We see those variations everywhere in nature - eyes without blind spots (octopuses), eyes that see more colours than we do (some birds, Mantis shrimp), cells that only detect the presence of light (remember, seeing is detecting light, nothing more), eyes that are holes with light sensitive cells so the animal knows where the light comes from and within species we find variations too - some humans are colour blind, some see better than others, we have different cone photoreceptor packing density and we have a huge variation in eye colours (all these are variations!), but because our 'tribes' keep interbreeding and it takes a huge time for a species to grow apart even if they are isolated I doubt that we would see a HumankindA who has different eyes than HumankindB and can't have kids with HumankindB (speciation) in the future.