Unlike humans who have a single lens in each eye, butterflies have compound eyes with multiple lenses, creating a mosaic-like image. While humans have the ability to focus on something far away and can see fine details, the vision of butterflies appears blurrier.
Butterflies can see a broader range of colors than humans, including ultraviolet light, which helps them e.g. to locate patterns in flowers that we cannot see. They are also able to see a wider view around them rather than just in front of them, allowing them to detect and escape threats.
The black ring structure in my photograph is the proboscis, tightly curled up.