The state or quality of having or producing clear visual definition and focus is used a lot in photography to direct the viewers attention to something. By using a depth of field you can create focus as it blurs the background and the object appears clearer. It’s practically married to sharpness because if an image that is completely sharp is said to be in focus but if an image that’s completely blurry is said to be unfocused. The same metaphor applies to your mind when you concentrate your mind is sharp and you are focused but when you’re confused you lack focus. There are a lot of situations where you don’t really want the entire image to be in focus but backgrounds tend to get in the way and they will distract your viewer from the point you are trying to make. There’s no better way to see how the depth of field can change than to simply try out a bunch of different aperture values. Your camera’s aperture priority mode is ideal for this because you can pick any aperture value you want and your camera will pick a corresponding shutter speed to give you the right exposure. When a photographer uses the term point of focus he is referring to that object in a picture at which he wants to draw the most attention. Every photograph has a specific location at which the photographer wants the viewer's eye to rest which can be seen through the lens of the camera which can be a single point or multiple points.