In a normal healthy eye images of objects we see, far away or close-up, are focused clearly onto the retina at the back of the eye, just as the lens in a camera focused the image onto a film. These images are then transmitted to the brain via the optic nerve.

In a healthy eye you can see clearly at all distances. When the images of objects are not focused on the retina, vision is blurred. This is can be due to a number of different visual defects.

Some of these defects occur when the clear structures of the eye are not as clear as they should be. These include opacifications in the cornea and lens (cataract) and in the vitreous gel.

Other defects are optical focusing problems where a person is longsighted or shortsighted so that the image is not focused in line with the retina due to the eyeball being too long or short.

Finally there can be problems with the retina and the transmission of the image to the brain for processing. Diseases such as macular degeneration and glaucoma affect the retinal tissue and its ability to function properly.