Are you familiar with this little wheel with a + and – symbol and a squashed looking T, on the edge of your viewfinder on your DSLR? It’s the diopter adjustment wheel, and put simply, you use it to make sure that the viewfinder is in focus for your eye.
If (like me) you wear glasses, this little wheel is a lifesaver in making sure you produce beautiful crisp sharp images without having to squish your glasses between your face and the camera every time you want to take a shot.
To adjust it, have your camera set to autofocus, and choose the central point of focus. Mount it on a tripod or set it on something stable, and focus the shot by depressing the shutter half way, or use your back button focus button if you have set it up. Without moving the camera (this is very important!) look through the viewfinder. Are the symbols (focal points and focal circle) sharp? Is the focal subject (the object that you chose to point the central focus button towards) of the image sharp? If not, move the wheel on the side of the viewfinder up or down until it is. It’s as simple as that!