Does The Focal Length F (Lens Length) Depend Only On The Physical Characteristics Of The Lens?

to be more specific, does it depend on the lens and the imager (distance between lens and imager? or does the incoming parallel rays always converge at the position of the imager (whatever this position is)?

The focal length is the distance between the lens glass (internal) and the image sensor. The focal length is not equal to the lens length (housing). With a fixed focal lens the focal length only depends on the physical characteristics of the lens. With a varifocal lens the focal length depends on the adjustment of the lens. The range of the focal length depends on the physical characteristics of the lens.

The beams do not converge at the position of the image sensor but internal in the lens. If I am correct the focal length is the distance between this converging point and the image sensor.

I found a pic: lens cross section

I was unable to upload so I included the link.

Thanks for the information.

When I compare your picture and the one in page 2 of class#1 document (see below) then it seems to me that:

- the sensor can be positioned at or after the convergence point. The sensor cannot be positioned before the convergence point;

- the convergence point can be, in some cases, within the lens. In these cases we have "focal length= (distance between sensor and point of convergence)= (distance between sensor and lens)

Please let me know if these two points are correct or not.

Its between the imager and the glass lens, not the physical characteristics of the lens..

No, it depends on the distance between the lens and the imager sensor

 Camera with a shorter focal length has greater optical power than one with a long focal length. It is the distance between the lens and the sensor. 

No, not just the physical characteristics it depends on placement of the lens.