Butterflies cannot focus their vision as what they see is at best a blur. Furthermore, they are sensitive to only the three most basic features of vision which would be light, color and motion. Butterflies can distinguish night from day. They might distinguish color along a very narrow band of the light spectrum. Hence a butterfly might see and feed from the nectar of a purple flower but be oblivious to the red flower nearby.
Butterflies are also sensitive to movement.
Butterflies do not have teeth or mandibles. Rather, their feeding mechanism is a long double barreled tube called a proboscis. Because they feed through what is essentially a double barreled straw, butterfly diets are exclusively liquids. The preferred diets will vary considerably depending upon the butterfly species. While people generally think of butterflies as feeding from the nectar of flowers, other common diets include mud, cow dung, water and tree sap.