The more round the lens, the more it focuses light originating closer to the eye. Light that comes from objects that the eyeball is pointed directly at focus on the fovea of the eye. It is a specialized place on the retina that does not have layers of nerves over the receptors, nor blood vessels. It has closely packed cones, no rods, and gives us our clearest vision in daylight. All this reminds me of the only triple pun that I know.
Some hawk eyes have a linear fovea that allows for more acuity.
Fish have spherical lenses that generally move in and out to focus. There are several types of invertebrate photoreceptors. I would like to just mention some of the more advanced eyes of invertebrates. The most advanced is that of the octopus or squid. It is very much like a vertebrate eye except the photoreceptors are on the surface of the retina, not near the pigment cells.