Abstract
Optical blur from defocus is quite frequently considered as equivalent to low-pass filtering. Yet that belief, although not entirely wrong, is inaccurate. Here, we wish to disentangle the concepts of dioptric blur, caused by myopia or mis-accommodation, from blur due to low-pass filtering when convolving with a Gaussian kernel. Perhaps surprisingly—if well known in optometry—the representation of a blur kernel (or point-spread function) for dioptric blur is, to a good approximation and disregarding diffraction, simply a cylinder. Its projection onto the retina is classically referred to as a blur circle, the diameter of which can easily be deduced from a light-ray model. We further give the derivation of the relationship between the blur-disk’s diameter and the extent of blur in diopters, as well as the diameter’s relation to the near or far point, and finally its relationship to visual acuity.
Highlights
A while ago we were wondering whether the low signal amplitudes in fMRI retinotopic mapping and visual evoked potential (VEP) recording from a psychiatric patient could be due to a severe lack of correct optical accommodation
We explore the relationship of defocus to visual acuity based on an empirical approach by Blendowske (2015)
As Artal (2014) points out, in eyes with normal optics, the amount of higher order aberrations and visual acuity are not related; it may even be that the normal pattern of aberrations provides the best performance (Artal et al, 2004)
Summary
A while ago we were wondering whether the low signal amplitudes in fMRI retinotopic mapping and visual evoked potential (VEP) recording from a psychiatric patient could be due to a severe lack of correct optical accommodation. P 1⁄4 Effective pupil diameter; f 1⁄4 focal length of the optical system (cornea and lens) within the eye; df 1⁄4 focal length error; b 1⁄4 blur-disk diameter; P 1⁄4 principal point; N 1⁄4 nodal point.
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