Which factor does NOT generally affect the focal length of a lens?

The focal length of a lens is primarily determined by the curvature of its surfaces, the material from which it is made, and its thickness. The curvature affects how light is bent as it passes through the lens, which directly impacts the focal length. Similarly, the lens' thickness influences the path of light rays, thus affecting focus.

However, the distance from the lens to the eye does not inherently alter the lens' focal length. Instead, it can change the effective magnification and perception of the image but not the fundamental optical properties of the lens itself. Thus, it is accurate to say that this distance is not a factor that alters the lens's intrinsic focal length, aligning with the correct answer.