Myopia & the Blur Effect

Question

I know myopia is caused by elongation of the eyeball. When the eye is too long, light entering the eye balls falls in front of the retina instead of on it.

However I do not quite understand how this leads to the "blur" we observe. If part of the light entering the eyeball "misses" the retina shouldn't the observed effects simply be a reduction in the brightness of the object - as if I were standing in a less illuminated place? For example, if I take off my glasses in broad daylight my retina is getting a far greater dose of light than when I'm indoors (wearing my glasses) under artificial lighting. Yet the world still looks blurry outside (without glasses) and clear indoors (with glasses).

What causes the blur effect? If less light is hitting the retina it should simply look less illuminated, not blurry.

Answer 1

You have some misconceptions about visual acuity. You need to understand how lenses work to understand why objects up close are blurry in myopia.

Sure, if one elongates the orbit, blurring will occur, but that's not usually the cause unless it's in childhood. As we age, the elasticity of the lens has more to do with it.

Light comes in through the pupil. The amount of light is what determines brightness; too little, and we can't see, too much and we look away. In between we "see". Being able to focus doesn't have any effect on brightness; the size of the aperture does, however (the aperture in this case being the pupil.)

Blurriness is caused by where the light rays striking the object we're looking at come into sharp focus in out eye. Sharpness falls within a very small range, then once the range is exceeded, the image becomes progressively blurrier because the rays scatter again. This is why looking through a pinhole improves our vision: less scatter. If your pupil could constrict down to a pinhole, that would be pretty amazing, but it can't.

The purpose of the lens is to focus the image. It can do so (with the help of ciliary muscles) by changing shape. But it can only change shape within a range. If it cannot change shape enough to focus the image where it needs to be - on the retina itself - you have blurry vision.

That's why additional lenses (or changing the contour of the cornea) help.