Can a focal length or focal point be constant on a particular lens?
Unless the lens itself is flexible (as is the case with one’s eye), then its focal length is going to be constant. Most of the lenses that we make are made of some rigid material. This means that the curvature of the lens is going to remain constant. This in turn means that the focal point will be constant. The only way by which to change the focal length of the lens would be to grind it to a different curvature or to immerse it in a media with a different refractive index. In the first case, you have basically made a new lens whose focal will remain constant at the new value. In the second case, immersing the lens in a different media will result in a different focal length, but once again, it will be constant as long as it is in that media. The only way in which you could truly get a lens with what I would consider a variable focal length would be if the lens is flexible enough to readily change shape. A change in shape causes a change in curvature, which results in a change in focal
