The focusing problem you describe may be an early symptom of presbyopia, an age-related change in vision. Presbyopia is not the same as farsightedness. In presbyopia, your eyes gradually lose the ability to adjust to see objects clearly at different distances. Although the most common presbyopic symptom is blurry close-up vision, you may also experience blurred distance vision when changing your focus from near to far objects.
If you're experiencing this problem after prolonged close-up work, such as reading or working at a computer, try resting your eyes every 10 to 20 minutes by closing them for 20 to 30 seconds. This may help minimize the symptoms. If you don't normally wear prescription glasses, you may consider getting a pair of nonprescription reading glasses that can be used for close-up work.
If you're concerned about these vision changes, make an appointment with your eye doctor. An eye exam can identify potential eye or vision problems that may require further evaluation and treatment.