Demystifying Depth of Field (DoF) in Photography
Depth of Field (DoF) is the distance range in a photograph that appears acceptably sharp and in focus. While a lens can only focus at a single precise distance, the zone of acceptable sharpness extends both in front of and behind that focus point. DoF is a critical creative tool in photography: portrait photographers use shallow DoF (blurring the background) to isolate their subject, while landscape photographers seek deep DoF to keep everything from the foreground to the distant horizon sharp.
Four variables dictate depth of field: focal length, aperture (f-stop), subject distance, and sensor size (specifically, the crop factor, which determines the Circle of Confusion). Longer focal lengths, larger apertures (smaller f-numbers), closer subject distances, and larger sensor sizes all decrease depth of field, leading to a blurrier background.
The limit of what is considered "acceptably sharp" is governed by the Circle of Confusion (CoC). This is the maximum diameter of a blurred point on the sensor that still looks like a sharp point to the human eye on a final print. For a standard 35mm full-frame sensor, the default CoC is 0.030 mm. For smaller sensors (like APS-C or Micro Four Thirds), the crop factor is applied to reduce the CoC size because the final image must be magnified more, amplifying any focus errors.