In digital photography, the term Depth of field is the zone within a photo that appears sharp and within focus. In a photograph, there is a point of focus, where the lens actually focused. But there is also an area both in front of, and behind, the point of focus that also appears sharper. That area correlates with the depth of field. In other words, Depth of field is the area in an image where objects appear acceptably in focus or have a level of acceptable sharpness. The main factors to determine depth of field are aperture(f-stop), distance between the lens and subject, and the lens' focal length.

Depth of field formula

DOF = 2u2Nc/f2

DOF - depth of field

u - distance of subject

N - f-number

c - circle of confusion

f - focal length

Aperture is a hole in camera lens through which light enters the camera and reach to the sensor. The f-stop values are normally f2.8, f3.5, f4, f5.6, f8, f11, f16, f22 etc. The smallest f-numbers means the widest apertures and therefore the shallowest depth of fields. And the larger f-numbers means to the narrowest apertures and therefore the deepest depth of fields. For landscape photography we need higher f number, e.g. f22. For portrait photography we need smaller f number e.g. f2.8.