- Near vision: ability to read a book/newspaper or thread a needle.
- Intermediate-range vision: ability to read a computer monitor or price on a supermarket shelf.
- Distance vision: ability to drive confidently and see street signs, watch a movie in a theatre or watch live sport.
The focus of the lens of the eyes is controlled by a muscle, which contracts to bring objects held close into focus. At the clinic, vision is assessed in two scenarios:
- With the eye muscle relaxed, distance vision is assessed.
- With the eye muscle contracted, the ability of the eye to focus close-up is assessed.
A patient’s vision can be described as their ability to see at different distances with the eye muscle relaxed.
Refraction is an important term and relates to distance vision. It refers to the type of lenses that need to be held in front of the eye to achieve the best distance vision.
A patient’s refraction has implications for their functional or day-to-day vision, as well as their ability to see clearly without corrective lenses at the three distances described above. There are obviously other factors that will influence a patient’s ability to see. The most of important of these is the degree of illumination of the subject. The better illuminated an object, the more likely it will be seen without corrective lenses.
Near vision is also measured at the pre-operative assessment. It is achieved by accommodation: eye-muscle contraction resulting in a change in shape of the lens that brings objects held close to the face into focus. In young patients accommodation works well and enables good reading vision without glasses. With time, accommodation deteriorates and good near vision requires reading or magnifying glasses: this is termed presbyopia. This process typically starts in the early 40s, progresses relatively quickly around the age of 50 and necessitates an increased dependence on reading glasses beyond this. Presbyopia, loss of close focus, is often erroneously referred to as long-sightedness.