The "minus lens method" is a technique used to measure the amplitude of accommodation, which refers to the ability of the eye to focus on objects at different distances. This method involves introducing a concave or "minus" lens in front of the eye and observing the resulting changes in the eye's focusing ability.
Here's how the minus lens method is typically performed:
The individual being tested is usually asked to sit or stand at a specific distance from a visual target, such as a letter chart.
Initially, the individual's eyes are relaxed and focused on a distant object, typically located at optical infinity (a point at a very large distance).
A concave or minus lens is then placed in front of one eye. The power of the lens is gradually increased until the target letters on the chart become slightly blurry.
At this point, the individual is asked to try and bring the blurred letters back into focus by exerting their focusing ability. This involves increasing the eye's optical power to compensate for the negative power of the lens.
The lens power required to bring the letters back into focus is noted. This power corresponds to the additional accommodation exerted by the eye to overcome the optical effect of the minus lens.
By measuring the lens power needed to compensate for the blur induced by the minus lens, the amplitude of accommodation can be estimated. The greater the lens power needed, the stronger the focusing ability or accommodation of the eye.
It's worth noting that the minus lens method provides an estimation of the amplitude of accommodation, but it is not the only method available. Other techniques, such as dynamic retinoscopy or optometers, can also be used to measure accommodation accurately. Consulting an optometrist or ophthalmologist is recommended for a comprehensive assessment of the eye's accommodative function.