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Factors affecting Visual Acuity
Visual acuity is measured in various different ways and that is dependent on the acuity task used. However factors affecting the visual acuity have to be taken into account. Visual acuity is affected by a number of ways such as diffraction, aberrations and photoreceptor density in the eye. There are a number of other factors too, which will be discussed more in depth in this article.
Other factors that affect visual acuity include:
- Refractive error
- Size of the pupil
- Time exposure of target
- Area of retina stimulated
- State of adaptation of the eye
- Eye movements
These factors will be discussed in detail below.
Refractive error is one of the important factors that affect visual acuity. Refractive error affects visual acuity depending on the correction, either Hypermetropic or Myopic or an Emmetropic eye. Visual acuity is affected depending if it causes a defocus at the retina, in the case of a Hypermetropic or myopic eye this is the case. Defocus is when fine details, sharp edges and contrast detail is blurred out.
In a Hypermetropic eye, the eye is too strong so the image is focused in front of the retina causing a defocus hence affecting visual acuity. The reverse occurs with a myopic eye, as the eye is too weak, the image is focused in front of the retina therefore again causing a defocus hence affecting visual acuity.
On the other hand to obtain a perfect image, image should be focused sharply on the retina. This is the case in an emmetropic eye; no defocus hence no visual acuity problem due to refractive error.
Having astigmatism also affects visual acuity. A phenomenon called the oblique effect states that oblique lines are seen less accurately, meaning that visual acuity is greater when lines are orientated horizontally or vertically. This is still the case even after a perfect correction of refractive error and astigmatism.
Size of Pupil
Eye takes time to see things. The nerve signals take a limited amount of time to travel along the optic nerve and instigate visual awareness. Therefore the size of pupil can determine how quickly or slow a signal can be stimulated thus determining how much visual acuity is affected. Large pupils let more light pass through into the eye so stimulating the retina more. However this does have a disadvantage; it affects resolution due to the aberrations that increase in the eye due to an increase in light. Small pupils reduce optical aberrations due to less light passing through into the eye however resolutions are diffraction limited. Therefore for good visual acuity a mid size pupil of approximately 3mm to 5mm is optimal as this is a compromise between the diffraction and aberration limits. Visual acuity also decreases with age due to decrease in pupil size, so a higher level of illumination is needed in older people to improve visual acuity.
For recognition tasks, visual acuity increases with illumination. However at very high illumination levels, acuity may decrease because of loss of contrast between object and its background, or reflections on the surface of the object decreasing the contrast between the background and object. Higher the contrast between the object and the background the greater the acuity is. When contrast is reduced, it becomes more difficult to read it against a darker background therefore the text need to be made larger in order for the illumination levels to be correct for good visual acuity.
Illumination levels greatly affect the older generation due to many people having cataracts. The cataract becomes a source of veiling glare within the eye therefore visual acuity decreases.
Time Exposure of the Target
To detect a small bright spot, detection is greatly dependent on the quantity of light rather than the exposure time. However to detect a line, the acuity is proportional to the exposure time. There is no simple acuity-exposure time relationship for the resolution of the target.
Area of the Retina Stimulated
Visual acuity is greatest at the centre of fixation, which in this case is at the fovea. Fovea has the greatest visual acuity due to the densely packed cones, so once the image is moved away from the centre of fixation visual acuity is affected. At a distance of 5 minutes of arc from the centre of fixation, there is a loss in visual acuity. At approximately 10 minutes of arc from fixation there is about 25% visual acuity loss. This shows that our vision is most accurate at the fovea and acuity falls rapidly as we move towards the peripheral retina. Therefore in order to achieve best visual acuity we must scan our visual field to place the fovea on the area of interest and to achieve this we must have excellent oculo-motor co-ordination.
State of Adaptation of the Eye
Highest level of acuity is achieved if the eye is tailored to the equivalent level as the test luminance of 34cd/m2 to 34,000cd/m2. Test luminance less than 34cd/m2 will achieve to some extent, better acuity. Under photopic conditions, the high density of cones at the fovea is accountable for the high levels of acuity. Under scotopic conditions it is a little altered, due to the Aii Amacrine cells appearing to limit resolution. Maximum scotopic acuity occurs at 5-15 degrees eccentricity which is corresponding to the cell density, while peak rod density occurs at about 15-20 degrees.
In steady eye fixation, the eyes are in constant motion; therefore moving objects are more difficult to see than stationary objects thus affecting visual acuity. Visual acuity will be greater when point of interest is constant or little movement however visual acuity will be at a great loss when object is in constant movement. Conclusion: Dynamic visual acuity is lower than Static visual acuity.
Visual acuity is measured in a number of different procedures however it is dependant on many factors. Visual acuity cannot just be taken as a figure without considering all the factors that affect it. Such as the refractive error, size of pupil, illumination, area of retina stimulated, adaptation of the eye, eye movements and time exposure of the target. Therefore without an understanding of this visual acuity cannot be taken accurately.
Atchison D. A., Smith G., and Efron N. (1979) The effect of pupil size on visual acuity in uncorrected and corrected myopia. Am J Optom Physiol Opt. 56:315-32
W,Karwowski. Encyclopaedia of ergonomics and human factors. Volume 1.