18.1 The Fundus Camera: Optics, Alignment & Correcting Artefacts
Key Takeaways
- The fundus camera uses an annular illumination beam and a central imaging path to avoid corneal and lens reflections.
- A central haze usually means the camera is too far forward; a peripheral crescent means it is decentred toward that side.
- Most fundus cameras need a pupil of at least about 4 mm, and non-mydriatic cameras rely on infrared alignment with a dark-adapted pupil.
- The dioptre compensation setting must be adjusted for high ametropia so the retina rather than the cornea is in focus.
- Dust and fingerprints on the objective produce fixed arcs and spots in the same position on every frame.
How a fundus camera works
The central problem of fundus photography is that illumination and imaging must share the same pupil, and any light reflected from the cornea or lens surfaces returns straight into the imaging path and washes out the picture.
The solution is annular illumination. The camera projects light as a doughnut-shaped ring through the periphery of the dilated pupil, while the imaging path collects light returning through the centre of the pupil. Because the illuminating and imaging paths occupy different parts of the pupil, corneal and lens reflections are excluded.
This single fact explains almost every fundus photography artefact. If the camera is too close, too far, too high, too low or too far to one side, the annulus and the imaging aperture overlap, and light leaks into the image.
Key components:
| Component | Function |
|---|---|
| Objective lens | Forms the aerial image of the retina; must be spotlessly clean |
| Annular illumination system | Delivers the doughnut of light through the pupil periphery |
| Flash and viewing lamps | Xenon flash for capture, continuous or infrared for alignment |
| Dioptre compensation control | Corrects the patient's refractive error so the retina, not the cornea, is in focus |
| Focusing crosshairs / split lines | The reticle focused first, to the operator's eye |
| Joystick and chin rest | Three-axis positioning |
| Filter wheel | Red-free (green), exciter and barrier filters for angiography, autofluorescence filters |
| Fixation target | Internal or external light the patient fixates to bring the desired field into view |
Setting up
- Focus the eyepiece reticle first, exactly as on a lensometer or keratometer. Turn it fully toward plus, then back until the crosshairs are just sharp. If this is skipped, the operator's own accommodation makes every image soft.
- Dilate the pupil unless using a non-mydriatic camera. Most conventional cameras require at least about 4 mm, and 6 mm or more is comfortable.
- Seat the patient with the forehead firmly against the bar and the chin in the rest, adjusting the height so the outer canthus aligns with the marker on the chin rest post.
- Set the dioptre compensation for the patient's refractive error. Cameras typically have a normal range plus a high-plus and a high-minus setting for aphakia and high myopia.
- Set the flash intensity — lower for lightly pigmented fundi and albinism, higher for heavily pigmented fundi and media opacity.
- Set the fixation target for the field required.
- Align, focus, capture. Approach from a distance, bring the annulus into the pupil, refine the working distance until the illumination is even, then focus on retinal detail — the vessels, not the disc margin — and take the picture.
Patient instructions that make the difference: "Keep your chin in the rest and forehead against the bar," "Blink normally, then hold your eyes wide open," "Keep looking at the little light even though it is bright," and a warning before the flash. A full blink immediately before capture gives a smooth tear film and a crisper image.
The artefact troubleshooting table
This table is the practical core of fundus photography and is worth learning as a unit, because each artefact has a specific geometric cause and a specific correction.
| Artefact | Appearance | Cause | Correction |
|---|---|---|---|
| Central haze / white-out | Overall milky veil across the image | Camera too far forward (too close to the eye); the annulus is striking the cornea | Pull the camera back slightly |
| Uniform dimness with a dark image | Underexposed, vignetted | Camera too far back | Move forward |
| Peripheral crescent | Bright white crescent at one edge | Camera decentred toward that side; the annulus is clipping the iris | Move the camera away from the crescent |
| Blue or white arc at the top or bottom | Arc of light | Camera too high or too low, or lid/lash obstruction | Adjust height; ask the patient to open wider, or hold the lid |
| Fixed dust spots in the same place on every frame | Small dark specks | Dust or fingerprints on the objective lens | Clean the objective with approved lens tissue and solution |
| Uniform soft focus | Everything slightly blurred | Eyepiece not focused, or wrong dioptre compensation setting | Refocus the reticle; change dioptre compensation range |
| Overall haze that improves after a blink | Milky, transient | Dry tear film | Ask for a full blink or instil a lubricant drop |
| Central dark shadow | Dark patch | Media opacity, or an inadequately dilated pupil | Redilate; reposition to photograph around the opacity |
| Lash shadows | Dark diagonal lines at the edge | Lashes in the light path | Hold the upper lid up, or ask the patient to open wider |
| Overexposed, washed-out image | Detail lost in highlights | Flash too high for a lightly pigmented fundus | Reduce flash |
| Underexposed, grainy image | Dark and noisy | Flash too low for a heavily pigmented fundus or media opacity | Increase flash |
| Reflections from an intraocular lens edge | Bright streaks | Pseudophakic eye, annulus striking the lens edge | Shift the camera slightly and recapture |
The alignment logic to memorise: the artefact tells you where the camera is. Haze in the centre means you are too close. A crescent at the edge means you are off-centre toward that edge and must move away from it. A dark image means you are too far back.
Non-mydriatic cameras
A non-mydriatic camera aligns and focuses using infrared light, which the pupil does not constrict to, then fires a brief visible flash for capture. The requirements are a dark room and a patient who is dark-adapted for at least a few minutes.
Practical points: photograph the right eye first, then the left, because the flash constricts the first eye; allow recovery time between frames; keep the room genuinely dark; and accept that the pupil is small, so field and quality are limited. Non-mydriatic imaging is excellent for diabetic screening programmes and for patients where dilation is undesirable, but it does not replace a dilated examination.
Videography
Video documentation covers motility (nine positions of gaze recorded as a sequence), nystagmus (including the waveform, null point and effect of convergence), lid function, pupil responses, and surgical footage through the operating microscope.
Technique points: use consistent framing and distance so sequences are comparable across visits, ensure even illumination without a hot spot on the corneal reflex, record long enough to capture a full cycle of an intermittent finding, state the date and the patient identifier at the start of the file, and obtain and document consent — video is more identifiable than a fundus photograph.
A fundus photograph shows a milky haze across the whole image. What is the correction?
A bright white crescent appears at the right edge of the image. What should the technologist do?
Why does a fundus camera use annular illumination?
Small dark specks appear in exactly the same position on every image from a session. What is the cause?
When using a non-mydriatic camera on both eyes, which practice gives the best results?