fitting height in single vision lenses (Spherical)

Hi everyone,

Sorry for my bad English I want improve.

I'm a student of optics and optometry, I work like "shop assistant" in an optical shop and... I learn :)

Be patient please I start my study only 6 months ago.the

I have a question...when I have to cut a lens for a patient we know that for avoid prisms on a lens I have to take the PD of the patient... the fitting height and calculate the positions of the optical center on the frame shape.

But often I see the optician (even in some books or on youtube tutorial even in English from different country ) take only the PD... the fitting height is taken only fo aspherical lenses and Pal's.

So it is so important take the correct fitting height on all lens or not?

thanks

I hope this is not boring for you guys.

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Originally Posted by dima

Hi everyone,

Sorry for my bad English I want improve.

I'm a student of optics and optometry, I work like "shop assistant" in an optical shop and... I learn :)

Be patient please I start my study only 6 months ago.the

I have a question...when I have to cut a lens for a patient we know that for avoid prisms on a lens I have to take the PD of the patient... the fitting height and calculate the positions of the optical center on the frame shape.

But often I see the optician (even in some books or on youtube tutorial even in English from different country ) take only the PD... the fitting height is taken only fo aspherical lenses and Pal's.

So it is so important take the correct fitting height on all lens or not?

thanks

I hope this is not boring for you guys.

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No, not boring at all. OCs should be taken as part of the fitting workflow. This is important for higher index materials and free form to maximize va. Keep in mind that changes in OCs will affect thickness, so you need to pay attention to RX/frame match. Rule of thumb that I live by - DVO @ center pupil. NVO @ 2 mm below center pupil. This is primarily for line of gaze. Others here may disagree, so I will wait to hear from them.

Quote:

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Originally Posted by dima

I have a question...when I have to cut a lens for a patient we know that for avoid prisms on a lens I have to take the PD of the patient... the fitting height and calculate the positions of the optical center on the frame shape.

But often I see the optician (even in some books or on youtube tutorial even in English from different country ) take only the PD... the fitting height is taken only fo aspherical lenses and Pal's.

So it is so important take the correct fitting height on all lens or not?

thanks

I hope this is not boring for you guys.

---

Good question. It's critical that we measure the pupil heights when there is anisometropia, specifically when there is a power disparity in the vertical meridian, eliminating vertical prism imbalance.

Moderate to high dioptric value lenses with pantoscopic tilt should have the vertical OC placed according to Martin's rule of tilt, lowering the OC .5mm per one degree of tilt, in the primary gaze, aligning the optical axis of the lens with the center of rotation of the eye, reducing power and astigmatic error.

In general, it's good practice to choose a frame that places the pupil 3mm to 5mm above the 180 line.

Hope this helps,

Robert Martellaro

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Originally Posted by lensmanmd

No, not boring at all. OCs should be taken as part of the fitting workflow. This is important for higher index materials and free form to maximize va. Keep in mind that changes in OCs will affect thickness, so you need to pay attention to RX/frame match. Rule of thumb that I live by - DVO @ center pupil. NVO @ 2 mm below center pupil. This is primarily for line of gaze. Others here may disagree, so I will wait to hear from them.

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I don't disagree, but in thinking about it, I don't think I've ever taken a height for NVO. In DVO, especially in higher Rx's I will take a height if the frame fits outside of the typical 2/3 ruling. I think it is more to do with most people choosing smaller or more centered fits for their readers, or maybe just because it has never caused problems at dispense for us, but I will keep that adjustment in mind should the need arise.

I generally, for low powers/lack of astigmatism, will not take an OC. In those cases I usually leave the centre on datum.

My reasoning has always been that, while you may introduce prism, it's negated by the fact that the prism will have the same diopteric value and base direction, so it cancels itself out.

Past low powers, and with significant astigmatism values (past -2.00 in my opinion) I will definitely measure an OC.

With low powers, I'll measure an OC if the patients eyes have more than a minor vertical difference between the two, to prevent prismatic effects.

I feel this is a patient by patient decision making process, subject to the experience and opinions of the opticians working with them. There may be a best way, but no one is really wrong.

Best practice would be to take pd's, oc's, vertex's, tilts, and wraps, every time we measure someone, but in practice, for a majority of patients, these measurements make minuscule to no difference at all. a -0.25 sph OU for example, you could just eyeball the lens centre while you're edging, and even if you're full centimeters out, the patient wont notice a difference.

My best explanation for opticians, including myself, making decisions to not measure things, is practiced laziness.

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Originally Posted by McAnerin

My best explanation for opticians, including myself, making decisions to not measure things, is practiced laziness.

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Same here... if someone is pretty well centered, I usually do not take OC unless higher RX... yes practiced laziness. I should not admit it, but it's true.

Thanks to everyone this help very much.

I take an OC height on every single SV order, if for no other reason than to reinforce to the patient that there are things you can't possibly get from online eyewear ordering.

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Originally Posted by AngeHamm

I take an OC height on every single SV order, if for no other reason than to reinforce to the patient that there are things you can't possibly get from online eyewear ordering.

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+1 excellent idea.

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Originally Posted by Robert Martellaro

...lenses with pantoscopic tilt should have the vertical OC placed according to Martin's rule of tilt, lowering the OC .5mm per one degree of tilt...



Hope this helps,

Robert Martellaro

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Does the 2-to-1 rule apply the same with freeform sv? Since the specified oc placement is usually pupil-high, does that mean the frame cannot be fitted with any panto (or retro) at all?

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Originally Posted by finefocus

Does the 2-to-1 rule apply the same with freeform sv?

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It depends on the design of the lens, but generally no. When in doubt, follow the manufacturer's recommendations.

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Since the specified oc placement is usually pupil-high, does that mean the frame cannot be fitted with any panto (or retro) at all?

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It depends on the software- some use default/fixed position of wear values only. For example, Spectrum uses default values of roughly 13mm for the vertex distance, a pantoscopic tilt 8 degrees, and a wrap/faceform/dihedral angle of 6 degrees.

However, if the measured values deviate significantly from the default values, typically due to facial structure and/or client preference for frame cosmetics, the lenses will not perform optimally. In this case we should select a lens that uses software that can accept custom position of wear values.

Best regards,

Robert Martellaro