When you change between looking at close objects and looking at distant objects, it is the lens in the eye that adjusts to keep the image sharp and detailed. After passing through the cornea, the pupil and the lens, the light beams end up on the lining at the back of the eye. The retina is like the film in a camera. The retina turns the light beams falling on it into electrical nerve signals. The retina does not work in the same way across the whole of the eyeball.
Light coming from different places falls on different parts of the retina.
If you look straight at something, the light will fall on the most central part of the retina. If you are looking at something 'out of the corner of your eye' then the light falls on the sides or edges of the retina. The centre of the retina is the most sensitive to detail and to colour. This means that you get the most detail when you look directly at something. You can still see things even when you are not looking directly at them, but not in so much detail and you can't judge the colour so well.