Accommodation and convergence: effects of lenses and prisms in ‘closed‐loop’ conditions

Experiments are described in which the steady‐state accommodation response to binocularly viewed targets at various distances in the range 0.2–5 m was measured by laser optometry in normal, near‐emmetropic subjects. Responses were determined with naked‐eye viewing and when equal pairs of either spherical lenses or base‐in or base‐out prisms were placed before the eyes. Snellen acuity was also measured under the same range of conditions. The results show that, although the most accurate accommodation responses were found under natural viewing, the response accuracy was remarkably robust against both lenses and prisms. There was, however, always some change in response level to any increment in lens or prism power. The results are discussed in terms of earlier theories and data on the general relationship between accommodation and convergence.