In video games, a relatively common graphics setting is “Depth-of-field”. Unfortunately, many games don’t offer any explanation as to what each graphics setting actually does, so people just enable or disable settings without understanding what they do.
When your eyes focus on an object, it and everything at the same distance, are in focus. If you want to look at a nearer or further object, however, your eyes need to focus on the new distance. Everything outside of this focal distance will appear to gradually get further and further out of focus. This is the nature of how lenses work, including the ones in your eyes and the lenses in cameras.
In video games, however, there isn’t a lens involved in the process of displaying an image on the monitor. This allows the entire image to be rendered perfectly in focus. Whether this is a good thing or not is a matter of personal opinion.
Depth-of-field can be used to draw attention to a specific part of the scene, such as the characters in a conversation while reducing the focus on events in the background. Some people regard depth-of-field as increasing the cinematic appearance of the game.
In scenarios where the player is in control of the camera, depth-of-field filters use the crosshairs or the center of the screen as the focal point. A blurring filter is applied to objects both nearer and further from the player in an attempt to match how your eyes work in the real world. Unfortunately, players don’t always want to look exactly at the centre of the screen and may want to look at or notice details in the background. With depth-of-field enabled in this scenario, players are unable to change the focus distance without physically moving the camera. Many people also find the changing of the focus distance to be distracting and to reduce the overall graphical fidelity of the game. As such a lot of players choose to actively disable depth-of-field, whenever it is an option.
Enabling depth-of-field creates a filter that must be applied to every frame that is displayed, which can come with a performance impact. While the exact amount of lost performance can vary, it is generally relatively low, in the order of a couple of frames-per-second.