Colored contact lenses that can make brown eyes vampire red or feline yellow to perfect a Halloween costume may also make it harder to see during a night out on the town, a Korean study suggests.

Researchers did vision exams on 30 people wearing clear lenses and then repeated the tests with colored versions that left varying amounts of untinted open space around the pupil, a region known as the optical zone.

The smaller the clear optical zone got on the colored lenses, the harder it was for people to see well, the study found. Objects at a distance got a bit blurrier, and participants also lost some ability detect contrast needed to spot dark things at night or pale items against a light background.

"For the most part, if you are going to wear colored contacts for a few hours on Halloween, the likelihood of a problem is very small," said Dr. Oliver Schein, an ophthalmology researcher at the Johns Hopkins Wilmer Eye Institute in Baltimore who wasn't involved in the study.

The trouble comes when people buy this costume accessory without seeing an eye care professional, said Dr. Edward Manche, director of cornea and refractive surgery at Stanford Health Care in California.

More On This...

"Many if not most of the people wearing the lenses obtain them over the counter, online or borrow them from friends," Manche, who wasn't involved in the study, said by email. "This often leads to poorly fitting lenses as well as use by people with no instruction on the care and wearing of lenses."

In the study by Tae-im Kim of Yonsei University College of Medicine in South Korea, and colleagues, participants tried out lenses with three different clear optical zone diameters - 4, 5, and 6 millimeters.

They found just a millimeter could mean the difference between normal vision and the development of slight changes in sight.

While the study, published in the British Journal of Ophthalmology, is small and additional research is needed to more firmly establish the connection between the clear optical zone diameter and any vision problems, the researchers recommend a minimum pigment-free zone of 6 millimeters.

The study found only a small change in vision even with a 4-millimeter optical zone, noted Dr. John Clements, a researcher at the Casey Eye Institute at Oregon Health and Science University in Portland.

"If prescribed by an eye care professional and used as directed, most of these lenses are relatively harmless," Clements, who wasn't involved in the study, said by email. "The most devastating damage occurs when someone gets these lenses over-the-counter for cosmetic use and abuses the lens. This behavior can lead to severe eye infections that may cause blindness."

The trouble, for example, with a vampire red lens for Halloween, is that contacts bought at the costume shop may carry an increased risk of infections or scratches on the eye surface because they may not fit properly or be made with safe materials, said Dr. Thomas Steinemann, an eye disease researcher at Case Western Reserve University and spokesman for the American Academy of Ophthalmology.

"People who wear costume contacts or colored contacts that they find at the dollar store or the flea market usually end up in my office needing treatment of some kind," Steinemann, who wasn't involved in the study, said by email. "I see patients all the time who come in who may wear colored lenses and they talk about how they're not comfortable and they can't always see well."