When I look at a piece of text, I can see all the text on the paper, no matter where I look, because my field of view covers it all. However, if I stare at a specific word, I cannot read the text a few lines above the position I am staring at, even though I can clearly see there is text there. I don't think it has anything to do with the text being out of focus, since I observe this phenomenon no matter what distance the paper is from me. What are the specific causes of this?
The field of view is determined by the light-receptive parts of the eye: the retinae (Kolb, 2012). The fovea is the region with the highest resolution. It is located in the central part of the retina and covers only about 2 degrees of the field of view (Lauweryns, 2012). The total field of view is roughly 180o (Fig. 1).
Fig. 1. Schematic of the human field of view showing the foveal region. Source: Medical Dictionary.
The reason for the regional differences in retinal resolution (referred to as visual acuity) is that the cone photoreceptors are densely packed in the foveal region, while rod photoreceptors predominate the retina outside the fovea region (Fig. 2) (Kolb, 2012).
Fig. 2. Distribution of rods and cones. Source: Webvision.
The cones sense color and are specialized to work under well-lighted (photopic) conditions. In contrast, rods are hardwired to converge onto a single optic fiber to collect responses for dim lighting (scotopic, or night vision). Because many rods (tens of rods) connect to a single optic nerve fiber, the spatial acuity is dramatically reduced (Fig. 3). Hence, you can read with only a few degrees of your vision. While reading, your eyes continuously scan the page for the next word to center it onto the foveal region.
Fig. 3. Many rods converge onto a single ganglion cell, while one cone synapses onto a single ganglion cell. Source: Slideshare.
This is the big handicap that people with age-related macular degeneration face. The macula (that includes the foveal region, see Fig. 1) degenerates and the cones are lost. Hence, they loose their high-acuity region and cannot read anymore without using magnifiers or other low vision aids.