Dispensing, Feature

Matching lens and frame curves

Poor lens curve and frame combinations can lead to dissatisfied customers and time-wasting remakes in the laboratory. MURRAY O’BRIEN details some of the most important considerations in this area.

Front curve selection to ensure the best solution for each frame and lens combination is crucial to successful optical dispensing. Making the wrong choice or, more commonly, not making a choice at all can cause heartache, cost money and lose customers.

Murray O’Brien.

It can also assist your laboratory in saving costly and time-wasting remakes.

In this article I hope to offer some practical advice on front curve selection. But to understand the issue, it’s important to note some of the problems that arise from poor lens curve and frame combinations:

1. Flat lenses into a frame with a high eye wire curve makes the temple ends too wide (loose fit on the head).

2. Highly curved lenses into a frame with a flattish eye wire curve makes the temple ends too close (tight fit on the head).

3. Spectacles become unattractive
or optically inappropriate. This is due to the extreme adjustment required because the frame is being distorted by inappropriate lenses.

4. Poor lens fitting results where the lenses are too highly curved for the frame design and tend to ‘pop’ along the upper eye wire.

5. Uncomfortable visual effects from inappropriate curves or inappropriate or extreme forced adjustment.

When should I think about it?

When the frame rep comes to your practice and lays out the new and exciting stock, what are the range of prescriptions you think these frames will take? The major considerations are:

Frame shape: The actual profile of the lens makes a huge difference to how accommodating a frame will be to a range of lens curves. Perfectly round is the easiest shape of all.

The closer any of the edges of the shape are to being straight, the less accommodating the frame will be to getting a good lens fit with the widest range of prescriptions. The very shallow, oblong shapes of 15 or so years ago were by far the most troublesome frames for optical mechanics to fit from a shape basis.

Frame material: Make sure the frame material being offered in the frame is easily adjustable. Cellulose acetate, monel and stainless steel are frame materials that are generally very easy to manipulate.

Some thinner titanium frames tend to be very springy and adding extra curve in the frame with the frame benders can be very difficult. Some injection moulded plastics are impossible to adjust and cast titanium can be so stiff and brittle that they are unadjustable. If the frames are not easily adjustable, give them a miss, even if they look great, they will cause you heartache and grief in the end.

Frame construction: Before purchasing frames, have a good look at the construction. If the frame is very flat, will you be able to add some curve to the eye wires and then be able to adjust the temples out?

Sometimes if you come across a new design ask the rep if you can test the adjustability. It’s better to be safe than sorry being stuck with a frame that can’t be adjusted appropriately and easily.

The prescription 

The dispenser must be fully aware of the patient prescription before becoming too involved in looking at frames.

If the prescription is at the higher end of the range, let’s say over three dioptres, we may have to give serious thought to the front curve.

As the customer tries on each frame, have a look at the frame closely and think about the practicalities of getting their lenses into the frame and what refractive index or design lens we may need to use for the best fit.

Use a lens clock and measure the shape of the insert lens. That will give an idea of the ideal front curve that will give a nice fit without much adjustment being required.

How far will the customer’s new lenses stray from the insert curve if we make them from 1.5 index? Will we need to specify a four base rather than the normal six base? Will we need to do 1.6 index and use an aspheric design lens to make it even flatter?

In some cases with minus lenses we may need to stay away from aspheric designs as they will be too flat for a given frame and render the sides too wide.

In conclusion, all we can do in such a brief article is to encourage you to think. Look at the frame, look at the Rx. Use your experience to consider how the lab will make the lenses then think about if you need to ensure some extra specifications to make the completed spectacles as good as they can be.

ABOUT THE AUTHOR: Murray O’Brien owns Designed Eyes in Rosebud, Victoria, where he works in full retail optics. He is also past president of the Australian Dispensing Opticians Association (Vic) and has previously worked in lens fitting work, specialising in rimless.