Original: Focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity.
Simplified: Focusing the image would also cause the sun's image to be focused on a few receptors. These could possibly be damaged by the intense light; shielding the receptors would block out some light and  reduce their sensitivity.