Problems with depth of field using Inner Surface PAL's

[G. F. Peck]

As more and more manufacturers are following the 'Freeform' route, we are faced with a seemingly endless choice of Inner Surface Progressive lenses. The literature accompanying these lenses clearly shows the benefits of wider fields of view when comparing a Front surface progressive with a modern Inner surface progressive and so their popularity is increasing.

However, a problem that will not go away is that of minimum fitting heights. All dispensers appear to be intent on pushing this to the limit and seem to insist on using the smallest frame depths available! This is where the problems start to creep in and we find that patients often have more trouble with the reading area on Inner surface progressives than on their Front surface counterparts.

Using the same type of illustration as is used to highlight the benefits of increased viewing widths with Inner surface designs, I believe that the drawing below (drawn to scale) shows Inner surface progressives have a reduced usable viewing depth. You will see from the drawing that reducing the back vertex distance would make matters worse and changing the pantascopic tilt does not help either.

Has anyone else found this problem? I’d be interested in any comments.

Kind Regards

Graham Peck (Optical Technician)
Langton Optometrists

Please note that the illustration used is taken from an actual prescription and all measurements are to scale. The original drawing that accompanied this query has been removed as it was not accurate enough.

[OCP]

As more and more manufacturers are following the 'Freeform' route, we are faced with a seemingly endless choice of Inner Surface Progressive lenses. The literature accompanying these lenses clearly shows the benefits of wider fields of view when comparing a Front surface progressive with a modern Inner surface progressive and so their popularity is increasing.

However, a problem that will not go away is that of minimum fitting heights. All dispensers appear to be intent on pushing this to the limit and seem to insist on using the smallest frame depths available! This is where the problems start to creep in and we find that patients often have more trouble with the reading area on Inner surface progressives than on their Front surface counterparts.

Using the same type of illustration as is used to highlight the benefits of increased viewing widths with Inner surface designs, I believe that the drawing below (drawn to scale) shows Inner surface progressives have a reduced viewing depth. You will see from the drawing that reducing the back vertex distance would make matters worse and changing the pantascopic tilt does not help either.

Has anyone else found this problem? I’d be interested in any comments.

Kind Regards

Graham Peck (Optical Technician)
Langton Optometrists

Hi Graham

This is very interesting, and I will check if my support base has any comment on this.

Regards

Mike

[YrahG]

As more and more manufacturers are following the 'Freeform' route, we are faced with a seemingly endless choice of Inner Surface Progressive lenses. The literature accompanying these lenses clearly shows the benefits of wider fields of view when comparing a Front surface progressive with a modern Inner surface progressive and so their popularity is increasing.

However, a problem that will not go away is that of minimum fitting heights. All dispensers appear to be intent on pushing this to the limit and seem to insist on using the smallest frame depths available! This is where the problems start to creep in and we find that patients often have more trouble with the reading area on Inner surface progressives than on their Front surface counterparts.

Using the same type of illustration as is used to highlight the benefits of increased viewing widths with Inner surface designs, I believe that the drawing below (drawn to scale) shows Inner surface progressives have a reduced viewing depth. You will see from the drawing that reducing the back vertex distance would make matters worse and changing the pantascopic tilt does not help either.

Has anyone else found this problem? I’d be interested in any comments.

Kind Regards

Graham Peck (Optical Technician)
Langton Optometrists

Depth of field isn't the description that I would have used for this phenomenon. Bringing the PAL to the back surface does require less ocular rotation to reach the reading segment, however the corridor isn't set in a FF pal or it doesn't have to be so if your stipulation is to have a full 18mm then the software should allow that (if the vendor allows). Also consider this your illustration shows a lens tilted at 10 degrees, their is no PAL I am aware of that should be fit orthoscopically, the slight amount of panto needed allows the wearer to split the aberrations or effects of a tilted lens between both the distance and the reading, it's like splitting the comprimise. My third point would be that the ability to reduce the amount of ocular rotation will allow more room for reading in the same frame at a more comfortable posture.

[AWTECH]

As more and more manufacturers are following the 'Freeform' route, we are faced with a seemingly endless choice of Inner Surface Progressive lenses. The literature accompanying these lenses clearly shows the benefits of wider fields of view when comparing a Front surface progressive with a modern Inner surface progressive and so their popularity is increasing.

However, a problem that will not go away is that of minimum fitting heights. All dispensers appear to be intent on pushing this to the limit and seem to insist on using the smallest frame depths available! This is where the problems start to creep in and we find that patients often have more trouble with the reading area on Inner surface progressives than on their Front surface counterparts.

Using the same type of illustration as is used to highlight the benefits of increased viewing widths with Inner surface designs, I believe that the drawing below (drawn to scale) shows Inner surface progressives have a reduced usable viewing depth. You will see from the drawing that reducing the back vertex distance would make matters worse and changing the pantascopic tilt does not help either.

Has anyone else found this problem? I’d be interested in any comments.

Kind Regards

Graham Peck (Optical Technician)
Langton Optometrists

Please note that the illustration used is taken from an actual prescription and all measurements are to scale. The original drawing that accompanied this query has been removed as it was not accurate enough.

The size of the reading zone is determined by the optics created between two lens surfaces. Depending on the result desired you will have the same area problem on the front or back. You are not allowing for the fact that a great design of a backside PAL will utilize what you have illustrated is not utilized. As the add power in a back surface PAL will cause the thickness of the add at the lower edge to be knife like, where a molded front design will need more material on the front surface molded into it to produce the reading area.

[G. F. Peck]

Thanks for that AWTECH,

Are you saying that regardless of whether we use a front surface design with 18mm minimum height or a back surface design with 18mm minimum height, like the one illustrated, both designs would work equally well??

In both cases the corridors would be 13mm and we would expect the reading add to come in at 18mm. For the same degree of occular rotation, with the front surface design there would be a direct line of sight through the reading add whereas with the back surface design the direct line of sight is restricted to a point 15.5mm down the progression - in other words the reading add is not reached.

Maybe I'm missing the point and perhaps my argument is too simplistic but if that's the case then why do we get problems with back surface designs that are usually cured simply by changing to a front surface PAL.

Ultimately, we are faced with a barrage of new designs, all promising to be better than the rest. In the interests of progress (and financial reward) we try these new 'wonder' lenses and if they work - fine! If they don't work, we either never use them again or we try to explain where the problems may be. That's all I'm trying to do.

Graham Peck

[AWTECH]

Thanks for that AWTECH,

Are you saying that regardless of whether we use a front surface design with 18mm minimum height or a back surface design with 18mm minimum height, like the one illustrated, both designs would work equally well??

In both cases the corridors would be 13mm and we would expect the reading add to come in at 18mm. For the same degree of occular rotation, with the front surface design there would be a direct line of sight through the reading add whereas with the back surface design the direct line of sight is restricted to a point 15.5mm down the progression - in other words the reading add is not reached.

Maybe I'm missing the point and perhaps my argument is too simplistic but if that's the case then why do we get problems with back surface designs that are usually cured simply by changing to a front surface PAL.

Ultimately, we are faced with a barrage of new designs, all promising to be better than the rest. In the interests of progress (and financial reward) we try these new 'wonder' lenses and if they work - fine! If they don't work, we either never use them again or we try to explain where the problems may be. That's all I'm trying to do.

Graham Peck

Your example does not consider that fact that lens design optical center for the reading area is designed into the lens. It is not designed on one surface only. A front design uses a complex front surface and a spherical back surface and a backside design uses a spherical front surface and a complex back surface. Both can achieve the same line of sight.

I don"t think the problems you are having are related to what you are concluding. Unless all of these issues are produced from the same design or different designs produced by the same lab. If that is the case there can be many reasons. As an example a great lens design produced by a lab with a lab management system with program errors could cause a great design to be produced incorrectly. Calibration of the and set up of the machine producing the designs etc. is also a possible reason.

[xiaowei]

Your example does not consider that fact that lens design optical center for the reading area is designed into the lens. It is not designed on one surface only. A front design uses a complex front surface and a spherical back surface and a backside design uses a spherical front surface and a complex back surface. Both can achieve the same line of sight.

I don"t think the problems you are having are related to what you are concluding. Unless all of these issues are produced from the same design or different designs produced by the same lab.

Hi Graham,

I have to fully agree with AWTECH, IMHO you are confusing things. Your somehow turning the usual "keyhole argument" around to proof just the opposite. This is not a different issue.

The "keyhole argument", usually goes this way: having a certain aperture ("fitting height") (say in mm) on the backside of a thick lens, nearer to the eyeball, will give a larger angular field of view compared to the very same aperture on the front side of that lens, further away from the eyeball.

This is IDENTICAL to saying that for a given angular field of view, less aperture would be required on the backside. HOWEVER, it is not valid to say that because the fitting height on the backside now has become smaller the design has become worse.

In fact, any resonable PAL design, as AWTECH also said, will take both surface into account and come up with a solution that will be almost identical for the viewer, be it a back- or a frontside design!

However, the above consideration will require that the backside design will need to be a bit more "compressed" compared to a similar frontside design, the difference beeing only marginal in usual cases and of no issue with modern free-from technology

Sincerely

[G. F. Peck]

Hi xiaowei,
I am starting to think that my original question has been misread. I have no argument with back surface progressives giving wider fields of view and with regards to the keyhole effect etc., I agree with all you have said.
However, my concern is not with width, it is simply that back surface designs don’t appear to be able to make use of the full depth of the lens in the same way that front surface designs do, due to the cut-off effect caused by the frame rims (or in the example shown – the lens edge).
If a back surface design has a minimum fitting height of 18mm I would expect the full reading addition to be effective from that point downwards (the same as you would expect with a front surface PAL). In the drawing attached to my original question you will see that the eye cannot view directly through this point in a back surface design as the line of sight would run through the flat edge of the lens. Admittedly, my drawing does not take ray tracing into account but I’m not sure it needs to.
All this leads me to believe that for a back surface design to have an effective reading area it would need to have a very short progressive corridor. If this is, in fact, what people are trying to tell me and that with a back surface design the reading addition is effective at a point higher up the progression corridor, then why are the reading checking circles on the lenses still in the normal position? Hope I'm not making this even more confusing?
Kind Regards
Graham

[Darryl Meister]

Graham, As the others have pointed out, your drawing actually demonstrates the advantages of the "keyhole" effect for increasing the apparent size of the viewing zones of the lens, ignoring prism and magnification for the moment, when the lens design is closer to the eye.

The point that you are (I believe) trying to make is that more ocular rotation is required to reach the near zone when it is located closer to the eye. And this is may or may not be correct, depending upon how the design was "transferred" mathematically to the back surface.

If the viewing zones are moved to the back surface by adding the optics of the front and back surfaces along the line of sight as it is refracted through the lens, the ocular rotation required should remain similar, which means that the corridor length may differ between the front and back surfaces. If the front lens surface at some X,Y point is added to the back surface at the same X,Y point, however, the ocular rotation angle may differ slightly for lenses of finite thickness.

[OCP]

Well I´m danish and maybe I don´t read all the posts clear enough to understand all your answers.
BUT I´m really think that most of you is misunderstanding the question from Graham Peck.

As I read his contribution, he is only call attention to the issue that, if you move the progressiv zone (the power) to the back of the lens, then the frame (especially acetat frames) will take some of the view (read "some of the power") if you place the fitting height in absolutly minimum fitting height. If you put the power in front of the lens, the acetat frame will not take any of the power when the eye are looking downwards.
Hope I could clear out some of this.

Mike

[G. F. Peck]

Thanks Mike,

Danish or not - you're a star. You got the point exactly!!!!

[Darryl Meister]

If you put the power in front of the lens, the acetat frame will not take any of the power when the eye are looking downwards.
Hope I could clear out some of this.

1. But you are still not necessarily reducing the wearer's field of view, as his own drawing illustrates.

2. Because of refraction at the back surface of the lens, the line of sight is actually deflected away the lens edge, reducing the difference shown in the drawing (apply Snell's law to it).

[AWTECH]

The available field of view for the acetate frame you describe is the same.

Whether a front side PAL or backside PAL the available field of view is the same. The actual optics of the lens design will determine what portion of the field of view is available for good vision.