The faculty of the eye to increase the optical power of the lens—may be used to focus on near objects, or to self-correct hyperopic refractive errors.
Astigmatism with the greatest refractive power in the horizontal meridian.
Any refractive error—any deviation from emmetropia.
Angle alpha (α)
The angle between the pupillary axis and the optical axis.
Angle kappa (κ)
The angle between the pupillary axis and the visual axis.
Disparity in the size of the retinal images formed in the 2 eyes of a patient.
Different refractive errors in the 2 eyes of a patient.
A refractive error in which the power of the eye varies in different meridians.
The ability to detect small spatial variations in the luminance of a visual stimulus; quantitatively, the reciprocal of the contrast threshold.
Contrast sensitivity function
The relationship of the contrast sensitivity of an eye as measured using grating test targets to the spatial frequency (spacing) of the gratings.
The minimum contrast at which a target is detectable.
The refractive state of an eye which focuses objects at optical infinity on the fovea with relaxed accommodation.
The image of the anatomical pupil as seen through the cornea.
Gullstrand model eye
A detailed schematic eye model developed by Allvar Gullstrand.
A refractive error in which objects at infinity are focused behind the retina, with relaxed accommodation.
A refractive error where the astigmatism varies significantly across the optical aperture of the eye.
Modulation transfer function
A function that describes the relationship between the contrast of a visual stimulus and the contrast of the image of the stimulus on the retina, as a function of spatial frequency.
A refractive error in which objects at infinity are focused in front of the retina, with relaxed accommodation.
The line connecting the optical center of the cornea and optical center of the crystalline lens.
A standardized letter or other character, typically defined by features that subtend one-fifth of the height of the character, such as the horizontal bars, and the spaces between them, of the letter E.
Pinhole visual acuity
Measurement of visual acuity as perceived through an occluder with pinhole apertures that reduce the impact of refractive error.
The loss of accommodation with increasing age.
Principal line of vision
The line passing through the fixation target, perpendicular to the corneal surface.
The line through the midpoint of the entrance pupil, perpendicular to the corneal surface.
Reduced schematic eye
A simplified schematic eye model with a single refracting surface at the corneal apex.
A refractive error where the astigmatism is approximately constant across the optical aperture of the eye.
Snellen visual acuity
The ability to identify small letters or other optotypes (presented at high contrast).
The number of stripes or cycles of a periodic test target per degree of visual angle.
The line connecting the fovea to the fixation target.
Astigmatism with the greatest refractive power in the vertical meridian.