Check out this great graphic detailing important steps in the evolution of a visual system, created by Voltier Creative:
I think it does a really good job of presenting the classical example of eye evolution by gradual changes, each stage of which is observable today in nature. The only thing I take any exception to is the “most evolved” statement regarding the mantis shrimp eye. Truth be told, many biologists use the same language when talking about any especially complex biological system. However, that doesn’t really sit right with me. Everything has been evolving for exactly the same amount of time. Nothing is more or less evolved, and complexity does not necessarily mean that more evolutionary “work” has gone into it. Modern animals with less complex eyes, like Euglena, are not less evolved; their visual system is continuously evolutionarily tuned to be as close to ideal for its lifestyle as possible.
A better way to put it would be something along the lines of, “mantis shrimp have the most diverse photoreceptor array of any animal.” Furthermore, mantis shrimp don’t really represent sight perfection either. Because of damning resolution limitations inherent of compound eyes, almost any animal with a camera eye has much better spatial resolution. Even through they have the second best spatial resolution among animals with compound eyes (behind dragonflies), mantis shrimp are still well below humans in that regard.