Binocular indirect ophthalmoscopy (BIO) uses a head-mounted light and binocular viewer with a hand-held condensing lens (usually +20D)
It gives a wide, stereoscopic, low-magnification view of the whole fundus to the ora serrata (with scleral indentation)
Image: real, inverted and laterally reversed, formed in the air between the lens and the examiner (an aerial image)

## Optics

The condensing lens is held about its focal length (5 cm for a 20D lens) in front of the patient's eye

Light from an emmetropic patient's retina leaves the eye parallel, so the lens forms the image at its own second focal point, 5 cm in front of it
The lens also images the examiner's pupils into the patient's pupil, so the whole lens fills with the fundus view

The examiner views the aerial image at about arm's length (40–50 cm). The headset eyepieces usually contain low plus lenses (about +2.00D) so the examiner need not accommodate
To draw the fundus as seen, draw the image and turn the chart upside down: the image is reversed top to bottom and side to side

## Magnification and field of view

Worked example: 60 ÷ 20 = ×3 with a 20D lens. A 28D lens gives 60 ÷ 28 ≈ ×2.1; a 14D lens gives about ×4.3

Axial magnification is the square of the lateral magnification (×9 with a 20D lens), so elevations look exaggerated
Field of view increases with the power and the diameter of the condensing lens: about 45° (roughly 8 disc diameters) with a 20D lens

Higher-power lenses: a wider field and less magnification, and they work better through small pupils
Lower-power lenses: more magnification and a longer working distance

Feature	Direct	Indirect (20D)
Image	Virtual, erect	Real, inverted, laterally reversed
Magnification	About ×15	About ×3
Field of view	About 5–10°	About 45°
Stereopsis	None	Yes
Periphery	Limited to the posterior pole and mid-periphery	To the ora serrata with indentation
Media opacities	View poor	Bright light penetrates better
Pupil	Works through a small pupil	Needs dilation
Patient's refraction	Changes magnification and field	Little effect

## Stereopsis

Both of the examiner's pupils must be imaged inside the patient's pupil
Mirrors in the headset reduce the examiner's effective interpupillary distance to a fraction of normal to achieve this
Stereopsis is therefore reduced, and it is reduced further by the small-pupil setting

## Technique

Dilate the pupil fully. Hold the lens with the correct face towards the patient (marked by a ring on the lens) to reduce reflections and aberrations
Tilt the lens slightly to move its reflections out of the view
The examiner stands opposite the area to be examined (e.g. at the patient's feet for the superior retina), with the patient looking towards it
Scleral indentation brings the far periphery and ora serrata into view, and shows whether a retinal break is flat or raised

BIO is the standard examination for retinal detachment, retinopathy of prematurity screening and peripheral retinal lesions
Slit-lamp biomicroscopy with a 90D or 78D lens uses the same indirect principle (see Slit Lamp and Fundus Lenses)
Laser can be delivered through the BIO headset, e.g. for retinopathy of prematurity
Wide-angle viewing systems for vitrectomy use the same principle and need an image inverter (see Operating Microscope)