Notes:

The other reason why eyes become habitually out-of-focus, thereby requiring glasses or contact lenses, is that the focusing power of the relaxed eye is a little bit too much or too little. In the first couple of years of life an infant's eye is continuing to grow and as it does so the optical system must compensate for the changing physical dimensions of the eye. There are automatic mechanisms in the eye which are not yet fully understood that coordinates this growth of the eye so that the eye remains well focussed as it grows. The problem is that sometimes this mechanism fails and the eye ends up either near-sighted, which means the optical system is too powerful and can't focus on distant objects, or it ends up far-sighted, which means its optical system is too weak.

For young people, being far sighted is not a great handicap because they can always increase the power of the internal lens to compensate just by contracting the internal mucscle slightly. However, as the lens grow older and loses the ability to change shape, far-sighted people need glasses even to see distant objects clearly.

The other common form of defocus problems in eyes is astigmatism, which is caused by lack of rotational symmetry in eyes. Astigmatism makes it impossible to clearly focus vertical and horizontal lines simultaneously. Fortunately, this kind of defocus is also easily corrected with spectacles or contact lenses.

The more difficult challenges in optometry relate to the other 3 major causes of imperfections in eyes: diffraction, scatter, and aberrations. Diffraction is an unavoidable factor that is due to the wave nature of light, but in a practical sense is not a limitation for vision except when the eye's pupil is tightly constricted down to a millimeter or so in diameter

Scatter can be a very serious problem in eyes that develop cataract or other diseases which reduce the transparency of ocular tissues. This is a large topic which I won't have time to discuss today if I am going to get to the main topic of this lecture, optical aberrations.