A focusing screen is a glass or plastic screen at the camera's image-forming plane. This is where the image can be viewed and focused, but your image needs to be bright enough to see properly. There are various types of screens available for different situations. These include plain matte, plain matte with grid, horizontal split image, horizontal split image with grid, diagonal split image and diagonal split image with grid. Most cameras have interchangeable screens which differ in brightness.
Single lens reflex (SLR) cameras are made so the distance between the focusing screen and the lens is the same distance as between the lens and film. Whatever appears in focus on the screen will be recorded in focus on the film.
Plain focusing screens look similar to traditional ground glass screens and give you an unobstructed image. They are useful for outdoor photography. Split image screens, situated horizontally or diagonally, give you centrally located focus assistance. These are ideal for wedding and portrait photographers. Plain and split image screens are generally available with an optional grid pattern for architectural and types of precision photography. Your screen needs to give you accurate focusing and depth of field assessment. Focusing screens range in price from about $30 to a few hundred.