Why is my viewfinder blurry on my Nikon D600?

ADJUST YOUR DIOPTER:

Next to your viewfinder, there is small dial with a plus(+) and minus(-) sign next to it. This is called the diopter and it measures the optical power of the lens. In other words, it is almost like putting a set of glasses on your camera to sharpen what your eye sees through the lens.

This will have no effect on your images (obviously, as you've noticed since your images are still turning out nice and sharp). Just simply adjust this so that what you are seeing comes into focus and you can effectively tell what is in focus and what isn't with your eye alone. This is important if you are doing any manual focus with your lens at any point.

The diopter adjustments are not drastic and most likely will not replace any need you have for wearing glasses. The adjustments are just enough to increase the clarity if it seems a little soft when you look through the camera.

As a little side note, if you are using someone else's camera with diopter adjustments, don't be surprised if you start to get a headache. Your eye is not used to the optical adjustment being made by their diopter. It's just like wearing someone's prescription glasses.

For more information visit Nikon D600 Manual.