iMac 27" Mid-2011 monitor blurry issue

I noticed that my iMac's monitor is a little blurry (resolution 2560 x 1440) especially when I look at the texts on any browsers and other apps. But, I look at the folder icons on the desktop screen which look fine. How can I make it clear on the window (i.e. Safari, Firefox, other apps) ? Or, is it me that I need a reading glasses?

Do you have the same problem?