Depth of field (DOF) refers to the portion of a photograph that is in focus and appears sharp. It is the distance between the nearest and the farthest objects in a scene that appear acceptably sharp in an image. The depth of field is determined by the aperture setting on a camera's lens, the distance between the camera and the subject, and the focal length of the lens. A shallow depth of field can be used to create images with a softly blurred background that draws attention to the main subject, while a deep depth of field will keep more of the scene in focus from front to back.
Here are a few online tutorials that provide a comprehensive explanation of depth of field and how to control it in your photography:
B&H Photo Video: "Understanding Depth of Field in Photography" (https://www.bhphotovideo.com/explora/photography/tips-and-solutions/understanding-depth-field-photography)
Photography Life: "Depth of Field Explained" (https://photographylife.com/depth-of-field-explained)
Adorama: "A Beginner's Guide to Understanding Depth of Field" (https://www.adorama.com/alc/a-beginners-guide-to-understanding-depth-of-field)
These tutorials will help you gain a deeper understanding of depth of field and how to use it to enhance your photos.