Curvature of Field

This means that the magnification of the image changes as one approaches the periphery. The result is a sharp image with peripheral curvature. Convex or plus lenses produce pincushion distortion; concave or minus lenses produce barrel distortion (Fig. 16.23).

Astigmatic aberration.

Apex

Apex

Base

Fig. 16.22 a Lenses may be regarded as composed of many prisms, which explains many of the optical phenomena of lenses such as dispersion. b A prism refracts a light ray toward its base twice (solid line). However, it appears to the observer that the object is shifted toward the apex of the prism (dotted line).

Base

Fig. 16.22 a Lenses may be regarded as composed of many prisms, which explains many of the optical phenomena of lenses such as dispersion. b A prism refracts a light ray toward its base twice (solid line). However, it appears to the observer that the object is shifted toward the apex of the prism (dotted line).

Barrel Distortion
Fig. 16.23 Viewing an object through plus lenses produces pincushion distortion of the image, whereas minus lenses produce barrel distortion.

17 Ocular Motility and Strabismus

Doris Recker, Josef Amann, and Gerhard K. Lang

Definition

Strabismus is defined as deviation of an eye's visual axis from its normal position.

There are two major types of manifest strabismus or heterotropia:

1. Concomitant strabismus (from the Latin "comitare", accompany). The deviating eye accompanies the leading eye in every direction of movement. The angle of deviation remains the same in all directions of gaze. This form of strabismus may occur as monocular strabismus, in which only one eye deviates, or as alternating strabismus, in which both eyes deviate alternately.

2. Paralytic strabismus results from paralysis of one or more eye muscles. This form differs from concomitant strabismus in that the angle of deviation does not remain constant in every direction of gaze. For this reason, this form is also referred to as incomitant strabismus.

Epidemiology: The incidence of strabismus is about 5-7%. Esotropia (convergent strabismus) occurs far more frequently than exotropia (divergent strabismus) in Europe and North America. Concomitant strabismus usually occurs in children, whereas paralytic strabismus primarily affects adults. This is because concomitant strabismus is generally congenital or acquired within the first few years of life, whereas paralytic strabismus is usually acquired, for example as a post-traumatic condition.

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