Resolution of the human eye (iii) – things that affect visual acuity

So now we have looked at the number of overall pixels, and the acuity of pixels throughout that region. If you have read the last two posts, you, like me, might surmise that there is no possibility of associating a value with the resolution of the eye. And you would probably be right, because on top of everything else there are a number of factors which affect visual acuity.

  1. Refractive errors – Causes defocus at the retina, blurring out fine detail and sharp edges. A good example is myopia (short-sightedness).
  2. Size of pupil – Pupils act like camera apertures, allowing light into the eye. Large pupils allow more light in, possibly affecting resolution by aberrations in the eye.
  3. Illumination of the background – Less light means a lower visual acuity. As cones are the acuity masters, low light reduces their capabilities.
  4. Area of retina stimulated – Visual acuity is greatest in the fovea. At 2.5 degrees from the point the eyes are fixated upon, there is approximately a 50% loss in visual acuity.
  5. Eye movement – The eyes move, like all the time (e.g. your head doesn’t move when reading a book).

Complicated right? So what is the answer? We have looked at how non-uniform acuity may affect the resolution of the human eye. The last piece of the puzzle (maybe?) in trying to approximate the resolution of the human eye is the shape of our visual scope. When we view something, what is the “shape of the picture” being created. On a digital camera it is a rectangle. Not so with the human visual system. Because of the non-uniformity of acuity, the shape of the region being “captured” really depends on the application. If you are viewing a landscape vista, you are looking at an overall scene, whereas reading a book, the “capture area” is quite narrow (although the overall shape of information being input is the same, peripheral areas are seemingly ignored, because the fovea is concentrating on processing the words being read). To provide a sense of the visual field of binocular vision, here is an image from a 1964 NASA report, Bioastronautics Data Book:

This diagram shows the normal field of view of a pair of human eyes. The central white portion represents the region seen by both eyes. The dashed portions, right and left, represent the regions seen by the right and left eyes, respectively. The cut-off by the brows, cheeks, and nose is shown by the black area. Head and eyes are motionless in this case. Not quite, but almost an ellipse. But you can see how this complicates things even further when trying to approximate resolution. Instead of a rectangular field-of-view of 135°×190°, assume the shape of an ellipse, which gives (95*67.5)*π = 20145, which converts to 72.5 megapixels for 1 arc minute sized pixels – which is marginally lower than the 75 megapixels of the bounding rectangle.

So what’s the answer? What *is* the resolution of the human eye? If you wanted a number to represent the eyes pixelation, I would verge on the conservative side, and give the resolution of the eye a relatively low number, and by this I mean using the 1 arc minute acuity value, and estimating the “resolution” of the human visual system at somewhere around 100 megapixels. This likely factors in some sort of compromise for the region of the fovea with high acuity, and the remainder of the field of view with low resolution. It may also take into account the fact that the human vision system operates more like streaming video than it does a photograph. Can the eye be compared to a camera? No, it’s far too complicated trying to decipher a quantitative value for an organic structure comprised 80% gelatinous tissue.

Maybe some mysteries of the world should remain just that.