The process of image retention is called after image and works like reaction time for the retinal tissue to react or to recover. This is a perfectly normal physiological phenomenon and will have no effect at all in future on your vision.
When eyes are exposed to bright light, the eyes take time to recover as you may have discovered when you go to a cinema in the day time. When you enter a dark cinema hall, you don't see much, but after about 10 minutes or so you are able to see a lot more in the same dark room.
Another example of this after image is when you see a fan running or spokes of a wheel running; after a certain speed the whole thing looks continuous. For the same reason when a pilot is flying a plane in almost total darkness at night in the cockpit, when he/she lands, the runway lights are bright; in order for quicker recovery they may have to wear red goggles. There are many examples of this in real life. In your case, make sure you are wearing proper reading and/or distance glasses to read comfortably. You cannot treat this after image as this is a physical property of all the tissues including nerve conduction in the body.