Why does my vision get blurry after reading for 30 minutes?

Great question. There could be more than one explanation. Several things happen when we read. They are automatic and we have no control over them. The pupils get smaller, the eye changes focus to near, and the two eyes come together slightly. If the lighting is poor, the small pupils may make it hard to see, causing you to notice the blur. The focusing power is measured in diopters; largely, it is controlled by the muscles in the eye when a focusing change is needed. If someone is nearsighted, they don’t use these muscles much and they are weak, causing blur after a while due to fatigue. If the person is farsighted they need these muscles to see in the distance and the muscles may fatigue with prolonged use at near because they are forced to work at near maximum capacity. Your eyes' ability to focus lessens with each passing year. This is why so many people require reading glasses in their 40s, so you may need reading glasses or bifocals, or a stronger prescription than what you now have.

In addition, the blink rate slows down when reading, so someone with dry eyes may become symptomatic when reading. Artificial tears used regularly are a great help. Use good posture, an ergonomically correct chair, good lighting, and don’t sit with the fan, heat, or air conditioning blowing directly on you.

See your ophthalmologist for a thorough evaluation and corrective measures.

This question was originally answered on Aug. 12, 2014.