Therefore older people often have problems operating controls when there is low illumination. If a person has limited dexterity as well then the problems become more difficult. Older people adapt at a slower rate to changes in the ambient illumination, which can give problems when the illumination on a control panel is greatly different to what is being viewed or controlled. Good vision can be obtained by correction with concave lenses in spectacles or contact lenses. Hypermetropia, or long-sightedness, is when the image is focussed behind the retina.
Convex lenses give good vision.
Astigmatism literally means without point focus. It may be either myopic, hypermetropic or 'mixed' - in every case the image is elongated or distorted. It is corrected with cylindrical lenses. The term 'accommodation' is used to indicate the ability of the eye to change its focus, and thus get a clear image on the retina. Presbyopia commonly occurs after the age of 40, and bifocal or varifocal spectacles are often required.