This has to be one of the most common questions I get asked when seeing patients for a variety of eye conditions that decrease vision. The first thing to do is understand exactly what glasses do and what are their limitations. First, we need to stop thinking of glasses as being too strong or too weak. Instead, we should think of glasses as being correctly or incorrectly focused. Think of the eye functioning the same as a camera. Light comes in through the lens, passes through the diaphragm and focuses on the film (or CCD sensor for digital cameras). The purpose of the camera lens is to bend light rays and focus them to a sharp image on the film. Bending the light more or less will result in the image being focused in front of or behind the film. If the image is focused in front of or behind the film, the picture will be blurred. The same applies to the eye. Glasses (or contact lenses) focus light on the retina. The retina acts the same as the film in a camera.
If you are nearsighted, (fig. 1) the image will be focused in front of the retina. Glasses will bend the light to focus it sharply on the retina. The opposite is true for farsightedness. Once the image is correctly focused on the retina, you cannot make the glasses “stronger” to improve the focus. It is either in focus or out of focus.
So, if my glasses are correctly focused, why can’t I see better? Let’s go back to our camera analogy and think of other reasons that your picture might not turn out very clear. What if there is a big thumbprint on the front of the camera lens? (Fig 2)
You can try to focus the camera lens all day and you’re not going to get a clear picture until you clean the thumbprint off of the front of your camera lens. This is very similar to any problem there may be with the front part of the eye. If there is an irregularity of the corneal surface, a cloudiness in the cornea, a scar on the cornea, a rash on the cornea, a cloudiness in the lens of the eye (cataract) or any other problem which damages the quality of the image entering your eye, things will appear blurred. No amount of focusing will solve the problem until the other issues are resolved first. In some cases, that may be impossible.
The same goes for the other end. If the film (or sensor) in the camera is bad, no amount of focusing will give you a sharp picture. Likewise, any damage to the retina such as macular degeneration or diabetic retinopathy will result in a less than perfect image.
…There is one exception.
You cannot make the focusing of glasses stronger, but you can increase magnification for near objects. This is very common in macular degeneration or a macular hole. With macular degeneration or hole, you develop damage to the very center part of the retina. This would be similar to punching a small hole in the center of the camera’s film. If you focus light right on the hole in the film, you won’t get a good picture. But what if you magnified the image much larger than the small hole in the film. You would be able to make out the picture despite there being a small hole in the center. This is what magnifiers or low vision aids do for macular degeneration. If you magnify things large enough, you can see around the damaged macula. Granted, the image will not be perfectly clear, but it does offer some improvement.
I hope this clears up some of the confusion about what glasses can and cannot do. Please contact your eyecare professional if you have any concerns.
This newsletter does not constitute medical advice. Make sure to contact your healthcare provider if you experience any of these conditions. Do not make any changes to your medications before consulting your physician.