magnification and components of the most commonly used optical systems are defined below:
Figure 3.1 Optics of magnification aids that use a two‐lens system. Note the focal length and working distance.
Source: Pieptu and Luchian (2003) and Cordero (2014).
Stereopsis: Perception of depth and three‐dimensional structure obtained by processing visual information delivered through the eyepieces to the surgeon's eyes. Stereopsis is achieved using an operating microscope or surgical loupe by manipulation of the eyepieces to accommodate the surgeon's interpupillary distance and visual deficits, also termed stereoscopy (Carr and Castellucci 2010; Socea et al. 2015).
Binocular head: Body of an operating microscope. Contains the eyepieces, lenses and/or prisms, and magnification changer (Figure 3.2). The binocular head may be straight, inclined, or inclinable (Carr and Castellucci 2010). Inclinable binoculars are adjustable through a wide range of angles to allow the surgeon comfortable head and neck posture and working position.
Eyepieces: Eyepieces contain the binocular lenses and/or prisms necessary for magnification. Eyepieces of operating microscopes are available in powers of 10× and 12.5× most commonly and contribute to total magnification achieved by the optical aid (Carr and Castellucci 2010). Eyepieces have rubber cups adjustable in height to accommodate surgeons wearing corrective eyeglasses and have diopter settings adjustable for each surgeon's vision deficits (Figure 3.3).
Figure 3.2 Binocular head–body of the operating microscope. Contains the eyepieces, lenses and prisms, and magnification changer.
Figure 3.3 Eyepieces contribute to the total magnification of an operating microscope. Note the rubber cups and diopter settings adjustable to an individual surgeon.
Focal length: A measure of how strongly a lens converges or diverges light. The distance from the center of the lens to the area on the lens where light rays originating from a point on the focused object converge (Pieptu and Luchian 2003; Cordero 2014) (Figure 3.1).
Interpupillary distance: The distance between the centers of the surgeon's pupils. Eyepieces of an operating microscope or surgical loupe are adjustable or customized to accommodate the interpupillary distance of the individual surgeon (Carr and Castellucci 2010; Mungadi 2010) (Figure 3.4).
Focal depth (depth of focus): Range of object position through which the object may be viewed at a set magnification level and remain in focus (Pieptu and Luchian 2003).
Working distance: The distance from the objective lens to the object. Working distance is dependent on the focal length of the lenses and ranges from 22 to 50 cm (Pieptu and Luchian 2003; Carr and Castellucci 2010; Cordero 2014) (Figure 3.1).
Figure 3.4 Eyepieces of an operating microscope are adjustable to accommodate the interpupillary distance of the individual surgeon.
Field of view: Extent of the operating field seen in focus through the optical system (Figure 3.5). Field of view changes with magnification level according to the formula: field of view diameter = 200/total magnification factor (Pieptu and Luchian 2003). The diameter of the field of view is inversely proportional to the level of magnification; the higher the magnification, the smaller the field of view (Carr and Castellucci 2010). In exotic animal surgery, it is possible at times to fit the entire patient into the field of view (Bennett 2000a).
Magnification changer of operating microscopes: A system of lenses between the objective and eyepiece lenses that allows for changing magnification manually by 3–6 steps at a time or for continuous adjustment of magnification (power zoom) (Carr and Castellucci 2010; Cordero 2014).
Magnification changer of surgical loupes: Interchangeable working distance optics available with some loupes can increase magnification power for special procedures (Chang 2015).
The Operating Microscope
The surgical microscope is considered by many surgeons to be the gold standard of operative optical aids, a mandatory instrument for performing many technically demanding procedures and an indispensible teaching tool (McManamny 1983; Pieptu and Luchian 2003; Al‐Benna 2011). A standard operating microscope is capable of magnifying an image from 6 to 40×, enabling many procedures that would otherwise be impossible to perform (Mungadi 2010; Stanbury and Elfar 2011). With the size amplification afforded by the microscope, visualization of tissue anatomy, pathology, color, and character is enhanced; suture and instrument placement is more precise; and small, bleeding vessels and other abnormalities are more readily appreciated (Bennett 2000b; Mungadi 2010). Although the degree of precise manipulation and dexterity possible with magnification is far superior to that achieved with the unaided eye, performing accurate surgical maneuvers with extensive magnification requires practice (Carr and Castellucci 2010; Eivazi et al. 2015). Operating microscopes are expensive and can be cumbersome requiring intimate familiarity with their precision parts to permit proper and efficient use during surgery (McManamny 1983; Mungadi 2010).
Figure 3.5 Field of view is the extent of the operating field seen in focus through the optical system. Here, the field of view is contained within the lighted circle as seen external to the microscope.
The optical system of an operating microscope consists of a binocular head containing the eyepieces, lenses, prisms, and magnification changer; the objective lens; an illumination source; a suspension system; and a foot pedal or handpiece for controlling magnification, focus, and x–y position of the objective lens (Carr and Castellucci 2010; Cordero 2014). During surgery, a skilled surgeon must adjust the level of magnification frequently as tissues are manipulated and sutures are passed and tied (Eivazi et al. 2015). For this reason, the author prefers a foot pedal to a handpiece for manipulation of magnification and x–y settings (
