DSLR focusing screens
Some decades ago, auto-focus was non-existent and photographers focused their lenses manually. Some still do, especially macro photographers.
Earlier photographers relied on focusing screens with special characteristics designed to help manual focusing. Modern focusing screens generally show the metering zones available with the camera as well as the focus points in use, but little more.
Many digital single lens reflex cameras allow the user to replace the provided focusing screen with others as needed. Some replacement focusing screens simply provide a grid to aid with compositions, while others can offer a split-screen or microprisms in the center.
The latter two greatly simplify manual focusing, and a seasoned photographer can focus manually about as fast as most auto-focus systems using these focusing screens.
There are companies that make custom focusing screens for many DSLRs. Customization includes features such as specialized grids and crop lines and brightness enhancement.Katz Eye Optics.