Interchangeable focusing screen
Many SLRs and medium-format cameras have interchangeable focusing screens. The benefit is that you can swap the standard split screen for a different pattern to suit the type of photography you are doing. The most common is a grid screen that has a grid of thin lines used to align horizontal and vertical points in the scene. This is a popular choice for architectural photographers and for copying. A plain screen is used for microscope and telescope work, and some prefer a fresnel for focusing with long telephoto lenses where the split screen version may black out.