Smoking more than 20 cigarettes a day may damage your vision by affecting blood vessels and neurons in the retina, a study warns.

The study, published in the journal Psychiatry Research, included 71 healthy people who smoked fewer than 15 cigarettes in their lives and 63 who smoked more than 20 cigarettes a day, were diagnosed with tobacco addiction and reported no attempts to stop.

The participants were between the ages of 25 and 45 and had normal or corrected-to-normal vision as measured by standard visual acuity charts, said researchers from the Rutgers University in the US.

They looked at how participants discriminated contrast levels (subtle differences in shading) and colours while seated 59 inches from a 19-inch cathode-ray tube monitor that displayed stimuli while researchers monitored both eyes simultaneously.

They also found that the heavy smokers had a reduced ability to discriminate contrasts and colours when compared to the non-smokers.

Previous studies have pointed that long-term smoking doubles the risk for age-related macular degeneration and causes lens yellowing and inflammation.

"Our results indicate that excessive use of cigarettes, or chronic exposure to their compounds, affects visual discrimination, supporting the existence of overall deficits in visual processing with tobacco addiction," Silverstein said.

Although the research did not give a physiological explanation for the results, Silverstein said that since nicotine and smoking harm the vascular system, the study suggests they also damage blood vessels and neurons in the retina.

He said the findings also suggest that research into visual processing impairments in other groups of people, such as those with schizophrenia who often smoke heavily, should take into account their smoking rate or independently examine smokers versus non-smokers.