Presbyopia - the lens in our eyes begin to harden at age 40, making it harder for our eye muscles to "squeeze" the lens to focus. Presents itself as blurriness and eye strain. We usually encounter a change in our Rx during this time too, sometimes needing a weaker Rx (counterintuitively).

Would love to chat about your experience in any case. I'm at jbornhorst [at] gmail.com if interested.