JasmineMasown said:
Wouldn't both of them appear blurry? I mean, we can't see individual light rays. Do you mean in any other way?
Let us first consider looking at the far away star.
There is no particular problem getting tight focus on an image coming in from infinity. The light from that star enters our eyes. (A portion of the plane wave a few millimeters in diameter comes in through our pupils). It is focused by the lens so that a real image is produced on the retina. Light that enters from the left and right sides of the pupil is deflected to arrive at the same spot. Light that enters from the top and bottom sides of the pupil is also deflected to arrive at that spot.
One (or maybe a tight cluster of a few) rods and cones are illuminated. A pixel on our retina is activated. That is what it means for the image to be sharp.
There are muscles in the eye that can stretch the lens in the eye to change its focal length slightly. In most humans, they can relax sufficiently to get proper focus for an image at infinity. Some near sighted individuals may need correction (glasses) to achieve this.
Let us switch to consider an LED at close range.
There is a limit to how hard the muscles can pull and how close a real image can be so that focus is attained. In my youth, I could get down to perhaps 3 or 5 centimeters. These days it is more like 15 to 30 centimeters.
For me, an LED held a few centimeters from my eyes is sure to appear blurry while the star will not.
My insufficiently stretched lens is not strong enough. Its focal plane would be behind the retina. Light from the LED that enters on the left side of my pupil illuminates a spot that is a bit leftward on the retina. Light from the right illuminates a spot a bit right. Top illuminates top. Bottom illuminates bottom. Instead of illuminating a single rod or cone or a tight cluster of rods and cones, a fuzzy region is illuminated instead. The LED will appear blurry.
You do understand that what we "see" is the result of activation of an array of illuminated pixels on the retina. An image which is then heavily post-processed by the optical cortex where we get things such as edge detection, auto focus, auto binocular tracking, color scaling, motion detection and various more sophisticated things.