continue to occasionally dip my toes into the FF waters.

[AWTECH]

Are Rotflex gadgets accurate? If they can't be checked with a lens bench or something how would one know they were calibrated accurately?

chip

This type of equipment has no moving parts, other than the slide in and slide out for the lens to be measured. Calibration is a done via software and can be done at anytime with no lens in place. If the software for this type of equipment is written correctly then it will either calibrate correctly or not. If not you contact the manufacturer or know how to fix the problem. (such as changing the expensive bulb).

[xiaowei]

By the way (and Harry-Darryl, correct me if I'm wrong here):

Spherical aberration is an *on-axis* aberration

Conceptually, Coma is spherical aberration's *off-axis* first cousin.

Barry

That's a good definition.

NO, NOT TRUE! SORRY!

What you are talking about applies to properly centered, rotationally symmetrical optical systems. These will/may have on-axis spherical aberration, but NO on-axis coma (already due to symmetry arguments), only off-axis coma (that´s what Seidel´s theory describes)

HOWEVER, a PAL is an extreme counterexample to a properly centered, rotationally symmetrical optical system. these systems also WILL have "on-axis" coma and trefoil due to the change of power along the progression channel within the pupil area, even Essilor CANNOT avoid it (as long as they don´t reduce the add to zero!), it´s only "Techno-Babel" (or maybe Techno-"Bubble"!?:) HAHA!)

In other, probably simpler terms, the "effective center of symmetry" of a PAL is way off the actual lens center due to the progressive zone, so you will ALWAYS use it "off-axis" -> Coma!

[Barry Santini]

Thanks, xiaowei, for your clarification. For ease of understanding coma, I was referring to, in a simple sense, a rotationally symmetric lens , which, of course, a progressive is not.

It's great to have you here to help clarify, explain and share your knowledge!

barry

[xiaowei]

Thanks, xiaowei, for your clarification. For ease of understanding coma, I was referring to, in a simple sense, a rotanially symmetric lens , which, of course, a progressive is not.

It's great to have you here to help clarify, explain and share your knowledge!

barry

Hi Barry, you´re welcome!

"Unfortunatelly", this is (part of) everday`s work. The company I work for usually designs fully rotationally symmetrical optical systems that are also highly corrected for off-axis coma etc. (because we sometimes use 10 or more lenses), however, due to residual manufacturing tolerances and surface errors, the whole system will end up somewhat decentered and unsymmetrical and will also show on-axis coma on the order of some waves or even only some tenth of a wave, often too much for our (customer´s) purposes as semiconductor process inspection with nm resolution. The most cost-effective compromise (besides getting the single lens element as good and centered as feasible) is to have a fine adjustment step under interferometric or high magnification control for (usually) one or two selected element(s) to move the overall "center of symmetry" of the system back on axis and then it will behave almost "ideal" again!

Sincerely