Why do small fonts strain the eyes?

[Ganesh Ujwal]

I understand that too much close reading will strain the ciliary muscles of the eyes. But what about small fonts? The contraction of the ciliary muscles is dependent on the distance not font size.

In other words, it is better to read from a closer monitor with bigger font or from a further monitor with smaller font?

 

[Doug Huffman]

With a small image small motion errors will cause misinterpretation and continuous focus efforts tiring that ciliary muscle. Keep your monitor distance comfortable, increase its brightness (smaller pupil larger depth of field) then increase font size. How about display contrast, anti-aliasing, greeking etc?