Best retinal focus for various fication distances in phakic and aphakic eyes

When an eye focuses on a distant target, it is often found that the long wavelength rays are focused on the receptors, but when the same eye looks at a target close to the eye, the green or blue rays are in focus in the retinal receptors. The authors investigated this phenomenon by means of a laser optometer to find out whether the shift in focus represented an attempt to reduce accommodative effort, or was used to aid distance judgement, or was simply a by-product of the control mechanisms. Six phakic individuals showed a shift of 50 nm over an accommodation range of 2.75 D. The single aphakic observer preferred a retinal focus within a very narrow wavelength range. The authors favor the view that the wavelength shift is an inability of the accommodative control mechanism to make a precise response to a given stimulus.