One of the more frequent visual attributes with which we are concerned is how well we can see. Generally what one means by this generic, rather broad idea, is whether of not the letters on a page of reading material, or whatever we are looking at are sharp and clear. In other words do we have good visual acuity? Or as the scientist might say, do we have acceptable spatial resolution?
If you looked at the demonstrations associated with the main diagram then you will already have been introduced to some of the factors that govern visual acuity. It will not hurt to repeat some of these and to expand on these ideas.
In the following we present a series of displays that help to explain some facets of how we see sharp, in focus images. It is probably a good idea to select the following demonstrations in the order in which they are listed below.
Most of us have had our visual acuity checked. The technical term that optometrists and ophthalmologists use is to refract the eyes. There are quite a few different kinds of tests that evaluate our ability to resolve spatial detail. If our visual acuity is good we are able to resolve very fine or small detail. If our visual acuity is not so good then we encounter difficulty. Frequently, this difficulty can be resolve by wearing spectacles.
Do you know what 20/20 or 6/6 means? If not, read on. Simply put, 20/20 means that you are able to see on an eye chart at 20 feet that which a person with normal visual acuity can see. If you require glasses because your optometrist says you have 20/60 vision, that means you are able to discriminate characters on an eye chart at 20 feet that a person with normal acuity can see at a distance of 60 feet. 6/6 means the same thing only in meters.
Above reference is made to normal visual acuity. What does that mean? The standard definition of normal visual acuity is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc.
Consider a circle which contains 360 degrees. One degree contains 60 minutes. Therefore, a visual angle of one minute of arc is 1/60 of a degree. The visual angle subtended by a spatial patterned is easily measured
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