The tolerance of placement of the image plane (the sensor or film) in relation to the lens. While depth of field refers to the zone of acceptable sharpness in front of the lens, focal depth refers to the corresponding zone behind the lens. A smaller aperture increases this tolerance, making sensor alignment less critical.
Heard on the job
The focal depth is incredibly tight on this f/1.4 prime, so the sensor needs to be perfectly shimmed.
Also heard as
- depth of focus
Do not mix it up with
- Focal plane: Focal depth is a tolerance range, while the focal plane is the specific flat surface where the image is sharpest.
Related terms
Focal plane
The flat surface perpendicular to the optical axis where an image is sharply formed.
Focal point
The specific point in space where collimated light rays converge after passing through a lens.
Focus
The point where light rays converge to form a sharp image, or the state of being sharply defined.