Dictionary of terms relating to
eyesight and vision

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Facility of Accommodation

A measure of the ease and speed of the eye(s) to change focus.

Fellow of the College of Optometrists in Vision Development (FCOVD)

Individual who is a licensed optometrist for a minimum of three years and directly involved in vision therapy for a minimum of 2 years, has completed a guided study program, submitted evidence of 100 hours of continuing education in functional vision (and vision therapy), and passed rigorous written, oral, and clinical examinations. Fellows are Board Certified in Vision Development and Therapy and must obtain at least 15 hours of continuing education annually in functional / developmental / behavioral / rehabilitation vision care.


The ability to recognize distinct shapes from their background, such as objects in a picture, or letters on a chalkboard.

Fine Motor Skills

The ability to coordinate hand and finger movements.


The ability to direct and maintain steady visual attention on a target. Fixations are a form of pursuits.

Fixation Disparity (FD)

Over-convergence or under-convergence, or vertical misalignment of the eyes under binocular (both eyes) viewing conditions small enough in magnitude so that fusion is present.


Also known as spots, are usually clouded or semi-opaque specks or particles within the eye that are seen in the field of vision. The eyes are filled with fluid which maintains the shape of the eye, supplies it with nutrition and aids in the focusing of light. Often, particles of protein or other natural materials are left floating or suspended in this fluid when the eye is formed before birth. If the particles are large or close together, they cast shadows which make them visible. This is particularly true when nearsightedness occurs or becomes more severe. In most cases this is normal but floaters can also be caused by certain injuries, eye disease or deterioration of eye fluid or its surrounding structures.

Form Constancy

The ability to recognize two objects that have the same shape but different size or position. This ability is needed to tell the difference between "b" and "d", "p" and "q", "m" and "w".


Center of the retina that can produce the sharpest eyesight and contains the most cones. (See diagram of the eye)

Fusional Vergence

A convergence response which serves to maintain (fusion) the union of images from each eye into a single image. The eyes will turn with a slow smooth tonic movement or a fast jumping movement called phasic.

Fusional Vergence Dysfunction

See "General Binocular Vision Disorder".


The union of images from each eye into a single image. There are three degrees of fusion. 1st degree fusion is the superimposition of two dissimilar targets. 2nd degree fusion is flat fusion with a two-dimensional target. 3rd degree fusion is depth perception (stereopsis) with a three-dimensional target.

Fusion Test

Determines the eyes ability to unite the images from each eye into a single image.