jwilzewske: vision

Showing posts with label vision. Show all posts

Thursday, December 15, 2011

Making an Ergonomic Holder for eReader Tablets for Piano

M y wife gave me a Samsung Galaxy Tab 10.1 Android tablet for my birthday. I use it for lots of things, but one of its best purposes is as a music reader on our piano.

A irTurn’s Bluetooth-based BT-105 is out--it has many nice, new features.

F or the BT-105, I've used an inexpensive Levenger Dovetail bookstand as a cradle, accessorized with a bit of Sugru® silicone to keep the tablet or the cradle from moving around with the vibration from vigorous playing/pedaling. I use that mostly with our MIDI keyboard.

B ut I still like and use my original AT-104 with its proprietary RF communication between the pedal transmitter and the USB dongle receiver. For one thing, there are no long-range Bluetooth interference/security issues with the AT-104.

T he 1280x800 pixel resolution of the 10.1-inch Widescreen HD WXGA TFT on the Galaxy is noticeably better than the 1024x768 resolution of the iPad 2’s 9.7-inch screen. (The screen on the Samsung tab has a 16:9 aspect ratio, compared to 4:3 on the iPad 2. In the landscape orientation, which is how I position the tablet on the piano music stand, the Galaxy display is only 4% more pixels vertically, but it’s got 25% more res than the iPad 2 horizontally.)

S o if I’m going to stick with my AT-104 AirTurn, then I need to get the USB receiver dongle attached to the Galaxy Tab, and the USB port is at middle of the bottom edge of the tablet. (I could rotate the Tab 180-degrees to be upside-down, so the USB port is on top. But then I’d have an unsightly dongle or pigtail cable coming out up there.)

T he Samsung USB adapter dongle is okay, but it sticks out 3cm beyond the edge of the tablet, and a male USB connector terminating a typical cable-end sticks out 5cm beyond that. You need some way to elevate the tablet about 10cm for the short male-to-female USB passthru/extender cable to the AirTurn Dongle to connect to the Samsung USB dongle.

E levating the tablet that much is actually an improvement in terms of good visual ergonomics. But you need to keep the tablet secure and level in that position.

A nd there’s another issue—which is the same as the issue that's affected the usability of laptop-based music eReaders for pianists: the front-to-back depth. It may be too deep for some piano music stands. And it’s definitely too deep for harpsichord or organ music stands. Or the rail on the music stand that is fine for 60 gm of paper sheetmusic is too wimpy for a shiny, slippery tablet that weighs 600 gm. There is a significant risk that the tablet will shimmy its way to a bad fall.

A further issue for me, at least, is the visually small form-factor of the tablet--specifically, the fact that the active display area of the 10.1-inch screen comes so close to the edge of the device—makes me want to have slightly wider “margins” framing the visual field that I’m looking at. With the tablet on the Dovetail bookstand, the “near-far” field at the left-hand and right-hand edge of the displayed music interferes with my eyes’ focus. Maybe that is not an issue for you, but it is for me.

S o I cut a 12x14-inch piece of black 3/16-inch Delrin acetal, put some 1/2-inch Delrin brackets on it with Sugru® to cushion them (see photo). You slide the tablet into the lower brackets and press the top edge in past the pan-head “crowns” of the 3 Sugru® bumpers--each one is about 1 mL of Sugru® that is molded to stand proud of the 5/16-inch hole by about 0.03-inch. The Sugru® is just the right Shore-A/durometer hardness to “give” just a little as the tablet edge moves inward and seats flat against the acetal sheet. The mechanical “detent” of the Sugru® is just enough to hold the tablet securely in place but still allow the tablet to be easily plucked out again with gentle finger force.

M y kluged black acetal stand leaves a nice visual margin all around, similar to the whitespace from the staves to the edges of normal sheetmusic paper, in the same focal plane as the image on the tablet. It makes it easier to read the Tab’s display, and it’s compatible with any piano, organ, harpsichord, etc., including ones that have only a tiny, narrow rail/groove for music to stand up in.

H ugh Sung’s Airturn site has plenty of nice stands and brackets to hold tablets and laptops on various types of mic stands or other uprights. Those are great for string players, wind instruments, singers, and other kinds of musicians. But if you’re a pianist and you’ve done ‘home-brew’ things yourself, to enhance the mounting of your tablet on your piano's music stand, please leave a comment below or email me!

Tuesday, December 9, 2008

Assistive Technologies for Musicians with Low Vision and/or Tremor: AirTurn, Tablet PCs, MusicReader

I am a pianist and about six months ago I was diagnosed with Age-related Macular Degeneration (AMD)—in both eyes. Actually, the two eyes are pretty different in how big the blind spot is. My right eye has a blurred area that makes music staves bend and notes blurry when I try to look directly at them. Dots disappear or blur into adjacent note-heads, 32nd beams look the same as 16th beams, and so on. The left eye is much worse, so what I see in the field that is affected relies almost entirely on what I’m seeing through my right eye. In spite of this, I can see almost well enough to read the page of music, so long as I am not looking directly at it—in other words, so the measures are off to the side of my ‘blind spot’. In the periphery, my vision is still pretty good. This adaptation—using my non-central visual fields for detailed seeing (I call them ‘non-central’; it seems wrong to call them ‘peripheral’, since it’s only a few degrees away from the central blind spot.) Also I find it works better for me to play on my MIDI keyboard with the chair raised higher, so I am looking down at the music instead of looking a bit upward, above eye-level, which is where the music is on my piano. It’s a far-from-ideal posture from a performance point of view. But looking downward at the music seems—for me, at least—to enable me to use parts of my field of vision that are less affected than other areas, which is a trade-off I’m happy to make.”
— Anonymous email to CMT.

Thank you, to the several people who emailed me about the CMT post about Pianoteq as a prosthetic ‘aid’, to enable people whose physical impairments would otherwise limit what they can do at the keyboard, to continue to perform on keyboard in the satisfying ‘manner to which they were accustomed’. Thank you, too, to the emailer above, for asking about visual impairments. There have been several other CMT readers who have had similar questions over the past several months, so now is as good a time as any to respond.

Y ou should know that you are not alone in your search for solutions to your visual acuity problems and your efforts to continue playing. According to current U.S. statistics, about 7% of all pianists between age 40 and 100 have AMD. Pianists who have (had) AMD include:

Arthur Rubinstein (1887-1982)
Helen Kolozsvary (1922-2007)
J. Wayne Streilein (1935–2004)
Madeleine Titus (1914-2008)
Earl Wild (1915-)
Jennie Lois Windle (1915-)
Sylvia Zaremba (1931-2005)

Age-related macular degeneration is the leading cause of blindness and visual impairment in the developed world. ‘Blindness’ as defined by the U.S. definition is a best-corrected visual acuity of 20/200 or worse (20/200 in the better-seeing eye, where the acuity of left eye and right eye are substantially different). ‘Low vision’ is defined as a best-corrected visual acuity worse than 20/40 but better than 20/200 (in the better-seeing eye, if left and right are different).

AMD is the leading cause of blindness in Americans 50 and older, and its prevalence in the U.S. is expected to increase to 18 million by 2020. There are similar epidemiologic statistics in Europe and elsewhere.

M acular degeneration is a disease of the eye causing the light-sensing cells of the macula to malfunction resulting in partial or complete blindness. The macula is the central portion of the retina directly opposite the lens. The retina is the portion of the eye responsible for receiving light and sending electrical impulses to the brain that precipitates with sight. Specifically, the macula is responsible for the sharp, straight-ahead vision used in seeing fine detail, reading printed material, driving, watching television, reading things at normal magnification on computer screens, and recognizing faces.

Loss of vision with age-related macular degeneration (AMD) normally occurs gradually and will usually affect both eyes, although not necessarily at the same rate or to the same extent in each eye. Macular degeneration is most often found in people over 60 years of age, although there are some hereditary forms of macular degeneration that affect children.

Gradual decline of central vision which tends to progress at different rates in the two eyes
Difficulty reading or performing tasks that require the ability to see detail
Straight lines appear distorted
Center of vision appears distorted compared to the rest of the scene.
A dark, blurry area or "white out" appears in the center of vision
Color perception changes or diminishes.

Y ou can test yourself with the Amsler Grid below. If you need reading glasses, put them on while you look at the Amsler Grid. The grid should be at about the same distance from your eyes that any other reading material would be. Cover one eye, then focus on the dot in the center.

Do any of the lines look wavy, blurred or distorted? (All lines should be straight, all intersections should form right angles and all the squares should be the same size.)
Are there any "missing" areas where you can't see the grid lines?
Are there any "dim" areas in the grid?
Are you unable see all corners and sides of the grid when you are focusing on the dot in the center?

Repeat the procedure with the other eye covered.

If you answered ‘Yes’ to any of the four questions above for either eye, make an appointment with your eye doctor right away. You can print the page, mark the areas of the chart/grid that you’re not seeing properly, and bring your marked-up grid with you to your eye exam.

If you’ve recently been diagnosed with AMD, there are many good books and online resources that you and your family members can take advantage of. Some of them are in the list at the bottom of this post. Laser treatments and other standard therapy alternatives are covered in those resources, and I won’t go into those here. But there are some newer things that I’ll at least mention, with some links at the bottom to enable you to explore on your own.

Over the past several years, people who have severe AMD have received off-label treatment with Avastin™ (bevacizumab), an anti-VEGF drug made by Genentech that was the first anti-angiogenic therapy approved for treating colorectal cancer. Ophthalmologists reasoned that if Avastin could block blood vessel growth in tumors, it might also affect angiogenesis in “wet” AMD. Genentech’s Lucentis™ (ranibizumab) was recently FDA-approved specifically for the treatment of “wet” AMD, and it is similar in molecular structure and action to Avastin. In a large, two-year study, this drug stopped vision loss in more than 90 percent of patients studied and restored vision in 33 percent. Scientists are now comparing the efficacy of Lucentis and Avastin, since Avastin is much less expensive than Lucentis.

The recently introduced anti-vascular endothelial growth factor (VEGF) intravitreal injections of Genentech’s ranibizumab (Lucentis™) and bevacizumab (Avastin™) generated a heated research argument: on the one hand Lucentis was proven effective and relatively safe in large prospective double-blinded studies, albeit this drug is expensive and might cost up to $1,000 per injection into the eye. Eyetech Inc’s pegaptinib (Macugen™) has the same anti-VEGF mechanism of action and is also quite expensive. On the other hand, Avastin is widely used worldwide as a low cost alternative for Lucentis, with an estimated cost of about $120 per injection, although its effectiveness and side effects were previously investigated only in smaller retrospective studies and only recently have been studied in larger prospective ones.

But those medications are used mostly in AMD that is already severe/advanced. They generally are not (currently) used in a pre-emptive or preventive fashion, or in early or mild cases of AMD. This may be in the process of changing, insofar as these medications—in clinical trials and in off-label observational studies—are evidently able to stop the progression of the disease, for two years or longer. How much longer is not known, since the clinical trials have not followed the patients for much longer than that yet.

L ooking to the right slightly seems to be better than putting the music in the center or putting it where I have to look a little off toward the left. I can’t imagine why this is, except maybe it has something to do with what parts of each eye are damaged by the AMD. My adjustments almost make it possible to play pieces that are very familiar to me. But I don’t know how much longer this trick will be adequate. I can imagine a couple of years from now the AMD might be worse and these little tricks won’t be enough anymore, and I’ll have to stop playing. Even now, it’s next to impossible for me to get a new piece and try to learn it. My sketchy non-central vision is good enough to remind me of what my fingers and mind already learned years ago, but it’s not good enough to enable me to learn an entirely new piece. But it seems to me that if I could just magnify things a bit—like I do on my PC when I’m reading an Acrobat document or something—then I would be able to manage. Increasing the size of the music would enable me to make-do with what I’ve got and the non-central vision would be enough if only the music were way bigger. Increasing the wattage of the music lamp doesn’t seem to help; just makes the glare worse. Does this make sense? Is there something different I should by trying? I saw the things you wrote about ‘assistive’ uses of Pianoteq for pianists with disabilities, and I thought you might have suggestions about pianists with AMD.”
— Anonymous email to CMT (continued).

Interestingly and in regard to the remark in the email above about ‘non-central’ visual fields mitigating the deficits in the foveal/central field, earlier this year Lingnau and colleagues at the Technical University of Braunschweig studied training people to use a parafoveal retinal location for reading. The study was designed to find out which part of the visual field is best suited to take over functions of the fovea during early stages of macular degeneration. A region to the right of the area you are needing to read leads to best (most accurate; quickest) reading performance and the most natural gaze behavior (so that the eyes do not get tired right away).

By contrast, reading performance was severely impaired when Lingnau and the other researchers asked the subjects to use a region to the left or below the area of visual fixation. An analysis of the underlying eye-muscle behavior revealed that ‘practice effects’ were accompanied by a larger number of ‘saccade’ eye movements in the text direction and decreased the visual fixation durations. Their journal article explained the observed performance differences at different retinal locations in terms of the neurophysiology of the interplay of attention and eye movements. The research has important implications for the development of training methods for AMD patients who aim to retain the ability to read detailed material such as sheetmusic. Most specifically, the research suggests that it is beneficial for AMD patients to use a region to the right of their central blind-spot, much as the CMT emailer has independently discovered.

W hat other non-medical, non-pharmacological things can a musician with AMD do, to compensate for the visual deficit?

Increase the contrast between light and dark objects;
Reduce glare with proper lighting (halogen bulbs are better than low-energy fluorescent, which in turn are better than incandescent);
Use curtains or shades to reduce ambient glare from light-sources near your piano;
Play for short periods, taking breaks that do not involve other visually-intense activity before returning to the keyboard;
Follow the nutrition (with vitamin supplements recommended in books and professional society / AMD support group websites; see links below) and lifestyle recommendations of ophthalmologists and other experts;
Replace your music with nice, high-contrast magnified sheet music—either photocopy and enlarge it yourself, or use a PC-based means to display digital sheet music (usually as Acrobat pdf files, either purchased or obtained free online or scanned using an inexpensive scanner-printer-fax attached to your PC).

M agnifying the music, either in hardcopy by enlarging it in a copier or by doing it digitally on your computer? As shown by Chung and colleagues in Houston, there is no need to increase the spacing between staves, to normalize reading speed in AMD patients. You just need to make the staves and notes and markings bigger; the space between the staves can be the same as in a normal score. Of course, if you’re making hardcopy 200%-size photocopies and cutting-and-pasting them to create a score that will fit on your music stand, you might go to the trouble of doing the paste-up staff-by-staff. But most of us would just scan the music as pdf Acrobat files, and then the spacing between the staves will be magnified by the same percentage as the staves themselves. That means that there’s more ‘white-space’ to scroll through, to reveal the next group of staves further down the page. That’s maybe a small thing to complain about if the alternative is not being able to see/play at all. But it is a significant annoyance, as I’ve found with my own use of a tablet PC for displaying my own music. Here is a little table that shows the amount of magnification you would need, depending on what visual acuity you have with your glasses or contact lenses on. In other words, the left-hand column is the ‘best-corrected’ acuity you have, not your visual acuity without corrective lenses.

Visual Acuity and Magnification Required

The thing that I like best about playing from digitized music on a tablet PC (or, for a bit more money, the MuseBook) is that you can set and adjust the magnification exactly how best suits you and the particular score you are reading. You can adjust the display contrast to be extra bright and contrasty if you want. You can tweak the RGB color controls on the monitor, to compensate for any color issues that your AMD is causing. You can’t do any of those things with hardcopy paper music.

Landscape-oriented PC vs. portrait-oriented paper

By far the most extensive source of information on tablet PC-based music and practical technology and techniques to use it is on pianist Hugh Sung’s website and blog. His site has many practical tips and video demonstrations that explain how to do various things.

Please also consider getting an electronic pedal page-turning device for your PC. The AirTurn™ device is one that I recently bought. I find that it is very well built—sturdier than other options that are available. I like the Boss FS-6 dual pedal version of AirTurn™ a lot. The FS-6 pedal has a nice ‘feel’ and the pedals are spaced at a distance that makes it convenient to click pages forward and backward without risking a mistake and advancing too far. Other pedals that I have tried over the past ten years are either too dainty or too industrial/stiff and take your concentration away from your playing; this FS-6 pedal is non-intrusive and easy to get used to. Here is a photo of Hugh Sung at Curtis Institute, holding one of the Boss FS-6 pedals...

The AirTurn™ AT-104 is an RF device, so there are no wires snaking up from the pedal up to the piano. Attached to the pedal you have a little battery-powered transceiver that sits on the floor. (Mostly, its power consumption from the battery occurs when the device is transmitting pedal events to the PC, so the battery lasts quite a long time, even with active pedaling and prolonged use.) And the other part of the transceiver ‘pair’ is a tiny dongle that plugs into a USB port on your computer, much like dongle you use with an RF mouse. It receives the signals from its mate on the floor, enabling the software in your PC to scroll or jump down the page or page-forward or -backward. The AirTurn™ system has 65,000 RF pairing IDs available; you select an unused ID so the AirTurn™ will not interfere with your RF wireless mouse or other 2.4GHz RF computer accessories you have.

The ability to mark-up and save annotated copies of your sheet music—save them as different filenames so your original music remains ‘clean’—is a great advantage. And you can keep changing your annotations—no erasing! Very nice.

Annotation of digitized sheet music, Hugh Sung

I am using my AirTurn™ with MusicReader™ software with my 1920x1200 and 1680x1050 monitors oriented in ‘landscape’ mode, and that works fine with my big “piano reading glasses” and even my bifocals. But MusicReader can be used with iRotate™ or other software to rotate the display to a vertical ‘portrait’ format instead of the default ‘landscape’ horizontal format.

I’m hoping at some point to try a dual-display monitor. Those are extensively used in financial trading—including people who do day-trading of stocks and commodities (or options and futures) at home—and by some gamers. The prices of dual-display monitors and the associated screen-splitting software have come down quite a lot over the past several years, due to their popularity. For music reading with diabetic retinopathy or other conditions that do not primarily involve a central macular deficit, maybe dual portrait-oriented displays—like the Zenview DuoPortrait 20 Pro pictured below—would be an advantage. But for other CMT readers—and maybe especially ones with early- to intermediate AMD where you are relying on ‘non-central’ visual fields—two landscape-oriented monitors may be better. In either instance, you will probably have to work out a different type of ‘stand’ for the monitors, because the ones that come with them from the factory (a) will not sit nicely on your Steinway and (b) will vertically position the centers of the monitors too high in your visual field. You will need either to go without a stand (monitors sitting on the piano in their bezel but without a stand) or find something that is squattier than the desktop-style stand that these products come with.

Note that the digitized sheetmusic is moved forward or backward by each pedal press; it doesn’t ‘jump’ across many pages to follow D.C. or al segno or other ‘marks’. In such cases you may want to make your own pdf (using a ‘full’ Acrobat license, not just AcrobatReader) that replicates each section that is to be repeated, and assemble the replica page/pages sequentially, so that instead of doing multiple pedal presses to jump backward you are doing a single pedal press to go forward into the repeat, or a single pedal press to skip forward over a section to the segno.

These products are not just for musicians with visual impairments. I do think that AirTurn™ and related products will significantly help older pianists who have ‘essential tremor’ or arthritis or other conditions that make reaching and turning music pages difficult. To quickly and easily and reliably navigate your digitized music by pressing pedal footswitches is a joy, compared to struggling and often failing to turn the paper pages accurately and with confidence.

One last thing. I put together a little spreadsheet that shows the AREDS scoring method that ophthalmologists use to assess the severity of your AMD and estimate prognosis for disease progression on a 5-year timescale. If you wish, you can click on the screenshot below and download the Excel file and play with it. Discuss with your ophthalmologist the specific findings and changes that she/he observes in your eyes and ask about what changes you might expect over the next 5 years based on the AREDS studies. Overly-quick office exams are not what you want. You want to go to the eye doctor prepared with questions to ask, so you can get specific answers and good prognostic advice.

Please have a look at the links below. Email me with comments, questions, or suggestions you may have. If there is enough interest, I will compile them into future CMT posts on ‘assistive technologies’ for musicians. Thank you!

Friday, November 16, 2007

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