I am having trouble understanding the meaning of focal length. I thought focal length was the distance between the lens and image sensor. These sources validate that definition: http://ksimek.github.io/2013/08/13/intrinsic/ http://imaging.nikon.com/lineup/dslr/basics/19/01.htm
However, I'm not sure how this works with the Lensmaker's Equation:
The lensmaker's equation calculates focal length as a function of the lens geometry and refractive index.
I'm wondering if refocusing a camera will change the focal length. By definition 1, if we move the lens closer to the sensor to focus, it will change the focal length. But by definition 2, the focal length is intrinsic to the lens and will never change.
How do these two definitions relate? Even if we assume the camera lens is thin, the thin lens approximation is still a function of the lens geometry, not the distance from the lens to the sensor.