The back focal length of a camera lens is the distance between the final lens element and the image plane.
As any photographer knows, the lens you use to take a picture has a critical impact upon the image produced. Though this is true for many reasons, the most obvious effect on the image comes from the lens' focal length, which is the measurement of how far away the objective lens is from the image plane--be it film or digital sensor. The back focal length is an element of this, measuring how far away the final lens element is from the image plane.
Instructions
1. Find the radii of curvature of the first and last lens surface in the system as well as the lens' thickness and refractive index. All of this information should be on the data sheet for the lens. Going forward, the radii of curvature will be referred to as "R," the lens' thickness as "d," and the refractive index as "n."
2. Determine the lens' focal length. This is the length printed on the barrel of a photographic lens in millimeters, such as 50mm for a normal-length 35mm film lens. It may be listed as the effective focal length, or EFL, on the lens' data sheet.
3. Subtract 1 from the lens' refractive index, "n."
4. Multiply the difference by the lens' thickness in millimeters, "d."
5. Divide the resulting number by the product of the lens' refractive index and the radii of curvature, "R," for the front lens surface.
6. Check the result against the actual distance from the lens' rear element to the existing imaging plane, if it is used in an existing system. This will let you know if adjustments need to be made to the lens to adjust its back focal distance, or if it will function with a specific camera setup, as many camera systems have different back focal length requirements.
Tags: focal length, refractive index, back focal, from lens, lens thickness