Above the fitting cross

[majucalicut]

1. Can any one say what length should I keep above the fitting cross when fitting a progressive lens?

2. What will be the power at the fitting cross? Is it same as distance power?

[Judy Canty]

My experience has been that you need about 10mm of distance vision above the fitting cross for patient comfort.

[Darryl Meister]

I would also say that providing at least 8 to 10 mm above the fitting cross is a good rule of thumb for many lens designs, for providing both adequate area for power verification and sufficient far vision utility for the wearer, although short-corridor lens designs for smaller frames may still work suitably well even with smaller values.

[SailorEd]

Above the fitting cross is the "Distance Reference Point" or Distance power circle. The distance from the cross to the top of the Reference Point is between 8 - 10 mm (check for the particular progressive you are dispensing because Kodak Concise is around 7mm). You should never go below that height and you should think very seriously about going at the minimum height.

P.S. It's ok to tell the patient that a progressive as fit in the frames will not work - they will appreciate your honesty.

[majucalicut]

If the power at fitting cross is equal to that of distance reference circle, why we need 8 or 10 mm. One of our customer fitted the lens just giving 3 mm above the fitting cross and the end customer is happy

[SailorEd]

If the power at fitting cross is equal to that of distance reference circle, why we need 8 or 10 mm. One of our customer fitted the lens just giving 3 mm above the fitting cross and the end customer is happy

What we're saying is it is the BEST policy to follow. I have some patients who do ok with a lined bifocal set at 5 mm above the bottom of the frame but that is not the optimum for which I would strive.

To fit someone with just 3 mm below the top of the frame SEVERLY limits upward vision and would cause him or her to raise their head up in order to see clearly things just above the horizontal line-of-sight simply because of the fact that the lens ends 3 mm above the cross.

So, yes, for someone it might work, but I certainly wouldn't make it a practice and I certainly WOULD advise the patient that they may want to try another frame and I would explain why. If the patient said that that would not matter to them, then I would certainly go ahead with the job and I would annotate the fact that I told them in their chart.

[k12311997]

If the power at fitting cross is equal to that of distance reference circle, why we need 8 or 10 mm. One of our customer fitted the lens just giving 3 mm above the fitting cross and the end customer is happy

I bet a plus Rx? I can't imagine a myope being happy with 3mm of distance vision.

[JanMueller]

I won`t go under 8mm. I guess progressives work best then.

[DC Optix]

I think we're all missing what's been asked here. Maju isn't looking for how much room should be above the fitting cross to function, I think the question is where to actually fit the progressive above the fitting cross.

My answer to that? The cross should be directly over the pupil for best performance and have a minimum of 8-10 mm above the pupil to the top of the frame. Some people here will argue that you should fit the cross anywhere from 1-3 mm below the pupil, but my personal opinion is that that thought is a hold-over from the days of fitting hard design lenses and it is no longer necessary.


Of course, I could be completely wrong!!! :)

[jimrask]

I think we're all missing what's been asked here. Maju isn't looking for how much room should be above the fitting cross to function, I think the question is where to actually fit the progressive above the fitting cross.

My answer to that? The cross should be directly over the pupil for best performance and have a minimum of 8-10 mm above the pupil to the top of the frame. Some people here will argue that you should fit the cross anywhere from 1-3 mm below the pupil, but my personal opinion is that that thought is a hold-over from the days of fitting hard design lenses and it is no longer necessary.


Of course, I could be completely wrong!!! :)

This kinda depends on what brand of lens you're working with. I know from experience that a Zeiss PAL should be fitted with the cross wright in front of the pupilcenter, unlike a Tokai PAL, wich must be fitted with the cross just at the bottom side of the pupil (2-3 mm difference)

[Darryl Meister]

If the power at fitting cross is equal to that of distance reference circle, why we need 8 or 10 mm. One of our customer fitted the lens just giving 3 mm above the fitting cross and the end customer is happy

Ultimately, this has more to do with good dispensing practice than with the optical limitations of progressive lenses. Progressive lenses don't necessarily need 8 mm of clearance above the fitting cross to "work" but, without it, there are several practical consequences:

1. Cosmetically, the eyewear will look poorly fitted if the eye is that close to the top rim of the frame; lens thickness and cut-out may also become issues in this case.
2. The pupil of the eye reaches up to 8 mm or so, which means that you would need at least half this amount (4 mm) of clearance above the fitting cross to prevent a vignetting-like effect from light entering the eye from outside the frame.
3. The eye is in a constant state of movement, scanning the environment, so the lens should provide a sufficient region of clear vision for these eye movements.
4. The distance prescription of the lens will be difficult to verify, since the distance checking circle is typically centered at 3 to 6 mm above the fitting cross.

Also, there is indeed typically some amount of unwanted plus power at the fitting point of progressive lenses, since the pupil of the focimeter or eye will pick up some of the progressive corridor. This is one reason why the distance checking circle is higher on the lens than the fitting cross.