Instrument myopia

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Instrument

microscopes.[1][2] For example, through a microscope, a person might focus the eyes to one meter distance although it can present an image at six meters distance.[1]

Characteristics

Ordinarily, when someone looks at an object at, say, one meter from the

crystalline lens of the eye. Although convergence and accommodation are separate processes, they normally operate synergistically.[4]

When someone looks into an optical instrument, such as a microscope, vision is far from ordinary. A microscope might force the person to use only one eye, it presents the person with a limited field of view, it presents a magnified view, and it allows the person to adjust the focus of the instrument for any viewing distance. Ideally, the person will choose a focus adjustment on the instrument that allows the eyes to have relaxed accommodation—that is to present the image(s) at a distance of about 6 meters, or optical infinity. However, most people tend to accommodate to nearer than 6 m.[1] It is this too-close focussing of the eyes that is instrument myopia.

Accommodating to a distance nearer than 6 meters makes the ciliary muscles work harder than they need to, leading to fatigue.[5] Some binocular microscopes are designed so that the vergence of the eyes is correct for a 6-meter viewing distance. Accommodating to a closer distance will tend to converge the eyes, leading to double vision.

According to Wesner and Miller (1986),[5] instrument myopia is promoted when the viewing is with one eye, when the field of view is small, and when the luminance is low, concluding that these are consistent with accommodation's going towards a person's dark focus, which is about one meter from the eyes.[6] Wesner and Miller also said it is possible that a person's knowledge that the viewed object is very close (on the microscope stage) contributes to instrument myopia. They said that instrument myopia is minimised by using a binocular microscope that forces the person's vergence angle to be small.[5]

History

Richards attributed early research into instrument myopia to H. Imbert in 1899.[1] [7] Early mention of the term occurred in 1970,[8] or earlier.

References

  1. ^
    S2CID 37513722
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  2. PMID 1185295
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  3. ^ Duke-Elder, S. (1973). System of ophthalmology London: Henry Kimpton.
  4. PMID 13463783.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  5. ^
    S2CID 20659472.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  6. PMID 1185296.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  7. ^ Imbert, Henri (1899). De l'état de l'accommodation de l'oeil pendant les observations au microscope [Of the accommodation state of the eye during observations through a microscope]. Paris: Librairie J.-B. Baillière et Fils.
  8. doi:10.1364/josa.60.000103
    .
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