Like, if you are staring at a moving object, your eyeballs turn along the direction of that object, smoothly and continuously. But when you try to do it when not looking at a moving object, you can’t, your eyeballs jumps in increments. Why is that? My hands can move smoothly whether or not they are following something. Why not my eyes?
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It’s worth mentioning that only a part of the back of our eye has any real detail capacity so we tend to move our eyes to keep that bit lined up with stuff that is important. And that a proportion of our field of vision as we experience it is practically illusory – we spend a lot of time functionally blind or a large area of our area of vision not at all detailed or accurate as it seems.
But if it’s of interest. And bearing in mind I claim no expertise.
There seem to be 4 types of eye movement.
Saccades are the voluntary/involuntary jerky movements in which we calculate where we need that focus next or covers a field of view making it seem all detailed by constantly moving here and there – very quickly. We are , I think, actually ‘blind’ each time the eye moves like this until it stops again.
Smooth pursuit movement more slowly follows a target keeping it lined up with that area of detailed vision and it’s generally only when there is something to follow – you have to train to be able to make this movement without an object. Sometimes when it loses the object it’s following a saccade brings the eyes back to the track. I’m not sure if we know whether this is an entirely separate ‘brain’ process or a different way of using the same processes.
The other two keep the eyes focused on somewhere despite our head moving and something to do with different objects at different distances.
So apparently a part answer is that we can train ourselves to move our eyes smoothly and continuously. And an unsatisfactory one is somehow that’s how evolution worked out for us. It’s always a bit dodgy making up evolutionary narratives but presumably it’s more efficient to move eye rather than your whole head and it’s useful to both be able to rapidly scan a whole field of visions looking for changes , and to be able to smoothly focus on and follow it , or be able to move ourselves and yet stay focused on something. So we have specialised process for each of them. Exactly how separate the underlying system for these things is seems to be under discussion.
Apologies for any errors of if it’s not ELI5 enough.
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