Aging is not fun as you have most likely all ready discovered if you are reading this post. At some point, if we are lucky enough to live long enough, we enter what our parents generation described as “Bifocal” age, scientifically known as Presbyopia (I like to call it “short arm syndrome”). The word bifocal to many people is synonymous with middle age and received by patients with a scowl when I bring up the diagnosis at the end of an examination. A bifocal lens is split into two parts – the top part usually for distance viewing (objects greater than 20 feet away) and the bottom part used for seeing near (12-20 inches from the bridge of the nose). Fortunately for those of use concerned with our image, bifocals are old technology, and unable to meet the demands of the modern workplace. The near part of the bifocal is set to low for computer use ; using it to view a computer monitor would require one to tilt their head back too far. They might clear up the vision at the computer by lifting their chin and find they need chiropractic as a result. The modern office worker needs a lens that can correct more of a range of vision from near to mid-range to far and back. Then there is the dreaded “line” in the bifocal lens; the line that tells everyone how old you are. It is because of the inability of the bifocal to correct vision comfortably at mid-range and because of the stigma the line creates that the most popular solution in eyeglasses are Progressive lenses, also known as “multifocal”.
Progressive lenses work by warping the lens so that the center area from top to bottom has a smooth and gradual curvature change. When the head is positioned straight and the eyes looking parallel to the ground, the user views distance. As the eyes decline the line of sight moves through sections of the lens that provide gradually closer and closer focus. The warping of the periphery of the lens is necessary to enable the aforementioned optics and is often the cause of difficulty during the adjustment process for many progressive users. In a bifocal, with the head looking straight, the user is able to look left and right with their eyes and encounter crisp, clear vision. With the progressive multifocal, the lens is warped in the periphery and looking left or right without moving the head puts the users line of sight in the “warped” area, which appears blurry or distorted to the user.
Many first time progressive users report initially noticing the warped periphery when they put their new pair on. For most, the eye and vision system adapts to the warped periphery and after a few weeks the glasses focus comfortably. In order to have the best chance at adapting to progressive multifocal lenses the following rules need to be followed: (1) the prescription must be correct (2) the optician must properly measure and mark the lenses prior to making them. (3) the proper frames must be chosen (4) the correct papillary distance needs to be measured “monocularly” (one eye) and binocularly (both eyes) and (5) patient expectations should be set in terms of the amount of time they should allow to adjust to the progressive multifocal. If any of these steps are not taken, it may result in non-adaptation or extended time required for the user to adapt. There is also a small percentage of people who may not be able to adapt, but new progressive technologies that require less distortion are constantly reducing the number of non-adapts.
For experienced progressive users, there are several things that can go wrong when purchasing a new set:
Different lens design: if you spent time adapting to one brand of progressives with its individual characteristics and an optician moves you into a different brand with different characteristics you may find there is an adjustment period. This often happens when the brand you are switched to has a different “base curve” or more narrow reading area then the pair you switched from.
Different frame height: If you switch frame and the “B” dimension of your frame is significantly different (the distance from the top to the bottom of the frame), usually from larger to smaller frames
Prescription change: If the parameters of your prescription change significantly, it may be necessary to adapt to the new prescription. This is especially the case if the “add”, or near power increases significantly. The lower the near power of the progressive lens, the wider the “corridor” that you look through and easier the adjustment. As we age, the more near power is needed, thus the corridor width decreases.
Incorrect Measurements/Errors by the Optician or with the Doctors Prescription
If the progressive is not measured and fit or if the frame is not correct for the lens chosen the progressives will likely be sub optimal in terms of performance. If the doctors prescription is off, this will also be the case. If the pupillary distance is measured incorrectly the glasses will never seem right or you may feel you have to twist your head in unnatural motions to get to the clear area of the lens.
Like all retail products, there is a pyramid of quality of progressive multifocal lenses. The best optical quality progressives with the widest corridor widths and best chance of successful adaptation usually cost more. Progressives with less than perfect optics may cost less but also may create unnecessary frustration through the adaptation process. Be sure you find an optical and optician you trust, someone with years of experience fitting advanced optical lenses and ask their advice! Hopefully these tips will enable you to ask the right questions.
Copyright 2010 – Dr. Alan N. Glazier, Optometrist, Rockville, MD – all rights reserved
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