↑ arvau

Wikipedia / Visual angle

Visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc. It also is called the object's angular size.

.

can represent the eye's nodal points at about the center of the lens, and also represent the center of the eye's entrance pupil that is only a few millimeters in front of the lens.

on the retina. The central line of the bundle represents the chief ray.

.

.

Measuring and computing

can be measured directly using a theodolite.

.

However, for visual angles smaller than about 10 degrees, this simpler formula provides very close approximations:

The retinal image and visual angle

As the above sketch shows, a real image. (See visual system is

is the distance from the nodal points to the retina, about 17mm.

Examples

.

.

Also, for some easy observations, if one holds one's index finger at arm's length, the width of the index fingernail subtends approximately one degree, and the width of the thumb at the first joint subtends approximately two degrees.

Therefore, if one is interested in the performance of the eye or the first processing steps in the visual cortex which determines the size of the retinal image.

Terminological confusions

In astronomy the term apparent size or angular diameter.

But in psychophysics and experimental psychology the adjective "apparent" refers to a person's subjective experience. So, "apparent size" has referred to how large an object looks, also often called its "perceived size".

Additional confusion has occurred because there are two qualitatively different "size" experiences for a viewed object., also called the object's perceived or apparent angular size. The perceived visual angle is best defined as the difference between the perceived directions of the object's endpoints from oneself.

, the object's physical width or height or diameter.

Widespread use of the ambiguous terms "apparent size" and "perceived size" without specifying the units of measure has caused confusion.

Representation of visual angle in visual cortex

The brain's primary visual cortex (area V1 or Brodmann area 17) contains a spatially isomorphic representation of the retina (see retinotopy of a given retinal image determines the extent of the neural activity pattern eventually generated in area V1 by the associated retinal activity pattern. Murray, Boyaci, & Kersten (2006) recently used Functional magnetic resonance imaging, also increases the extent of the corresponding neural activity pattern in area V1.

of one was about 17% larger than for the other, due to differences in the background patterns for the disks. It was shown that the areas of the activity in V1 related to the disks were of unequal size, despite the fact that the retinal images were the same size. This size difference in area V1 correlated with the 17% illusory difference between the perceived visual angles. This finding has implications for spatial illusions such as the visual angle illusion.

See also

Notes

References

  • Baird, J. C. (1970). Psychophysical analysis of visual space. Oxford, London: Pergamon Press.

  • Joynson, R. B. (1949). The problem of size and distance. Quarterly Journal of Experimental Psychology, 1, 119–135.

  • McCready, D. (1965). Size-distance perception and accommodation-convergence micropsia: A critique. Vision Research. 5, 189–206.

  • McCready, D. (1985). On size, distance and visual angle perception. Perception & Psychophysics, 37, 323–334.

  • Murray, S.O., Boyaci, H, & Kersten, D. (2006) The representation of perceived angular size in human primary visual cortex. Nature Neuroscience, 9, 429–434 (1 March 2006).

  • McCready, D. The Moon Illusion Explained.