The operating microscope is a binocular compound microscope with a long working distance, coaxial illumination and variable magnification
Like the slit lamp, it uses erecting prisms so that the surgeon sees an erect, correctly orientated image (see Slit Lamp and Fundus Lenses)

## Optics

In a simple compound microscope, the objective forms a real, inverted image and the eyepiece acts as a loupe on it. The final image is virtual and inverted, so Porro prisms (or similar) are added to erect it
Modern microscopes are infinity-corrected

The surgical field sits at the focal point of the objective lens, so light leaves it parallel
The working distance roughly equals the objective's focal length, usually 150–200 mm (e.g. f = 200 mm for anterior segment surgery)
The parallel beam passes through a magnification changer: a set of Galilean telescopes on a drum (stepped) or a motorised zoom
The tube lens in the binocular head forms an image, which the eyepieces (e.g. ×10 or ×12.5) magnify

Parallel light between the objective and the tube allows beam splitters to be added for an assistant's scope, video and camera without changing the focus

## Magnification and field

Total magnification = (tube lens focal length ÷ objective focal length) × magnification changer × eyepiece

Worked example: a 160 mm tube, a 200 mm objective, the changer at ×1 and ×12.5 eyepieces: (160 ÷ 200) × 1 × 12.5 = ×10
Changing to a 175 mm objective gives (160 ÷ 175) × 12.5 = ×11.4, with a shorter working distance

A longer focal length objective gives a longer working distance but less magnification
As magnification increases, the field of view and the depth of field both fall, and the image becomes dimmer. Closing the aperture increases the depth of field
Stereopsis comes from two separate optical paths that view the field from slightly different angles

## Setting up

Set the interpupillary distance and each eyepiece's dioptre setting for the surgeon's refraction
Make it parfocal: zoom to the highest magnification and focus, then zoom out and adjust the eyepieces until the image stays sharp. The image then stays in focus at every magnification
Foot pedal controls: focus, zoom, X–Y movement and the light

## Illumination

Light (halogen, xenon or LED, often through a fibre-optic cable) is directed through the objective
Coaxial illumination: light travels along, or very close to, the viewing axis

Light reflected from the fundus returns through the pupil, giving a bright, even red reflex (retroillumination)
The red reflex is essential in cataract surgery for the capsulorhexis, removing cortex and seeing the posterior capsule
Oblique illumination instead gives better contrast for surface detail but no red reflex

Phototoxicity: intense microscope light, especially blue and near-UV, can cause a photochemical retinal burn. It looks like an oval lesion near, but usually not at, the fovea, corresponding to the light's position
The risk is higher with long operations, high intensity and coaxial light focused on the retina
Reduce it: the lowest intensity needed, blue- and UV-blocking filters, tilting the microscope or the eye slightly, covering the cornea or pupil when the view is not needed, and keeping operations short

## Wide-angle viewing in vitreoretinal surgery

Contact or non-contact wide-angle lenses use the indirect ophthalmoscopy principle (see Indirect Ophthalmoscope), giving a large field of view
The image is real and inverted, so an image inverter (e.g. a stereoscopic diagonal inverter) in the microscope turns it the right way up again
Endoillumination through a light pipe provides the light, rather than the microscope

Surgical loupes are Galilean telescopes (about ×2 to ×3.5) for lid and oculoplastic surgery (see Telescopes and Loupes)
Digital three-dimensional "heads-up" systems display the microscope image on a screen viewed with polarised glasses. They allow lower light levels and better posture
Intraoperative OCT can be built into the microscope (see Optical Coherence Tomography)