Compensated Rx
I understand the concept behind compensated Rxs, but I don't understand how the computer comes up with the Rx. On the Varilux W2+ and the W3+, vertex is not taken into consideration, so why would the power change at the optical center? Why does one axis change by 1 degree, but the other changes by 18 degrees? Recent example:
Prescribed Rx
-8.75 -1.00 x 30
-9.50 -.50 x 10
+2.50 add
Compensated Rx
-8.39 -1.39 x 31
-9.25 -0.72 x 173
OD add +2.57
OS add +2.56
Because of tilt and wrap. When you tilt a lens you induce cylinder. The program is compensating for that induced cyl.
The optical center isn't sitting still. It moves around with the frame. X-Y-Z axis. The compensations are so the light coming through the center of the pupil is correct. The optical center doesn't matter; the pupil center does.
And the vertex is being taken into consideration, only the vertex measurement is being assumed to be "standard" which is defined by the lens design company and is usually between 12.5mm to 15mm depending on the designer.
Also here is some great info by the optical gawd D-meister
http://64.50.176.246/files/memo_on_u...on_of_wear.pdf
Vertex distance is not being used in the compensating calcualtions, just tilt and wrap. If your manufacturer takes center of rotation into account then the vertex distance is used in addition to the center of rotation to compensate add power along with oblique errors. In order to compensate for the vertex distance the refracted vertex distance would need to be known and most prescriptions do not list this value.Originally Posted by Tallboy
What modern lens design compensates an RX without taking into account ECR lensgrinder? IOT compensated designs all do, VSP's UNITY VIA lenses do, Varilux lenses most certainly do, The Autograph lenses do, as well as any compensated SV custom aspheric/atoric lens. I know the Zeiss Individual does.
Not trying to be snarky, I just can't imagine a lens that is delivered with a compensated RX that is not taking ECR into account in the design. Do you know of one off hand?
Of those you listed I have tested the IOT and Autograph lenses, with more than one independent lab. Neither comped for vertex distance. Just run a -15.00 through the software, vertex of 7mm and vertex of 20mm, and see what it spits out. In my testing it did not change the power. I guess it could be the lab software not being installed or preforming correctly. I haven't done any testing with Varilux or Zeiss lenses yet.
Okay but the lens design changes, I guess thats what I was referring to. Funny that it actually doesn't change the RX at all. I knew vertex was involved in estimating the eyes rotation but I was not aware it affected nothing else!
I should have been a little clearer, it is not being compensated for power changes in regards to fitted vertex versus refracted vertex, because no one gives the refracted vertex when the lens is ordered. There may be a few labs that will compensate for this value before they produce the surfacing file if the refracted vertex is given, but it is not done from a manufacturers stand point. The reason to have the vertex is to add this to the center of rotation to compensated for mean oblique power, oblique astigmatic error, etc. It is also used to calculate errors through the corridor along with the given tilt and wrap.
This depends on the LMS software. Ours does have a field for fitted Vertex vs refracted Vertex, but it is a hidden field. I know where it is, but most CS agents don’t. As lensgrinder stated, ODs don’t write refracted vertex. I guess they just assume that we know! Doesn’t help us rats a bit. What say you DRK?
Quick question! Which comes first in the order of operations? Tilt compensation or vertex compensation?
Which LMS are you using, if you don't mind?
Gotta be vertex.
If your lab is compensating the power for vertex distance then they would perform that calculation prior to tilt and wrap calculations. If you perform the calculation order for the vertex compensated prescirption.
VisionStar
Biggest problem is that ODs do not provide refracted vertex. They assume that opticians and Labs know. Problem is, refracted vertex varies from office to office. We can assume 13, but it ranges from 12.5 to 14. This makes it tough for Labs to provide the required compensation. Is this too much to ask for?
I'm still confused. With the W2+ and W3+, you don't provide the pantoscopic tilt or wrap measurements. When you order the lenses uncut, why would those measurements come into play?
Can you explain a little more, please? Perhaps I'm at a disadvantage, never having worked in a lab. On the Varilux W2 and W3, the optical center is set at the center of the pupil. So, if I'm reading the lenses at the optical center, it seems to me that the script should be as ordered. I can understand it varying as we move away from the optical center, or if vertex or panto measurements were taken into consideration, but they're not. I'm really trying to understand this. Thanks!
Most freeform use average POW for calculations, unless specific POWs are provided. The majority of wearers are comfortable with this.
Free Form designs are all compensated. Not all compensated designs require POW measurements(customized)
I didn't attend the funeral, but I sent a nice letter saying I approved of it. Mark Twain
There are quite a few that are not compensated, but still can be customized with POW. Again, most of these designs utilize average POW as a default.
I believe if you don't provide POW for Varilux compensated lenses the defaults they use (ie: the defaults they use on the non "fit" designs, which are exact same designs just without you providing custom POW) 5 degrees for the wrap, 9 degrees for the Panto and 14 mm vertex. I may be off a bit, but those are the numbers I remember.
You want the truth?
You can't handle the truth!
The sad truth is, phoroptors have a very crappy means by which to measure vertex. You need like a super secret spy glass to read the vertex on most of them...
In a flat world (I'd like to live in that world) we only align the optical center horizontally and vertically at the pupil center (which is assumed to be the visual axis, but really it's not...it's where the reflection of the pupillometer mire is...that is, about 1/2 mm nasal to the pupil center, but even that's not exactly right) and you're done!
But wait, we have a 3D world. And even worse, those eyeballs...they're moving around all the darn day! That's where it gets complicated.
Digression: do we "need" to make it this complicated? Probably not in almost all cases, but what other near-theological discussions could we have without optical minutiae?
Lenses have to be tilted (panto) so that the eye is the same distance from the lens in downgaze as the distance is in straight-ahead gaze. Rats. Now what? When you tilt the lenses down at the bottom, you invoke a lens power error (more minus and more WTR cyl). Hopefully the OD set the phoroptor in some semblance of six degrees panto to refract, so that the phoroptor lenses match the tilt of the final lenses.
What's more, then you have to drop the OC by 1 mm per 2 degrees of panto. This keeps the ray of light coming through the optical center intersecting with the (nebulous and rather variable) center of rotation of the eye, so that when you look up and down, the power is stable. (I really don't understand that, but maybe the Great Santini, who knows stuff like this, will post).
Now, tilting lenses with faceform is another problem, and you can't get a phoroptor to wrap. Plus frames have different wrap, anyway. So for that one you have to change the power of the lens so that when light goes through a tilted lens, it comes out the way you want it to. That's calculable rather easily (with a program!). In single vision. Variable power lenses have variable compensations...so that's a much more complex calculation that presumably Shamir et al understand and build into their programs.
Vertex, IMHO, is a toughie because glasses always slip and squirt, and as mentioned, Garbage In from the prescriber because who knows what the intended vertex is, in the first place.
So, long story short, we have to move the optical center of the lens in all sorts of directions so that the light coming into the pupil is correct.
Curious as to what you mean. Free Form; as explained to me by the experts, all are compensated designs by definition. POW means that you are customizing the frame, Rx, to the individual as the frame is fitted to them. Average POW loses it's meaning in the translation; as you are now incorporating default or borrowed statistical data. Simply put one is either customized or not yet both retain their Free Form status.
Last edited by Paul Smith LDO; 03-27-2018 at 02:20 PM.
I didn't attend the funeral, but I sent a nice letter saying I approved of it. Mark Twain
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