Depending on your age, it could be the onset of presbyopia. I am having the same experience right now - my eyes have "reversed" to some extent (I have always been rather nearsighted)...now, I have the same difficulty with scopes. Iron sights are a real problem - as the most difficult distance for my eyes to focus....seems to be exactly in the range of where the sights on my rifle are mounted. I am working on a solution - probably by having a special pair of glasses made (progressive lenses, with the "middle range" set for about 3 - 5 feet).