Ophthalmoscope and Retinoscope

The ophthalmoscope illuminates the retina through the normal iris defect that is the pupil. Light rays forming the image of the retina re-emerge through the pupil. The viewing aperture (window) of the ophthalmoscope contains a lens that modifies light rays to assist the user. In the procedure, one looks at structures lying in the innermost aspect of the globe, collectively known as the eyegrounds: retina, retinal blood vessels, optic nerve head (disk), and to a limited degree, subjacent choroid.
The pupil is frequently dilated pharmacologically to render retinal inspection easier, and for examination of the macula. One paralyzes the pupilloconstrictor muscle of the iris with nonabsorbable, short-acting topical parasympatholytic drugs, resulting in a larger pupillary aperture. In comparison to the ophthalmologist, the internist, neurologist, or pediatrician concentrates particularly on funduscopic manifestations of systemic disease and less on local ocular disease.
Synonyms for funduscopic examination include funduscopy, ophthalmoscopy, and direct ophthalmoscopy. Only ophthalmologists perform retinoscopy and indirect ophthalmoscopy, which require other equipment and provide different information.

Retinoscopes are optometry devices which are used to shine light into a patient's eye and observe the reflection off from the retina to measure the refractive error in the eye. The working principle of retinoscopy is Focault's principle.

Retinoscopy determines a patient's refractive error by establishing what corrective lens places the far point of the eye at infinity. Infinity is simulated at the peephole of the retinoscope where all light beams reflecting from the patient's eye are gathered. The far point is the point where light from the retinoscope focuses after reflecting off the retina. It is defined as the point in space that is conjugate with the retina in a non-accommodating eye.

When the far point is located behind the retinoscope or behind the retina, no change in the direction of light will be observed, creating a "with motion" reflex. When the far point is located between the retinoscope and the patient's eye, this point acts as a fulcrum, creating an "against motion" reflex. When the far point is in the plane of the retinoscope, neutrality is observed with no motion of the reflex.
There exist four main refractive states of the eye: emmetropia, myopia, hyperopia, and astigmatism. Emmetropia (normal vision) is a refractive state wherein parallel light rays are focused on the fovea of a non-accommodating eye and emerge as parallel light rays. If this is the case, neutralization will be observed.

Myopia consists of light rays that come to a point of focus in front of the retina in a non-accommodating eye and emerge as converging light rays. "Against motion" is observed in uncorrected myopic patients. Hyperopia consists of light rays that come to a point of focus behind the retina in a non-accommodating eye and emerge as diverging light rays. Hyperopia produces "with motion" on retinoscopy in optically uncorrected patients. Lastly, with astigmatism, light rays form two points of focus in the eye. Astigmatism can produce either a "with" and/or "against motion" reflex.

The location of the far point can be altered based on the correcting lens placed in front of the eye. Patients with emmetropia will already have the far point at infinity since the rays reflect parallel from the eye and require no correcting lens. Meanwhile, patients with myopia will have a far point between the patient's eye and infinity since the rays leave the eye converging, requiring a minus lens to push the far point outwards. Patients with hyperopia will have a far point beyond infinity since the rays reflect diverging from the eye, requiring a plus lens to pull the focal point towards the eye.