Is it normal that i sometimes can't focus my eyes?

Sometimes when I need to be very diligently staring at something (say watching a forest horizon for movement) I am unable to focus my eyes. Everything just gets blurry and I quickly focus again but it just goes back to blurry. It is like I have no control over my eyes spacing out. It happens when I am trying to pay attention to very minute detail that usually doesn't involve using my hands. Sometimes it happens while I am reading, my eyes just don't focus and they want to space out. Is this indicative of some sort health problem? What's up?