# Ophthalmoscopy Technique > Ophthalmoscopy for Optometry exams: direct versus indirect technique, magnification and field, dilation cautions, disc assessment and the ISNT rule. - Canonical URL: https://prepelephant.com/topics/allied/optometry/ophthalmoscopy-technique-ss - Exam / course: Allied Health · Subject: Optometry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Ophthalmoscopy Technique", PrepElephant, https://prepelephant.com/topics/allied/optometry/ophthalmoscopy-technique-ss ## Direct answer The red reflex comes first, from about a metre out: approach through it and the direct ophthalmoscope delivers an erect, virtual, 15-times magnified monocular view of disc and posterior pole — perfect for detail, hopeless for extent. Indirect ophthalmoscopy is its mirror image: a +20 D condensing lens before a dilated pupil produces a real, inverted image at roughly 3× (the eye's 60 D divided by the lens's 20 D), with stereopsis, a field near 37 degrees and far-periphery access by scleral depression. Dilate when the view demands it (tropicamide with or without phenylephrine), but never before judging the anterior chamber — and examine the fundus as a sequence: red reflex, vessels, disc, macula, periphery. ## What you must remember - Direct ophthalmoscopy: erect, virtual, about 15× magnification, monocular, roughly 10-degree field (about two disc diameters) — the tool of disc detail and macular pathology. - Indirect: real, inverted, about 3× with +20 D (magnification = eye power ÷ condensing lens power), stereoscopic, 37-degree field, brighter through media opacities; +28/+30 D trades magnification for wider field. - Dilation: tropicamide 0.5–1 per cent (about 20 minutes onset, 4–6 hours), phenylephrine 2.5 per cent added when needed; warn drivers about glare. - Never dilate without assessing peripheral anterior chamber depth (Van Herick) in eyes at risk of angle closure — an attack precipitated by dilation is the classic iatrogenic emergency. - Direct technique: start at a metre on the red reflex (an opacity shows as a dark shadow whose movement with gaze localises it fore or aft), close to about 2 cm, patient fixing a distant target. - Find the disc by following a vessel nasally — about 15 degrees nasal to the fovea; arteries narrower than veins (about 2:3) with a bright reflex. - Disc assessment: vertical cup-to-disc ratio, the ISNT rule (Inferior rim thickest, then Superior, Nasal, Temporal), pallor, haemorrhages, peripapillary atrophy. - Macula: two disc diameters temporal and slightly below the disc; the patient looks directly at the light to bring it into view. - Indirect technique: headset binoculars with the condensing lens, patient's gaze to eight positions, scleral depression for the periphery. ## A diabetic fundus exam, walked through The task: grade diabetic retinopathy in a 54-year-old. Dilation follows a Van Herick check showing a safe angle. The direct scope starts at one metre — the red reflex is uniform, so no significant media opacity. Closing in on the right eye, sweep systematically: superior arcades, temporal macula, inferior arcades, nasal field, back to the disc — microaneurysms as tiny red dots, dot-and-blot haemorrhages darker, hard exudates as yellow circinate rings. The macula is examined last with the patient staring at the light: foveal reflex intact, no exudates within a disc diameter — no clinically significant macular oedema on this view, though OCT has the final word. The disc is judged for new vessels — a red-flag escalation. Indirect ophthalmoscopy then completes what direct cannot: peripheral sweeps in all eight gazes reveal a patch of pre-retinal haemorrhage — new vessels somewhere. The drawing afterwards uses standard conventions so the surgeon reads a map, not a memory. Direct for character, indirect for extent — the two are complements. ## Where students slip The comparison is the exam, and the swaps are predictable: direct gives about 15× with a small field; indirect gives less magnification with a wide stereoscopic field — candidates who write "indirect magnifies more because the machine is bigger" have learned only the machine's price. On the direct scope, the beginner's classic is searching temporal for the disc — follow a large vessel nasally and the disc arrives. ISNT gets quoted without order; "which rim is thickest?" wants "inferior", and its violation by notching is a glaucoma sign. Finally the safety question: dilating an unchecked elderly hyperope risks acute angle closure — Van Herick first, dilation second, with the patient warned. ## Frequently asked questions ### Compare the images of direct and indirect ophthalmoscopy. Direct: erect, virtual, about 15×, monocular, roughly 10-degree field. Indirect: inverted, real, about 3× with a +20 D lens, stereoscopic, field near 37 degrees with peripheral access. ### Why is the indirect image inverted? The convex condensing lens forms a real, inverted aerial image of the illuminated retina between lens and observer; the headset restores binocular viewing, not orientation. ### Which condensing lens is routine for indirect ophthalmoscopy? The +20 D lens, balancing about 3× magnification with a wide field and workable distance; +28 or +30 D widens the field at lower magnification. ### What is scleral depression and when is it used? Gentle indentation of the peripheral sclera through the lids to bring the extreme periphery, including the ora serrata, into view — essential in detachment and peripheral lesion searches. ### What is the ISNT rule? In the normal disc the neuroretinal rim is thickest Inferiorly, then Superior, then Nasal, then Temporal; thinning or notching that violates the order suggests glaucomatous damage. ### What precautions precede pharmacological dilation? Assess peripheral anterior chamber depth (Van Herick) in at-risk eyes — shallow chambers may precipitate acute angle closure — and warn drivers.