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Back Focus

Back focus refers to the distance between the rear surface of a lens and the image sensor or film plane of a camera when the lens is focused on an object at infinity. This distance is crucial in photography and optical systems, as it affects the ability to achieve sharp focus at various distances.

Understanding Back Focus:

  • Definition: Back focus is measured from the rear element of the lens to the focal plane (image sensor or film plane) of the camera when the lens is focused at infinity.
  • Importance: Proper back focus adjustment ensures that when the lens is focused on objects at different distances, the image remains sharp and in focus according to the intended focal point.

Key Points:

  • Adjustment: In some optical systems, especially in professional cameras and lenses, back focus can be adjusted to ensure accurate focusing at different distances.
  • Critical in Imaging: Incorrect back focus can result in images that are out of focus or not properly aligned with the intended focal plane, leading to blurry images or loss of sharpness.

Factors Affecting Back Focus:

  • Lens Design: Different lens designs and types (e.g., prime lenses, zoom lenses) may have varying back focus distances.
  • Camera Setup: Back focus can be influenced by the camera body and its alignment with the lens mount.

Practical Applications:

  • Photography: In photography, especially in portrait, landscape, and macro photography, precise back focus adjustment ensures sharpness and clarity in images.
  • Broadcasting and Cinema: In professional video and cinema cameras, accurate back focus adjustment is critical for maintaining sharp focus during filming.


Back focus is a fundamental concept in photography and optical systems, determining the distance between the rear lens element and the focal plane when the lens is focused at infinity. Proper adjustment and understanding of back focus are essential for achieving sharp and accurate focus across different distances and settings, ensuring high-quality images and consistent performance in various photographic and imaging applications.

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