The scientific explanation is this:
“Astigmatism is an imperfection in the curvature of your cornea — the clear, round dome covering the eye’s iris and pupil — or in the shape of the eye’s lens. Normally, the cornea and lens are smooth and curved equally in all directions, helping to focus light rays sharply onto the retina at the back of your eye. However, if your cornea or lens isn’t smooth and evenly curved, light rays aren’t refracted properly. This is called a refractive error.” American Academy of Ophthalmology
“In astigmatism, images focus in fromt of and beyond the retina, causing both close and distant objects to appear blurry.”
“ In a normal eye, the cornea and lens focus light rays on the retina.”
I have a Corneal astigmatism:
If the cornea does not curve perfectly – if one half is flatter or steeper than the other – the light that hits it will not refract properly and the retina at the back of the eye will receive an imperfect image. The person will have blurred vision from that eye – astigmatism. People with astigmatism commonly have an oblong-shaped (oval-shaped) cornea rather than a perfect sphere shape. A ping-pong ball is a perfect sphere, while an American football or a rugby ball has an oblong shape. If the cornea’s curve is like an oblong the light rays will focus on two points in the retina, rather than just one.”
(Medical News Today, http://www.medicalnewstoday.com/articles/158810.php)
Astigmatism is often accompanied by short/far sightedness luckily for me i only have a small amount of shortsightedness in my left eye which also happens to be the eye that has the the rugby ball curved cornea, my eyes do however tire pretty easily. My condition doesn’t so much make my vision blurry, more distorted particularly when i am tired. The image from my right eye is clear, the image from my left eye is a bit distorted almost like looking through very still water, its as if my eye has focused on a space in the air in front of an object as opposed to the object itself. The images from both my eyes are combined in my visual system to form one image, this combination can make judging distances challenging, like playing pool or driving, and if i look at the same distance for prolonged periods ill get a headache from having to consciously focus my eyes; if i continue and my eyes become strained and i will have difficulty focusing at all and will often walk around with everything out of focus limiting myself to focus on select points according to relevance. This is not ideal but im used to it, on the plus side these visual quirks have also produced some interesting effects on what and how i see. Knowledge and study of vision, through doing a BFA at Elam and a BA in psychology, has helped me understand these quirks and explore them further.
There are certain instances when my eyes are very tired (from looking at the same distance for long periods ie. painting or looking at a computer screen) where my eyes where not prepared for the onslaught of spatial differences in my field of vision. One marked example occurred one afternoon a few years ago when i was coming home after painting at Elam for about seven hours; was walking through a park with various trees and objects spaced at various distances before me. My vision was swimming and space completely flattened out before me, i still understood that i was looking at a 3D scene but it was as if my eyes, after looking at flat images all day and concentrating on manufacturing 3D illusions on 2D surfaces, had defaulted to the interpretation systems used for flat images instead of the interpretation systems for a 3D scenes. I was consciously aware of certain visual cues, in particular occlusion and perspective, that told me where objects were located in space but at the same time the scene before me appeared very flat. If you havent had this experience yourself I liken the sensation to looking at a hologram.
A photograph captures the point of view (POV) of a camera, upon seeing the print the cameras POV becomes the observers POV. Building a sense of scale and orienting objects in the space using that scale and if included the horizon line in addition to visual cues available in 2D images (perspective, shading, occlusion, haze (aerial perspective). In this way we are able to interpret three dimensionality from the flat image. I liken what i was experiencing to viewing a photographic print, as if my point of view was that of the camera but rather than looking through the lens i was looking at the resulting photographs. The experience is fairly disorientating and certainly headache inducing but very interesting for someone that is invested in understanding images and vision. Possibly its my stigmatism that allows this state to occur, it happens naturally in the right conditions, i think it could be because the balance between the two images from my right and let eyes is pushed slightly out of kilter when my eyes become tired. So to compensate my visual systems rely more heavily on visual cues used for flat images (not so much stereopsis/relative motion) resulting in my apperception of a visual scene being distorted. I spend a lot of time looking at images in the form of paintings and photographs, and also using the computer and watching tv, i wonder if this prolonged concentrated exposure to images has influenced how my visual system functions? Im sure other people will have had similar experiences.
I have tried to train myself to be able to control this perceptual shift a process that has reminded me of when i played soccer and i had to learn to look ahead but still see the ball at my feet using my peripheral vision. I have practiced shifting my perceptions back and forth between a 2D/3D. Previous to this year of study i have done this while creating paintings on canvas, seeing the image i was creating as completely flat then shifting to make the image occupy a three dimensional space within the canvas. This past year i have developed a process of making that has allowed me to experiment with the opposite, taking three dimensional objects in space and shifting my perception see the composition as a flat image.