Big depth of field is a photography term that describes an image in which the entire scene, from foreground to background, is in sharp focus. 

This is achieved by using a narrow aperture (a higher f-number) to increase the depth of field, which is the distance between the nearest and farthest objects in a scene that appear acceptably sharp. 

A big depth of field is commonly used in landscape photography, where the goal is to capture the full breadth and detail of a scene, as well as in product photography, where sharpness and detail are important for showcasing the product. 

A big depth of field can also be used creatively to create a sense of depth and context in an image, or to emphasize a particular element of the scene by keeping it in sharp focus while blurring the background or foreground. 

Achieving a big depth of field requires careful attention to composition, focus, and lighting, and is influenced by factors such as the lens focal length and the distance between the camera and the subject.