Back focal length is mainly a mechanical/compatibility specification, not something that directly changes focal length, aperture, or angle of view.

In simple terms, it describes the distance from the rear of the lens optics to the image plane/sensor area. A shorter back focal length usually means the rear of the lens sits deeper inside the camera body. That matters most on cameras with a moving reflex mirror: if the lens protrudes too far, it can interfere with the mirror.

That’s why this spec is important on SLR-style systems, and much less important on cameras with no mirror, such as many industrial, cine, microscope, or surveillance cameras using C-mount lenses.

So for two lenses with the same focal length and aperture, a different back focal length usually does not mean a different field of view. It mostly reflects a different optical design and how much clearance the lens needs behind the mount. In everyday use, the main concern is whether the lens physically fits and clears anything inside the camera body.