Optical deficiencies can be split into a few groups, and I shall deal with them in turn, along with the possible implications for the ornithologists out there.
When we view an object, the image of that object has to be focused on the retina. This is primarily achieved by the cornea, and is supplemented by the lens. If the cornea's power is too high, the object will be focused in front of the retina, if it is too low, it will be focused behind the retina.
This occurs when the object that is in view is focused in front of the retina. This means that distant object are blurred, but objects at a nearer distance may be in focus. For bird-watching, any level of myopia over around 1 dioptre (the range of most binocular adjustments) will give a blurred image.
Correction: this is generally by spectacles or contact lenses.
This occurs when the object being viewed is focused behind the retina. The term "long-sighted" is quite misleading as a distant object may well be blurred (more so as you get older) as well as near objects. For bird-watching it will depend upon age and degree of hypermetropia as to whether it affects your viewing pleasure. Generally if you require spectacles to drive or watch television, wear them for bird-watching.
Correction: again, spectacles or contact lenses.
This is where the same eye has two prescriptions, typically one horizontally and one vertically (but not always). Generally astigmatism is combined with either long or short-sightedness, but prescriptions can be wholly astigmatic. Astigmatism alone causes varying degrees of blur, dependent upon the amount. If your level of astigmatism gives blurry vision you will need to wear a correction for successful bird-watching.
Correction: spectacles or (here's the tricky bit) contact lenses. Standard soft contact lenses will not correct astigmatism over about 0.75 dioptres, so for clearer vision you will need to use a rigid gas-permeable lens or a "toric" soft lens. Disposable lenses are available for simple short-sighted and long-sighted prescriptions . For astigmatic prescriptions they are available in certain powers (generally up to about 3.00 dioptres or so), but cost more.
Approximately 10% of the population have a binocular vision abnormality, this basically means that they have little or no binocular vision. This may be caused by a number of things - strabismus (squint) or injury are the most common causes. Obviously, if one does not have binocular vision, binoculars lose some of their three-dimensional effect.
When you get to about 45 years of age, the lens in the eye no longer focuses as well as it used to, and so reading spectacles are required. Existing spectacle wearers may opt for bifocals or varifocals. For bird-watching I would recommend that if you wear bifocals or varifocals, it will be worth your while getting a pair of single-vision long-distance spectacles.
When you have your eyes examined, you are given a spectacle prescription (if you are not, go back and ask for one - it must be given in all cases - that's the law in the UK). All powers are in dioptres (this is the reciprocal of metres) and indicate the focal length of the lens.
Generally such prescriptions are tabulated and will have the following information.
From this prescription spectacles can be made, or you may opt for contact lenses (which will have a different prescription!).
For the ornithologist, there are a few advantages of contact lenses. They don't steam up when you go from a cold to a warm place. They don't get dribbles of rain on them. They don't acquire dirt in the way that spectacles do. You get a better field of view. When using binoculars you use them in the same way as the clear sighted amongst us use them, no need to roll back the eye-piece cups. This also has implications for the field of view when you look through an optical instrument.
Downsides of contact lenses? Well, you are introducing a foreign body into the eye, this can (rarely) have complications. Some people hate to go anywhere near their eyes with their fingers (this makes them tricky to wear). Occasionally they may move in the eye, this can lead to momentary blurriness. They require a daily maintenance routine (although I'd like to think you all clean your specs at least once a day). They may be expensive, especially if you are clumsy (but most are disposable these days and are fairly cheap to replace).
I hope some or possibly all of this information is of interest. I am pretty certain that it is possible to purchase an adapter for some makes of cameras/binoculars that may be glazed with your prescription, this gives the convenience of having apparently perfect vision without need for spectacles or contact lenses, but I have little knowledge on this subject (an appendix for someone?).
If clarification or amendment is required, feel free to post it on the group, or mail me personally.