Defocus aberration

Nearly all image-forming optical devices incorporate some form of focus adjustment to minimize defocus and maximize image quality.

The degree of image blurring for a given amount of focus shift depends inversely on the lens f-number.

The penalty for achieving this extreme depth of focus is very dim illumination at the imaging film or sensor, limited resolution due to diffraction, and very long exposure time, which introduces the potential for image degradation due to motion blur.

However, in a clinical setting blurry vision means the subjective experience or perception of optical defocus within the eye, called refractive error.

The following are some examples of blurred images that may result from refractive errors:The extent of blurry vision can be assessed by measuring visual acuity with an eye chart.

These corrections sometimes have unwanted effects including magnification or reduction, distortion, color fringes, and altered depth perception.

This allows the optometrist or ophthalmologist ("eye doctor") to determine the extent refractive errors play in limiting the quality of the patient's vision.

Optical defocus can result from incorrect corrective lenses or insufficient accommodation, as, e.g., in presbyopia from the aging eye.

(d = diameter in degrees visual angle, p = pupil size in mm, D = defocus in diopters).