December 2013
Volume 13, Issue 15
OSA Fall Vision Meeting Abstract  |   October 2013
Precompensating for defocus by spatial filtering
Author Notes
  • Footnotes
     Moderator: Heidi Hofer, University of Houston
  • Footnotes
     Blur has long been regarded as a necessary evil, something that we are stuck with as a result of the laws of optics. But with regards to blur, the visual system again shows its facility to turn lemons into lemonade by adapting away some of the perceptual effects of blur and exploiting blur as a cue to depth. This symposium examines the visual mechanisms involved in the detection and discrimination of defocus and higher-order aberrations, along with the subsequent processing of these signals.
Journal of Vision October 2013, Vol.13, T4. doi:
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      Jack Yellott; Precompensating for defocus by spatial filtering. Journal of Vision 2013;13(15):T4. doi:

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      © ARVO (1962-2015); The Authors (2016-present)

  • Supplements

Beyond magnification, what can be done typographically to mitigate defocus for presbyopes forced to read without glasses? Ideally, topographical precompensation would correct defocus for a given object O(x,y) (e.g., a printed letter) by transforming it into another object O'(x,y) which yields O(x,y) when convolved with the pointspread function P(x,y) of the defocused eye: P*O' = O. (O, P, and O' are nonnegative real functions representing light intensity.) This can always be done for P itself, regarded as an object, by replacing it with the point object delta(x,y), since P*delta = P. In general an object O can only be completely precompensated for defocus in this way if its Fourier transform Fo contains the transform of P as a factor: Fo = FpFo', where Fo' is the transform of another object. This constraint (together with nonnegativity) implies that an object's spectral contrast |Fo(u,v)/Fo(0,0)| can only be precompensated if it does not exceed the eye's modulation transfer function (i.e., only if |Fo(u,v)/Fo(0,0)| <= |Fp(u,v)/Fp(0,0)| ). For levels of defocus routinely created by presbyopia this means only objects that are already severely blurred can be precompensated for the contrast-reduction component of defocus—ordinary printed letters need not apply! However the phase component of defocus—phase reversal (“spurious resolution”)—can always be precompensated by selectively pre-reversing portions of the object's phase spectrum. This can dramatically improve legibility by correcting retinal image shape. The talk will demonstrate how well phase precompensation for defocus works for letter-like objects, and discuss theoretical and experimental questions posed by phase-prefiltering.


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