If you wear glasses, you may find that using the viewfinder on you digital camera is difficult. They lenses in the view finder are made to focus at a certain point, and often glasses may interfere with this.
One thing you can try is to adjust the diopter on the eyepiece to match the prescription for your eyewear. Adjustinging the diopter will adjust the focus of the view finder so that when you look through the view finder, the image is in focus without the aid of your eyeglasses. If the diopter adjustment is not enough, some camera manufacturers also sell an diopter accessory for further adjustment.
If you do not use glasses but find that the focus is off on your camera when using the autofocus mode, but the images are in focus when you review them later, or when using manual focus the image in your viewfinder is in focus, but out of focus when you review them later, check your diopter to be sure that it has not been modified. To do this, I press my focus spot selector to light up the focus indicators in the view finder, then adjust the diopter until they are sharp.
Maintaining all facets of your equipment will help ensure your images are everything you want them to be.
Until next time, happy shooting.