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Vladimir Lemberg; Focusing Precision under Different Magnification Settings in Operating Microscope with Coherent Image Coupling. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5851.
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It is important to achieve precise focusing on the object (corneal surface) during refractive surgical procedure. The focusing precision is determined by the ability of the surgeon to detect the error in focus. In the operating microscope the object is coherently coupled to the image, therefore, if the object is defocused the image can be refocused. The purpose of this work is to theoretically investigate the ability of the human eye to detect the focus error through the microscope and examine the depth-of-field as a function of magnification settings and the instrument exit pupils.
Rotating a microscope magnification changer drum and changing the lenses in the drum inserts the internal apertures into the optical path. The apertures have different diameters for the different magnification settings. The smallest internal apertures introduced by Galilean changer are the aperture stops and their images are the microscope exit pupils. On the highest magnification setting the aperture stop forms the smallest system exit pupil and the view is likely vignetted. Since the eyepiece lenses and tube lens forms an afocal (telescope like) system, one can calculate the exit pupil diameter (entrance pupil for the eye) for the different Galilean magnification changer settings. The entrance pupils for the eye as a function of the magnification settings are examined and changes in depth-of-field are inspected closely.
The performed calculations show a reduction of the eye sensitivity for the ocular adjustments on the highest microscope magnification settings. The analysis using the defocus blur disc model reveals a reduction of the differences in the depth-of-field due to the unequal internal apertures in an optical path for the various microscope Galilean magnification changer settings.
Dioptric ocular adjustment for the ametropic eye has to be performed at the smallest magnification settings where the microscope exit pupil has maximum diameter and the eye depth-of-field is minimal. The different diameter internal apertures in an optical path at various microscope magnification changer settings reduce the differences in the microscope depth-of-field.
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