jwilzewske: Better Eyeglasses for Chamber Musicians?

I use glasses when playing. I can see the music in the central part of my visual field, but my peripheral vision is out of focus. My sense of where the keys are isn’t as good as it should be. Also, when I glance down the keyboard for jumps I can’t focus. And if there’s a D.C. or D.S. or a repeat that stretches back across a page-break, then I find myself having to move my head and neck to get focus on the part of the page that I’m headed to.

LASIK (keratomileusis) isn’t a good option for older, farsighted musicians like me. LASIK works best for myopia or nearsightedness and has little benefit for presbyopia (the fancy medical term that denotes the inability to focus on near objects due to hardening of the natural lens in the eye).

And graduated-focus glasses for all-purpose use don’t work for reading music, because the magnification for reading is at the bottom of the lens and because the variable-magnification in that area has a relatively narrow-angle field-of-view.

Some musicians seem to do pretty well with bi-focal contact lenses, provided the diopter correction you need is not too large and provided you don’t have too much astigmatism. Still, a single-vision pair of glasses is what I prefer.

But it’s hard to find an optometrist in the U.S.—even in large cities—who really comprehends what a musician needs. Possibly the only ones who truly do understand are the ones who are optometrist-musicians, a somewhat rare breed. So, typically, you visit your friendly conventional non-musician optometrist and tell him that your eye-to-music working distance is X centimeters, and you want ‘music glasses’. When you pick up the glasses you find that, despite the pleasant discussion, he’s given you a “generic” pair of book-reading glasses. Or maybe you tell him you want straight bi-focals with the top part of the lens optimized for an eye-to-music distance of X cm and the bottom part of the lens focused at eye-to-keyboard distance Y, and he looks at you like the request is a giant inconvenience or impossible to do. I doubt that these frustrating experiences are unusual.

What to do? First of all, you need to shop around and find an optometrist who will agree to make glasses for you that are designed for the typical working distance between your eyes and the music. Somebody, optometrist-musician or otherwise, who’ll talk with you and is interested in your special needs and is not trying to persuade you to immediately accept some generified mass-market compromise.

Actually, you can finagle a solution that re-purposes a mass-market eyeglass design. And some of these (see links below) may be quite inexpensive. But you may need to have some detailed and quantitative idea of what you need for your specific playing situation before you walk into the optometrist’s office. That’s what this blog post is about. This post is meant to enable you to have the information you need to have a successful dialogue with your optometrist and get a pair of eyeglasses that is matched to the geometry of your playing environment.

You see, where vision is compromised, the musician has to adapt her/his posture to minimize eye strain. If the musician is using reading glasses designed for a 40 cm normal book-reading distance, she must lean toward the music stand that may be 70 cm away or more, in order to clear the image. But if she leans in, then there is back and neck strain. And if she leans in, the field of vision in lenses designed for book-reading may be too narrow to encompass the entire page or scan easily from one page to the next. Extra head and neck movements are needed to compensate for the narrow field of vision. That’s wrong!

What size lenses would be ideal for your particular eye-to-music viewing distance and your eye-to-eyeglass-lens distance? Well, first you need to say how far away your music stand is from your eyes? How far down your nose do you wear your glasses when you’re playing? How strong is your prescription? All of these things figure into the equations that determine the minimum lens size you’ll need.

And there’s more. The eye’s rotatory movements occur around a center-of-rotation within the globe. Eyeglass optics design usually assumes that this center-of-rotation is in a constant or fixed location, but detailed physiologic studies show that this isn’t fixed or stationary at all, especially when the head is moving vigorously. The center-of-rotation moves medially and laterally inside the eye-socket. And the center-of-rotation of the eye has significant velocity when it does this—in fact, the center-of-rotation moves quickly in a semicircle in the plane of the eye’s rotation. So even simple eye movements are complex, and your eyeglass design should take these things into account—quantitatively, if possible—inasmuch as these movements augur for bigger lens widths. In straight-ahead viewing, the center-of-rotation is located about 13.5mm behind the apex of the cornea on the line of sight.

So how much does the center-of-rotation deviate from this, in its little elliptical path? Optometrists won’t know, but engineers who design aircraft cockpit displays do! Almost all of the conventional optometry and optical engineering literature regards the eye as a sphere whose degrees of freedom are only rotational. An aspect of eye movement that’s seldom considered is translation—linear motion of the globe. At near distances and when the two eyes’ convergence is 20 degrees or more, the eye muscles make the eyeballs translate temporally by up to 0.5 mm. This may seem like a small amount, but it actually is pretty large when you consider the angles involved at small eye-to-glasses (relief) distances of only a few millimeters.

Another stumbling block is that much of the conventional optometry literature neglects the dynamic nature of how a musician scans the music—far more important for sight-reading than for playing pieces that you know well. Mostly-involuntary movements of the eyes—in slow ‘drifts’ rather than ‘microsaccades’—are thought to be mostly responsible for accurate fixation and fine vision when you scan a page. But let’s say your two eyes need different corrections—different diopters. If the refractive difference for the two eyes along the optical axis between the eye and the part of the page you are scanning is big, then the eyes and brain may not maintain the drift movements necessary to achieve or sustain a clear, unambiguous view of that part of the page. In this situation, the experience you get is similar to a blind-spot—a scotoma—and this amounts to a surprisingly large percentage of the angle that’s bounded by the rims or edges of the eyeglass lenses. So the calculation of minimum lens width may have to take into account the refractive difference between your eyes, as well as the amount of time you spend scanning off-axis for the typical number of pages you have open, the typical eye-to-music distance you have, and so on.

The conventional optometry and optical engineering literature also fails to take into account that a musician moves or rolls her/his head in the natural course of playing their instrument (or singing, or conducting), up to about 25 degrees. The optometrists’ conventional lens design rules fail to recognize that there are torsional movements that the eye executes, to compensate for your head motions. The compensatory ocular torsion (ocular counter-roll) is opposite in direction to the head role, up to about 4 degrees. These dynamic motions interact with regard to the visual field you experience through the eyeglasses when you’re playing music. The optometrists’ design rules are expecting a stationary reader, not a mobile musician. Let’s calculate eyeglass lens width with provisions for movement! What we need is an eyeglass lens-sizing calculation that takes into account (a) the lens edge-related scotomata (what optometrists call the ‘jack-in-the-box effect’) that reduce the effective lens-width [i.e., that reduce the usable lens area in the periphery, near the eyeglass rims or lens edges] and (b) the range of head roll and ocular counter-roll that’s inherent in performing.

What we need, too, is some insight into lens materials. It’s all too likely that a musician will be ‘sold’ a pair of glasses by a salesperson who is persuasive about eyeglass features that affect appearance or weight but who is unaware of the properties that are the ones most relevant to the musician’s use of the glasses.

Advantages of increased refractive index materials:

Thinner lenses give nice aesthetics and weight reduction.
In myopia, high index minimizes thickness at the outer edges of the lens. A thinner edge means less light entering into the edge of the lens, which in turn eliminates internal reflections and the visual distraction/confusion from reflections.

Disadvantages of increased refractive index materials:

Worse dispersion (lower Abbé number) and chromatic aberration.
Inferior light transmission and reflection properties (Fresnel Reflection Equation), making an anti-reflective coating more important.
Greater sensitivity to grinding and polishing precision and manufacturing defects.

Of all of the properties of a lens material, the one that dominates overall optical performance is dispersion, measured by the Abbé number. It sounds like a geeky term, but don’t let that stop you. Ask the optometrist to show you the datasheets on the lens materials you’re considering. Easy!

Low Abbé number lenses have significant chromatic aberration—color fringes above/below or to the left/right of high-contrast objects, like black music note heads on a white page, especially in larger lens sizes and stronger prescriptions (±4 diopter or greater). Generally, lower Abbé numbers are a property of high refractive index lenses, regardless what the material is—glass or plastic. Also, note that the Abbé effect on chromatic aberration is not ‘linear’. That is, a change from 30 to 38 Abbé will not have a noticeable benefit, but a change from 40 to 48 Abbé could be beneficial for a musician who requires a strong correction, who moves her/his head a lot, and who looks ‘off-axis’ quite a bit of the time.

Some people don’t sense color-fringing directly but instead perceive ‘off-axis blurring’. Abbé values as high as 42 produce chromatic aberrations that are noticeable with lens widths bigger than 45mm, especially if the correction is stronger than ±4 diopter. At ±8 diopter, even glass with Abbé 58 gives enough chromatic aberration to interfere with your playing.

The human eye has its own Abbé number, of course. The eye’s Abbé number is independent of the eyeglass lenses’ Abbé number. Your eye:

Moves to keep the visual axis close to the achromatic axis (which is, by definition, free of dispersion);
Is pretty insensitive in the periphery (at retinal points distant from the fovea, where the cone cells responsible for color vision are concentrated).

Your eye moves to look through various parts of a corrective lens as it shifts its gaze. Some regions of the lens that your eye traverses can be two centimeters or more away from the optical center of the lens. So, despite the eye’s own Abbé properties, the eyeglass lenses’ Abbé value can’t be ignored. People who are sensitive to the effects of chromatic aberrations, who have stronger corrections, who often look off the lens’s optical center, or need larger lens widths to accommodate the geometry of their eye-to-music working distance and page-size will be more affected by chromatic aberration and will therefore be better suited to lens materials that have high Abbé numbers and lower refractive index.

To minimize chromatic aberration interference with the clarity of your seeing the music, you would want to:

Try to use the smallest lens height that’s comfortable and still covers the vertical extent of the music page at your typical working distance from the page. Usually, chromatic aberrations are more noticeable as the pupil moves vertically below the optical center of the lens (for example, when you look at the keyboard or down the nether regions of your cello’s fingerboard). Keep in mind, though, that a smaller lens height will make you move your head vertically more, especially while performing pieces that require short and intermediate distance viewing. This could lead to more neck strain.
Restrict your choice of lens material to the highest Abbé value at acceptable thickness and weight.

What else? As your gaze shifts from looking through the optical center of the eyeglass lens, the lens-induced astigmatism value increases. In a spherical lens, especially one with a strong correction whose base curve is not in the best spherical form, the lens-induced astigmatism can significantly impair the clarity of your vision in the periphery. In this regard, classical musicians work in stressful, high-glare environments that often require them to maintain asymmetric, off-axis postures for prolonged periods during performances and rehearsals. Maladaptations like astigmatism and anisometropia that are exacerbated by stress and by asymmetric head and body postures haven’t been researched much. Harris’s old paper (link below) discusses a study that shows that sight-readers in music use a different eye scan pattern to read music from the pattern that they use to read written language. To my knowledge, no subsequent publications on this have appeared to-date.

Taking all of the above into account, I’ve prepared an Excel spreadsheet that lets you input your distances and your diopter correction (via scroll-bars) and calculates the minimum lens width that would enable you to visualize the page(s) on your music stand with a minimum of head and neck motion. This two-eyed calculation takes ocular and facial anatomy into account—for example, the fact that, when you gaze to the right, your left eye is only using a modest percentage of the left lens, and vice versa when you gaze to the left. The equations in the spreadsheet also incorporate the so-called ‘Prentice Rule’ estimate of edge scotoma minus the angle of the pupil subtense, to get the size of the ‘jack-in-the-box’ scotomata at the periphery. This adds significantly to the required lens width for corrections greater than +6 diopter and normal pupil diameters between 4mm and 7mm. Click on the screen shot below, and it will open a window where you can play with the spreadsheet or download it to your computer for off-line use.

I hope this spreadsheet and the links below are useful for you. As always, if you have a criticism or suggestion, please post a comment below. Thanks!

Spreadsheet to Calculate Minimum Lens Width for Music Glasses

jwilzewske

Friday, November 16, 2007

Better Eyeglasses for Chamber Musicians?

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