Progressive Lens Drop

[EyeFitWell]

This is kind of a spin-off of HappyLady's post below regarding her Davis Lab.
I noticed that the patient wanted a 2mm drop instead of a 4mm drop, and that got me thinking.
I had always guessed that the "drop" had to do with the relative locations of the laser marks and the fitting cross. I never thought that the drop would affect vision at all (as long as the lens was still fit properly for the right drop).
Can someone shed some light on this one? What difference does it make to the patient looking through the lenses?

[Robert Martellaro]

I never thought that the drop would affect vision at all (as long as the lens was still fit properly for the right drop).

I believe that's accurate. The only exception might be with anisometropia- significant power disparities at ninety degrees will induce vertical imbalance on the distance gaze, more so when the distance from the FC (fitting cross) to the PRP (prism reference point or 180 line) is 6mm in the Gradal Top as opposed to 2mm for the Percepta. Another concern might when there is considerable power on the vertical meridian, let's say -10.00D, a 6mm difference will induce 6^ of base up prism compared to 2^ BU with a 2mm difference, which would change the wearer's perspective slightly, probably making the floor slant down and away, with objects displaced downwards.

Which brings me to something I've been thinking about lately- should we lower the fitting cross on high minus (or higher on high plus) to account for the eyes turning towards the prisms apex .3mm per prism diopter? With the example above, Gradal top, 6mm from FC to PRP, .3mm x 6^ = 1.8mm. FC measures 20mm on a plano lens so order 18 high?

[EyeFitWell]

Robert,
Am I correct in assuming from your post that when the drop changes, so does the distance from the FC to the PRP?
And if that is true, does that mean that a lens with a larger drop would essentially have more prism in the lens when in wearing position? (Although it would be cancelling prism)

[Robert Martellaro]

Robert,
Am I correct in assuming from your post that when the drop changes, so does the distance from the FC to the PRP?

Well, I assumed that the drop is the distance from the FC to PRP.

And if that is true, does that mean that a lens with a larger drop would essentially have more prism in the lens when in wearing position? (Although it would be cancelling prism)

Right.

Regards,

[EyeFitWell]

Ah, I see.
I always thought the drop was the distance from the FC to the engravings, which may or may not be in line with the PRP. That's why I couldn't figure how it would make any difference whatsoever to the patient how much of a drop it was. (I was thinking maybe some optician somewhere had convicned this patient that he needed a 2 drop, and he became insistant on it.)
Thanks for the info!

[Happylady]

Isn't the drop related to how far from the fitting cross it is until the power starts to change? A lens with a 2 mm drop will start the change faster then a lens with a 4 or more mm drop. This is what I have understood. If I am mistaken I want to know.

[EyeFitWell]

Happylady-
That's a good point, and I honestly have no idea. I would look at the minimum fitting height to gague where the power changes.
I wonder if there is an example of two progressives where the minimum fitting heights were different (say 18 vs 24), but the drop was the same. What would that mean about the lenses?
Thoughts, anyone?

[Robert Martellaro]

Ah, I see.
I always thought the drop was the distance from the FC to the engravings, which may or may not be in line with the PRP. That's why I couldn't figure how it would make any difference whatsoever to the patient how much of a drop it was. (I was thinking maybe some optician somewhere had convicned this patient that he needed a 2 drop, and he became insistant on it.)
Thanks for the info!

Your welcome.

I didn't even know there was a name for the distance from the FC to the 180 line. Must be something the labs are using, borrowed from the terminology used concerning segmented multifocals?

As far as I know, the PRP is always on the 180 line, although the reference marks designating it can vary from symbols to logos. It's important to know that there can be multiple symbols/logos, e.g. a symbol/logo to designate the 180 line, and then logo/trademark below the nasal symbol to identify the manufacturer, sometimes with numbers/letters designating index, and/or base curve. The add power is always below the temporal reference mark.

Regards,

[Robert Martellaro]

Isn't the drop related to how far from the fitting cross it is until the power starts to change?

To the best of my knowledge there is no direct correlation between the "drop" and the length of the corridor, where the progressivity starts, or the rate of change.

Regards,

[EyeFitWell]

Thanks for the information, Robert!
I love your sig quote. It made me laugh.

[Happylady]

To the best of my knowledge there is no direct correlation between the "drop" and the length of the corridor, where the progressivity starts, or the rate of change.

Regards,

I know it doesn't correlate to the lenght of the corridor or the rate of change. However if there is a 2 mm drop, doesn't that mean the change STARTS 2 mm below the fitting cross? If there is a 4 mm drop, then the change starts 4 mm below the fitting cross. Otherwise, I don't understand what the drop is all about.

[Robert Martellaro]

I know it doesn't correlate to the lenght of the corridor or the rate of change. However if there is a 2 mm drop, doesn't that mean the change STARTS 2 mm below the fitting cross? If there is a 4 mm drop, then the change starts 4 mm below the fitting cross. Otherwise, I don't understand what the drop is all about.

It's probably related to Martin's rule- position the OC .5mm below the pupil center per one degree of panto, assuring that the optical axis of the lens intersects the center of rotation of the eye.

[HarryChiling]

I was always under the impression that the design of the lens dictated where the FC would be. The distance should have about 120 degrees of clear vision or close, that point is the FC and the measure from there to the PRP is the drop.

Here is an example:

[Robert Martellaro]

Harry,

I think it would be more accurate to say that the design of the lens dictates the area of clear distance vision, and that the design is referenced to the FC, which makes sense because that's where the pupil is.

For instance, two of my old glasses were Percepta with a 2mm distance from the FC to the PRP, and a pair of Sola Max, with a 4mm separation. One might think that the Max would then have clear distance off-axis vision, instead the Percepta is noticeably clearer than the Max. Moreover, the add power comes into play faster with the Max, just the opposite of what one might expect.

Most of the PAL designs as of late seem to have adopted a separation of 4mm, which makes sense from an optical viewpoint assuming 8 to 10 degrees of Panto, and probably to give some consistency across the industry.

Regards,

[HarryChiling]

I think it would be more accurate to say that the design of the lens dictates the area of clear distance vision, and that the design is referenced to the FC, which makes sense because that's where the pupil is.

I do better explaining things with visuals than in words sometimes, but yes you are correct the design dictates how clear the periphery of the lens is, and depending on the designer and their parameter for how wide the distance vision should be, I think that's what dictates the drop height. I may be totally off though as this is just an educated guess with a pretty picture attached to it (well at least I think it's pretty ;) ). It does make for interesting conversation though and kudos to EyeFitWell for bringing it up. I am hopeing that any minute now Darryl or someone that would have more knowledge on the subject will chime in.

[MarcE]

I have seen a thread where Darryl address the standardization of lenses over the past few years at 4mm ( I think).
I'm not understanding this thread. Can someone explain it to the us slow folks.
Many soft lenses will measure distance Rx +0.12 at the fitting cross.
So these designs have a negative "drop", right?

So what are we defining "Drop" as?

[Joann Raytar]

I wondered about this one too. I always thought that it was the drop to the add that affected vision.

[EyeFitWell]

Marc and Jo,
I guess my questions when starting this thread are:
1. What exactly is the drop? Is it the FC to the PRP? Or FC to engravings? Or is the PRP always in like with the engravings?
2. Does the drop directly correlate to the lens design and provide insight into: a. where the progression starts, b. how fast the progression is. If it does indicate something about the above, what does it indicate? Is a 4mm drop lens faster or slower than a 2mm lens?

[HarryChiling]

Marc and Jo,
I guess my questions when starting this thread are:
1. What exactly is the drop? Is it the FC to the PRP? Or FC to engravings? Or is the PRP always in like with the engravings?
2. Does the drop directly correlate to the lens design and provide insight into: a. where the progression starts, b. how fast the progression is. If it does indicate something about the above, what does it indicate? Is a 4mm drop lens faster or slower than a 2mm lens?

1. The FC to the PRP is being refered to as the drop in this post, I don't know if this is the technical term or not but it seems to fit. The PRP is always in line with the engravings this is a standard that the companies use. This point on the lens is just where you woud go to measure the amount of prism.

2. The drop has nothing to do with where the progression starts, The drop is a measure of how far up the lens you have to go from the PRP to get to the desireable portion for ditance. It has no indicator as to how fast the progression starts, otherwise all 4mm high progressives would have a seg height of 21, which we know isn't true. The drop has no significant value as I can see to the optical performance of the lens, it's just a measurement used to properly fit the progressive.

[Fezz]

Keep in mind that the some lenses do not follow the same drop guidlines. An example is the Sekio Succeed, Proceed, Exceed. The fitting cross is on the same horizontal as the lens engravings(and where most progressives have the PRP. I believe Pentax(AF Mini)used to do the same.

[Darryl Meister]

With rare exception, the prism reference point is generally located in the middle of the two semi-visible reference engravings (or logos) on the 180-line of the lens. Ideally, the optical center of the lens is surfaced at this location in the absence of prism. So, if you're concerned about prism or the effects of prism on optics, this distance becomes potentially important. However, since many (if not most) progressives now utilize prism-thinning anyway, the optical center is typically at some random distance above or below this location. Consequently, the initial "drop" distance between the fitting cross and prism reference point seldom makes any visual or optical difference to the wearer, one way or the other.

There are some optical and mechanical reasons for choosing this distance, though I suspect that the choice is seldom based on these. For instance, using a longer fitting cross "drop" distance reduces the amount of prism-thinning required for the job; reduces vertical prism imbalance at near caused by anisometropia; and reduces the likelihood of blocking wave aberrations encroaching on the near zone of the lens. A longer fitting cross distance also increases blank size cut-out in larger frame sizes.

In many cases, the prism reference point is also the design center of the progressive lens, or the center of the coordinate system used during the lens design process. The choice of this design center "drop" distance was probably dictated more by legacy lens design applications, the use of values common to earlier progressive lenses on the market, and some rather arbitrary choices as to where the optical center of a progressive lens should reside (at least in the absence of prism). The same holds true for the distance reference point at the center of the distance checking circle.

For instance, the AO and SOLA standards were once 2 mm. Essilor has been using 4 mm. Several manufacturers, including Carl Zeiss Vision, have started using 4 mm as well, in order to bring some level of standardization to the market in order to make life a little easier on lab techs and dispensers.

[HarryChiling]

Thanks Darryl, you saved at least me from chaseing my tail in circles. :bbg: Alot of how I visualize a progressive lens comes from your CE on www.opticampus.com and I recommend anyone who hasn't checked it out to do so.