18.2 Standard Fundus Photography: ETDRS Fields, Stereo Pairs, Red-Free & Wide-Field Imaging

Field definitions

A conventional fundus camera images a 30-degree or 45-degree field, positioned by moving the patient's fixation, not the camera.

The ETDRS seven standard fields — derived from the Early Treatment Diabetic Retinopathy Study and still the reference protocol for diabetic retinopathy grading — are 30-degree fields:

Fields 4 to 7 are positioned so their edges are tangent to the disc and macula fields, tiling outward from the posterior pole.

Minimum practical protocols: many clinics use a two-field protocol (disc-centred and macula-centred) for routine documentation, and a four-field or seven-field protocol for diabetic grading. The protocol used must be recorded, because a lesion missing from a two-field set may simply have been outside the photographed area.

Fixation technique. Use the internal fixation target where the camera provides one, moving it to place the desired field. When the required field lies beyond the internal target's range, use an external fixation target — a small light the patient fixates with the fellow eye, positioned to rotate the photographed eye into position. External fixation is essential for far peripheral fields and for patients with central scotomas who cannot see the internal target.

For a patient with a dense central scotoma, external fixation with the fellow eye is often the only way to obtain a steady, centred macular photograph.

Stereo photography

Stereo pairs give the depth information a single image cannot: disc cupping, macular elevation, retinal detachment height and tumour contour.

Technique (sequential stereo):

1. Dilate widely — a 7 mm or larger pupil makes stereo far easier.
2. Align and capture the first image with the camera shifted slightly to one side within the pupil.
3. Without changing focus or the patient's fixation, shift the camera laterally a few millimetres to the other side of the pupil and capture the second image.
4. The two images are viewed with a stereo viewer and must be mounted in the correct left-right order.

Consistency matters more than perfection. Serial disc stereo pairs are used to detect progressive cupping over years, so the same shift magnitude, same field and same magnification must be used each time.

Simultaneous stereo cameras use a beam splitter to capture both images at once, eliminating the interval between exposures and the risk of patient movement between them.

Monochromatic and filtered imaging

Different wavelengths penetrate to different depths, so filters make different structures visible.

Red-free (green) imaging is the one to know cold: it is standard for documenting diabetic retinopathy and for nerve fibre layer assessment, and its hallmark is that blood appears black against the green-grey background.

Wide-field and ultra-widefield imaging

Strengths of ultra-widefield: a single image, no dilation needed in many systems, excellent for peripheral diabetic non-perfusion, retinal vein occlusion, retinopathy of prematurity, peripheral degenerations and tumour screening, and far faster than seven-field photography.

Limitations to record and understand: the peripheral distortion means apparent lesion size and area in the far periphery must be interpreted with caution; eyelashes and lids commonly obscure the superior and inferior extremes, so ask the patient to open wide and consider steering images; colour rendition differs from conventional photography because the image is built from two or three laser wavelengths rather than white light; and media opacities still degrade the image.

Anterior segment and external photography

Although covered in detail in the next section, note here that the same camera body often serves both, with different attachments, and that the field and magnification must be recorded for any serial comparison.

Practical quality standards

Before accepting an image set, check each frame against these criteria:

1. Correct field — the intended landmark is centred and the protocol's fields are all present.
2. Focus — retinal vessels are crisp at full zoom, not just the overall image.
3. Exposure — detail visible in both the disc (the brightest structure) and the surrounding retina.
4. Even illumination — no crescent, no central haze, no vignetting.
5. No obstruction — no lash shadows, no lid encroachment.
6. Correct laterality label — right versus left, verified against the patient, not against the order in the worklist.
7. Correct patient identifiers on the file.

A mislabelled laterality is the most consequential photography error, because treatment decisions and even laser or surgery may follow from it. Confirm laterality at capture and again at filing.

Consent and communication

Photography is usually covered by general consent to treatment, but the patient should always be told what is being done, why, and that a bright flash is coming. Warn about the afterimage — a purple or green blob lasting several minutes — before the first flash, not after, and reassure that it resolves. Warn a dilated patient about photophobia and near blur, advise sunglasses, and confirm they have transport arrangements where dilation affects driving.