This is always an interesting question, but sometimes tricky to explain. The thing is, we all have different eyes and eye strength. The question is more about what is actually causing weakness of the eyes when reading.
When we read our pupils get smaller which helps with our focus on the smaller print and our eyes change focus to accommodate the smaller print for near sightedness. If our pupils are smaller we are allowing less light to enter the eyes. 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. It is controlled by the muscles in the eye when a focusing change is needed. If someone is near sighted, they don’t use these muscles much and they are weak, causing blur after a while due to fatigue. If the person is far sighted 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.
Vision Works has just the answer for you. Pop into a Vision Works near you and get your eyes tested for R90 a pop. Love your eyes, it’s the right thing to do!