A Fun Fact or Two About Depth-of-Field


“You don’t have to make it perfect, Doc.  You just have to make it better.”  This lesson was taught to me many years ago by a wise old patient named Krauss (HIPPA be damned).  I was reminded of this lesson yesterday, searching for the iPad under my car seat, and coming across a copy of the October 2014 issue of Optometry & Vision Science on Wavefront Refraction and Correction inadvertently hidden.

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An article that caught my eye was entitled Depth-of-Field of the Accommodating Eye, and it’s abstract concludes with the following statement:  “The visual system takes advantage of the DOFi to change the refractive state less than necessary to form the paraxial image at the retina when it comes to focusing a near target (5 to 6 D of AD). This indicates that the main purpose of accommodation is not to maximize retinal image quality but to form one that is good enough.”  There it was — nature having a built-in rule that my wise patient understood.  Our visual systems are not typically in search of perfection.  Here’s the powerful conclusion of the article:

“Our data support the hypothesis that the difference between the ideal and real accommodation response is mainly attributed to the presence of DOFi.  That is, when accommodating to near objets, the visual system takes advantage of the full DOFi to change the refractive state of the eye by about the minimum, even if there is enough potential accommodation to improve the retinal image quality.”

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The brain exhibits a certain tolerance to out-of-focus images, a feature known as depth-of-focus (DOF).  The corresponding distance range in which the objects are seen clearly – or at least as we’ve learned, clearly enough to identify – is referred to as depth-of-field (DOFi).  A variety of optical and neural factors influence the DOFi, one of the most important being pupil size.  The eye is not a camera, but they do share certain principles in common – and camera apertures or F stops are pretty good correlates to pupil size when it comes to DOFi.  Here’s a graphic from a wonderful item regarding “acceptable clarity” by Jennifer Valencia.

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Think about this the next time you prescribe lenses, or use lenses in therapy.  The brain relies on purposeful aberrations and apparent errors to provide a certain amount of flexibility in visual system responses.  Regarding accommodation, the goal is to attain clarity good enough to accurately identify the target within the region of space selected for attention.  Remember, the goal in image quality is good enough – not necessarily perfection. This provides a nice framework for the term “Identification”, introduced to us by another wise old man named Skeffington.

 

3 thoughts on “A Fun Fact or Two About Depth-of-Field

  1. Thanks Len, I feel much better reading your blog today. Helps me to get by the 86 year old patient last year who was not satisfied with his new lenses. He felt the image quality was not perfect, even with the blue tech lenses I prescribed.
    You never know what is good enough for one person vs the other person. I do remember reading somewhere a study that was done on 80 year old people where they placed them with 30 year old people for a time and the 80 year old participants become in their minds 30 years old. I wonder if their sight improved. It all boils down to if you believe it and say it enough it might come true.
    August

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