We have surprisingly low visual acuity (resolution) in parts of the visual field that are not at the center of where we are looking — the center of gaze. We are not aware of this because we usually move our center of gaze to whatever we want to look at.
The center of gaze, called the "fovea," has a higher density of cones than anywhere else on the retina. In fact, there are no rods at all. (In the diagram at right, the cones are shown in green.) The fovea evolved to have the highest possible visual acuity, and the cones are as small as they can possibly be, and still be alive. Moreover, in the fovea, the retinal ganglion cells have smaller receptive fields, and in the periphery, they have much larger receptive fields.
The density of cones in our center of gaze is shown in the graph above. The peak is on our fovea. The edges of the graph are our peripheral vision.
The fact that our vision has the highest acuity in the center of gaze does not mean that vision in the rest of the visual field is inferior — its just used for different things. Foveal vision is used for scrutinizing highly detailed objects, whereas peripheral vision is used for organizing the spacial scene, and for seeing large objects. Our foveal vision is optimized for fine details, and our peripheral vision is optimized for coarser information.