Definitions: The common acuity notation is called Snellen Relative Acuity. For 20/20 (6/6), it assumed the ability to read a black letter which fits into a block 8.73 mm high by 4.38mm wide on a white background at a distance of 20 feet (6m) . < P> The mathamatical way of looking at it: the numerator (top number) is the testing distance and the denominator is the distance at which the letter read by the viewer subtends 5 minutes of arc.
A practical view: For example 20/200: what should be seen at 200 feet away requires the viewer to be at 20 feet.
None of this has a direct, uniform relationship to the diopter power of the lens correction which provides the viewer with 20/20 vision. There's much more to vision than 20/20 !
There are other factors which influence visual acuity. Contrast, background, brightness, color, pupil size as well as the cortical brain processing of information all play important roles in the overall presentation of the quality of our visual world. Fo r example, the higher the contrast between the object and its background, the easier it is to see. The brighter the illumination, the clearer the image, up to the point at which the light overwhelms the image with glare and then the image is seen less cl early. Brightness works both ways, to enhance and to limit acuity.
The same thing occurs with colored images: the greater the saturation or brightness of the color relative to its background, the easier it is to see. The closer an objects color is to the hue of its background, the more difficult it is to perceive. We can use these factors to design pictures which fool the brain as to what's really there---optical illusions. (check out Viewable Graphics of Illusions when they become available.)
Pupil size also effects our visual acuity. Small pupils, less then 2.5 mm, create a depth of focus (a range of clear vision) larger than what might be expected for a given optical error. If you have a vision correction, try looking through a pin-hole you cut into a piece of cardboard without your glasses. The image will be strikingly clear! This is what you create when you squint your eyelids down to a small slit. The first cameras used the principal of pin-hole focusing to place a focused image on the f ilm. Today, photographers can vary the f-stop, or lens aperture setting, just as our pupil varies the aperture to the inside of our eyes. The optical effect is the same: alteration of the depth of focus.
Finally, the brain has to interpret the information sent to it by our retinal photoreceptors. Some people are better at processing this information than others and "see" better with the same degree of correction as would someone else.
As you see, there's more to visual acuity than just 20/20 or 6/6 !