short corridor progressives

[oceanboy]

hi. There`s no doubt that the new progressives (life xs, b`active,solamax, etc.) are great for small frames. 16mm fitting height and all, but there seems to be a definate compromise in the mid range- width of field in particular. This is especially true of the life xs. Has anyone else run into this????P.s Is anyone going to the vision convention in toronto????? Silmo is involved this year!!!!! I`m going to be a kid in a candy store.Lee / Vancouver.

[sticklert]

Lee,

Your mid range will always be compromised with a short corridor lens. I always recommend not putting anyone above a 2.00 add. Then you should be OK. I would guess you are having more problems with higher add powers and I would not be surprised if they were Hyperopes too?

You can't have it all with a Progressive Lens. You have to compromise something when you are fitting specific design parameters like a Short Corridor Lens. In some cases with higher add power RX it is better to use a General purpose lens with a minimum of 18-22mm and bump the add power.


Todd

[Judy Canty]

I disagree. Simply "bumping" the add forces the wearer to read within the intermediate corridor which is a much smaller area than is comfortable for most wearers. The key here is to educate the customer about what the options are in progressive lens designs and how they will work in specific frames. If the intermediate distance is problematic, then your customer may want to invest in a pair of mid-diatance progressive lenses. Asking one progressive lens to answer all visual needs is like asking one good pair of shoes or one well made suit to work for every occasion every day of the week.

[Pete Hanlin]

I would like to echo Judy's comments. Today's PAL designs (e.g., Panamic) are too sophisticated for tricks like "bumping the add power."

Sticklert hits on a valid point about mid-range. Remember, the shortest "no-line" lens is a blended bifocal. In the space of about 1mm, the optics go from distance to near power. Some of the "short-corridor" lenses that are hitting the market are versions of previous designs by the manufacturer that have been "compressed" into a shorter progression. Assuming the manufacturer optimized the progression in the original product, one has to question to what degree this compression compromises the design. Other manufacturers have actually used practically the same design for their "short corridor" PAL- they've simply moved the fitting cross down into the corridor (which has the same effect, and poses the same challenges, as fitting a PAL "high").

Again agreeing with Judy, you cannot hope to acheive quality vision for all needs with one design.

Coincidentally, both Comfort and Panamic reach 85% of their total add power at 12.5mm below the fitting cross. While Varilux recommends a minimum fitting height of 18mm, wearers tests against "short-corridor" PAL designs show that Varilux designs out-perform the short-corridor lens even when fit below 18mm.

In my opinion, it comes down to giving professional care to your patients. They cannot be expected to realize the visual compromises they will be making with a PAL that is fit too low. As the professional Optician, it is YOUR task to guide them to a good fit- and recommend a specialty pair of lenses if needed.

I don't pretend that this is easy (I used to dispense myself, and remember the 44 year old woman who wants the 36mm B frame and works with computers). Sometimes you have to compromise and dispense a less-than-ideal fit. In those cases, studies show you are best off with a solid PAL design that is fit on pupil center.

[Jubilee]

So Pete, is what is true for the Varilux also true for the Natural? I noticed in the 2002 progressive identifier that the recommended fitting height for a Natural is 20 mm. Yet for all of our A/Rs from Essilor, we receive the Natural unless the seg is 17 or lower. I assume since it is a company owned lab, that the opinion is an 18 seg is ok in a Natural, but I get flack from some of my fellow associates saying that too much of the reading is getting cut off. So if I have some numbers for them... maybe we can work this out.

Cassandra

[Robert Martellaro]

Welcome to optiboard.

If you design a PAL with an extremely short corridor there will be a decrease in corridor width and an increase in distance peripheral vision acuity, if the lens is to remain a reasonably soft design. The Kodak Concise is a good example; full add power well with-in the lens at 18 high, more distance peripheral blur than any lens I've tried, with the intermediate even more diffcult to use than regular progressives. I would not worry about the intermediate because as Judy said if you work at that distance a separate pair is required (computer/music glasses). The distance blur might be another matter. It certainly would not be my first choice for a pilot, driver, sports etc.

If you see an Rx that shows, for example, a +2.00 add with a note that says "+2.50 if progressive", ignore the note and use the "true" add. Bumping was good advice 25 years ago but is unacceptable with todays lenses. Judy said that if bumped, the reading zone is displaced up into the intermediate where the horizontal field is narrower. It will also cause posturing to occur as the wearer raises and lowers the chin to catch the near point focal length.

Robert

[sticklert]

I don't want to get shredded on this thread but. You cannot convince me that simply bumping the add by .25 is going to displace the reading area enough to cause any discomfort or even a non-adapt. It is my understanding that a patients convergence is mostly based on the the base curve add power combination. So lenses that are designed with a variable inset, will accommodate for that .25 bump (which is almost every current progressive dispensed today). Second my suggestion was obviously not meant to be done on every patient. But for example you have your pt come in who is wearing a +2.00 OU with a 2.50 add that you fit short corridor brand X at 16mm. They come back and are having a problem seeing the dashboard of their car clearly but can read fine and see distance great, Oh and they do not want to change frames! What do you do? I would suggest putting them in one of the newer soft design progressives still fitting it 16mm bump the add .25 and they have full add, a wider intermediate by at least 10-20% and the can now see the dash board of their car. I always suggest going with mfg recommendations but we all know their are exceptions to the rules.

[Pete Hanlin]

I always suggest going with mfg recommendations but we all know their are exceptions to the rules.
I agree, which is why I said "Sometimes you have to compromise and dispense a less-than-ideal fit.

So Pete, is what is true for the Varilux also true for the Natural? I noticed in the 2002 progressive identifier that the recommended fitting height for a Natural is 20 mm.
The Essilor recommendation for Natural's minimum fitting height is 18mm. Natural reaches 85% of its add power at 14mm below the fitting cross and 100% at 16mm. Comparatively, Varilux Comfort reaches 85% of its add power slightly higher than 12.5mm below the fitting cross. The "near verification circles" you see on Essilor fitting guides represent the fitting height necessary for an ideal fit. It was never intended as an indication of where 85-100% of the add power begins. An Essilor minimum fitting height does not necessarily mean the patient will necessarily "non-adapt" below the indicated height- it simply provides the eye care professional with a guide.

It is my understanding that a patients convergence is mostly based on the the base curve add power combination. So lenses that are designed with a variable inset, will accommodate for that .25 bump (which is almost every current progressive dispensed today).
Although "variable inset" is claimed in the description of many PAL designs, analysis of actual lenses has shown that many of these lenses have very little variation in inset (and some have no appreciable variation in inset). Panamic has the greatest amount of inset variation of all the designs we have tested to date. BTW, the same holds true for "multi-design." It is an easy claim to make, but upon inspection, there are "multi-designs" that do not appear to have appreciable variation in design across the add power and base curve range.
You cannot convince me that simply bumping the add by .25 is going to displace the reading area enough to cause any discomfort or even a non-adapt.
Let us assume we are fitting a patient in a Natural PAL with a fitting height of 16mm. The patient requires a +1.75 add, but has been "bumped" to a +2.00 in order to provide the full add power at a point slightly higher in the lens. As a result, the patient will reach the +1.75 add at a point approximately 14.5-15mm below the fitting cross.
If we have fit the Natural at 16mm and have bumped the add as described above, we have succeeded in raising the "reading zone" by 1.5mm. We will be providing the patient with approximately 1-1.5mm of full reading power. However, the patient is now viewing through the lower portion of the progression- which has a field that is 10-15% narrower than the reading zone. Accordingly, we should not be surprised if the patient complains of a "narrow reading zone." I would suggest that finding a frame that fits 17mm and leaving the add power at the prescribed power is a much better solution (even if we use the same frame and fit the design 1mm high).

Realize that this is precisely why we have minimum fitting heights! Once we start "playing around" with the fit, we are going to be compromising vision either in the distance or the near. For a lens to be a Progressive, it simply has to have room to progress! One of the reasons Comfort was so successful upon its launch (and continues to be the most prescribed PAL worldwide) was its combination of a soft design with a short progression (which is the basis of one of its patents). This is a very popular combination for a PAL design (soft & short) because it is comfortable to use (the patient doesn't have to tilt the head as far to read, but peripheral distortions are kept to a minimum).