"There's far more to contact lenses than meets the eye," as ophthalmologist Spencer E. Sherman and writer Nancy P. Bruning say in their new Consumer's Guide to Contact Lenses.

"Contact lens technology and manufacture are booming . . . Unfortunately, with the widespread availability of contact lenses has come a great deal of confusion and misinformation.

"Of the annual 1.5 million new contact-lens wearers, an astounding 30 to 40 percent drop out during the first 12 to 24 months . . . This doesn't have to be. A well-informed consumer has the best chance of wearing contact lenses successfully."

The American Optometric Association outlines these advantages and disadvantages of each kind:

Standard Hard Lenses

Initial Cost (including eye exam and follow-up care): $175 to $275.

Description: The "granddaddy" of contacts, available since the 1950s, made of a clear, tough, inflexible acrylic plastic.

Advantages: The most versatile lenses, correcting most vision problems including high degrees of astigmatism. They have the longest lifespan of all contacts, lasting some wearers 10 years or more. They are the only contacts that may be modified when a prescription change is needed, eliminating the expense of new lenses. Available in bifocals and tints.

Disadvantages: Require a longer adaptation period than other lenses, ranging from 11 days to four weeks. They must be worn fairly consistently each day to maintain adaptation, can slip or fall off the eye and may cause a temporary "spectacle blur," making it difficult to see out of glasses after removing the lenses.

Oxygen-Permeable Hard Lenses

Initial Cost: $225 to $325.

Description: Introduced in the late 1970s; made of several types of slightly flexible plastics that allow oxygen to pass through them.

Advantages: All the advantages of standard hard lenses, plus they require a shorter adaptation period and reduce or eliminate spectacle blur.

Disadvantages: The same as for hard lenses; do not ordinarily come in tints.

Daily-Wear Soft Lenses

Initial Cost: $225 to $325

Description: First approved in 1971, and now many different versions made of various flexible water-absorbing plastics.

Advantages: Usually more comfortable at first than hard lenses; short adaptation period of about a week. Can be worn on and off without requiring readaptation, eliminate "spectacle blur" and are difficult to dislodge. In some cases, available in bifocals and tints.

Disadvantages: Not all vision problems can be corrected; require more daily care than hard lenses and are less durable -- requiring replacement every year or two.

Extended-Wear Soft Lenses

Initial Cost: $350 to $450.

Advantages: In addition to all the advantages of daily-wear soft lenses, can be worn continuously for periods of up to two weeks or longer in special circumstances. In some cases, more frequent removal and cleaning is recommended.

Disadvantages: Do not correct all vision problems; require relatively frequent follow-up care, not available in tints or bifocals, the least durable of all contacts and a short (roughly one year) life span.