Distorted vision is a symptom with several different possible causes.
Vitreous detachment 
The vitreous is a gel-like fluid which fills most of the eye. As people age, this vitreous becomes more and more liquefied. The vitreous has loose attachments to the retina, and more firm attachments to the optic nerve. At some point in a person's life, the vitreous liquefies enough to shift position in the eye. When this occurs, usually between age 50 and 70, the back edge of the vitreous will pull forward away from the retina, leading to a "vitreous detachment". This is generally a normal process, although it may happen abnormally early in cases of high nearsightedness or trauma. As the vitreous detaches, it tugs on the retina. This is perceived as a flash of light, similar to a lightning flash in the corner of the vision. It may occur especially with eye movement, since the vitreous moves in the eye. Debris pulled off of the optic nerve and retina are then seen as floaters, suspended in the vitreous above the retina. Sometimes this is described as a cobweb, a net, a string, or a fly over the vision.
These symptoms usually resolve over a period of days to weeks, although some people will continue to see the floaters for a longer period of time. The important thing is to determine that the retina is healthy as the vitreous detaches. This requires a careful dilated examination of the retina to look for tears, or other areas which may be at risk for tearing. A retinal tear can then lead to retinal detachment, if not treated. Thus, people experiencing these symptoms should be examined by an ophthalmologist as soon as possible. (Note, only about 1 in 10,000 cases of vitreous detachment lead to retinal detachment, but it still is one of the most common causes of retinal detachment.)
A tear in the retina can occur with vitreous detachment (see discussion above), with trauma or eye injury, or in areas at risk for a retinal tear, such as "lattice degeneration". The symptoms of a retinal tear usually are of a flash of light in the peripheral vision followed by floaters. The floaters may be debris, but may also be blood, if the tear extends through a retinal blood vessel. Symptomatic retinal tears should be treated by laser to prevent retinal detachment. Sometimes a retinal tear is discovered incidentally as part of an eye examination. These may or may not need to be treated.
Uveitis refers to a large group of disorders which cause inflammation within the eye. A similar condition, iritis, usually refers to an inflammation involving the front structures of the eye associated with pain, redness, and sensitivity to light. In this discussion, uveitis could have these symptoms, but mainly consists of inflammation involving the back structures of the eye (the retina, choroid, and optic nerve). Inflammatory debris liberated into the vitreous leads to the visualization of floaters. If this liberation continues, the vision may become substantially hazy and blurred. There are numerous conditions leading to uveitis, and many have floaters and blurred vision as predominant symptoms: sarcoidosis, toxoplasmosis chorioretinitis, ocular histoplasmosis, multifocal choroiditis, pars planitis, endophalmitis, syphilis, candidiasis, viral uveitis, Vogt-Koyanagi-Harada syndrome, and HIV related unveitis.
Entoptic phenomenon 
The entoptic phenomenon is a normal phenomenon that some people may become suddenly aware of. This sudden awareness may lead to the idea that there is a problem with the eyes, when actually there is not. The entopic phenomenon can be seen especially when looking at a bright blue sky. Small, rapid pin-point sparks of light can be seen darting about in the central vision. Some people may think that these sparks are floaters. In reality, they represent white blood cells moving through the blood capillaries of the retina. This is a normal finding, and actually may indicate normal retinal function.
Migraine headaches may be preceded by a visual "aura", lasting for 20 to 30 minutes, and then proceeding to the headache. Some people, however, experience the aura but do not have a headache. This visual aura can be very dramatic. Classically, a small blind spot appears in the central vision with a shimmering, zig-zag light inside of it. This enlarges, and moves to one side or the other of the vision, over a 20 to 30 minute period. When it is large, this crescent shaped blind spot containing this brightly flashing light can be difficult to ignore, and some people fear that they are having a stroke. In reality, it is generally a harmless phenomenon, except in people who subsequently get the headache of migraine. Since migraine originates in the brain, the visual effect typically involves the same side of vision in each eye, although it may seem more prominent in one eye or the other.
Some people get different variations of this phenomenon, with the central vision being involved, or with the visual effect similar to "heat rising off of a car". Some people describe a "kaleidoscope" effect, with pieces of the vision being missing. All of these variations are consistent with ophthalmic migraine.
Corneal edema 
Seeing rainbows around lights, especially at night, usually indicates swelling of the cornea. This may occur from a variety of causes which are discussed under Corneal Edema. Cataract can sometimes cause this also.
Colour vision is perceived mainly by the macula, which is the central vision portion of the retina. Thus any disorder affecting the macula may cause a disturbance in color vision. However, about 8% of males and 0.5% of females have some version of "colour blindness" from birth. Usually this is a genetically inherited trait, and is of the "red-green confusion" variety. The reds, browns, olives, and gold may be confused. Purple may be confused with blue, and pastel pinks, oranges, yellows, and greens look similar. Usually both eyes are affected equally.
There are many obscure macular retinal disorders that can lead to a loss of colour vision, and many of these syndromes are inherited as well. There may also be a problem with a generalized loss of vision with these problems as well. Other retinal problems can lead to a temporary disturbance of colour vision, such as Central serous chorioretinopathy, Macular Edema of different causes, and Macular Degeneration.
Certain types of cataract can gradually affect the colour vision, but this is usually not noticed until one cataract is removed. The cataract seems to filter out the colour blue, and everything seems more blue after cataract extraction. Optic nerve disorders such as Optic Neuritis can greatly affect colour vision, with colours seeming washed out during or after an episode.
Distortion of vision refers to straight lines not appearing straight, but instead bent, crooked, or wavy. Usually this is caused by distortion of the retina itself. This distortion can herald a loss of vision in macular degeneration, so anyone with distorted vision should seek medical attention by an ophthalmologist promptly. Other conditions leading to swelling of the retina can cause this distortion, such as macular edema and central serous chorioretinopathy.
An Amsler grid can be supplied by an ophthalmologist so that the vision can be monitored for distortion in people who may be predisposed to this problem.
Tunnel vision implies that the peripheral vision, or side vision, is lost, while the central vision remains. Thus, the vision is like looking through a tunnel, or through a paper towel roll. Some disorders that can cause this include:
Glaucoma - severe glaucoma can result in loss of nearly all of the peripheral vision, with a small island of central vision remaining. Sometimes even this island of vision can be lost as well.
Retinitis pigmentosa - This is usually a hereditary disorder which can be part of numerous syndromes. It is more common in males. The peripheral retina develops pigmentary deposits, and the peripheral vision gradually becomes worse and worse. The central vision can be affected eventually as well. People with this problem may have trouble getting around in the dark. Cataract can be a complication as well. There is no known treatment for this disorder, and supplements of Vitamin A have not been proven to help.
Stroke - a stroke involving both sides of the visual part of the brain may wipe out nearly all of the peripheral vision. Fortunately, this is a very rare occurrence
- Anthony Pane, Peter Simcock, Practical ophthalmology: a survival guide for doctors and optometrists