There are two types of retinoscopes used in clinical optometry: (1) Streak retinoscope — projects a linear streak of light; preferred because the streak orientation helps identify the astigmatic axis easily; (2) Spot retinoscope — projects a circular spot of light; older design. The streak retinoscope (e.g., Copeland, Welch Allyn) is the standard instrument in India. Working principle: the plane mirror retinoscope uses the principle of the plane mirror, and the concave mirror retinoscope uses a concave mirror. Working distance is typically 66 cm (1.5D) or 50 cm (2D).

Optical Coherence Tomography (OCT) uses low-coherence interferometry with near-infrared light to create high-resolution cross-sectional images of individual retinal layers — completely non-invasive, no dye, no radiation. Gold standard for: macular pathology (AMD, DME, ERM), glaucoma (NFL thickness), retinal detachment monitoring. Why not others? Fluorescein angiography requires IV dye injection (invasive). Pachymetry measures only corneal thickness. Fundus photography provides a 2D surface image — cannot show individual retinal layer architecture.

A standard slit-lamp is designed for anterior segment examination — cornea, anterior chamber (cells, flare), iris, pupil, crystalline lens — all accessible without any additional lens. The macula and posterior segment cannot be focused upon directly because the eye’s own optics (+60D converging power of the phakic eye) diverge the slit-lamp beam before it reaches the retina. Additional aids needed: Non-contact — 90D or 78D Volk lens. Contact — Goldmann 3-mirror lens, Superfield lens. These lenses neutralize the eye’s power, allowing posterior segment imaging via the slit-lamp.

VEPs measure the electrical response of the occipital visual cortex to light stimulation. In demyelinating optic neuritis (e.g., in multiple sclerosis), myelin loss along the optic nerve slows conduction velocity, resulting in a prolonged P100 latency — the hallmark finding even after symptoms resolve. VEP is the most sensitive test for subclinical optic nerve damage. EOG tests RPE/photoreceptor function (Arden ratio). ERG tests photoreceptor-to-bipolar cell function. EVG and EAG are not established clinical tests — distractors here.

Diffuse illumination uses a wide, defocused beam (with a ground glass diffuser) to provide broad, even lighting across the entire anterior segment and periocular structures. It is ideal for examining gross ocular adnexa — eyelid margins, lash follicles, and meibomian gland orifices in blepharitis. Slit-lamp illumination techniques summary: Diffuse: broad overview, adnexa. Optic section: narrow beam — corneal thickness, layer-by-layer analysis. Broad beam: overview of cornea/anterior chamber. Specular reflection: corneal endothelium, tear film. Sclerotic scatter: corneal opacities. Retro-illumination: corneal vascularisation, lens opacities.

15× magnification = Direct ophthalmoscopy — NOT indirect. Indirect ophthalmoscopy provides only 2x to 5x magnification depending on the condensing lens used (20D lens → ~3x, 28D → ~2x). The trade-off is a much wider field of view (25°–40°), making it ideal for peripheral retina, retinal detachment surgery, and screening. Comparison table: Direct → 15× magnification · narrow field · virtual upright image. Indirect → 2–5× magnification · wide field · real, inverted image · stereoscopic · binocular. Options B, C, D are ALL true for indirect.