focal length calculation

I am trying to calculate the focal length of the lens in my digital camera.

I understand that a smaller number such as 18mm is wide angle and a larger
number such as 200mm is a tele-foto lens.

If I calculate the angle of view of the camera, can I calculate the focal
length?
If so, how - and do I use the horizontal angle or the vertical plane
(assuming the camera is used in landscape).

Is this calculation an absolute or does it differ for 35mm cameras and
digital cameras?

Thanks

Also, If i add a 0.5x adaptor, does this half the focal length, or does it
half the area (i.e. the square of the increase in focal length)?


"TS" wrote in message
...
I am trying to calculate the focal length of the lens in my digital
camera.

I understand that a smaller number such as 18mm is wide angle and a larger
number such as 200mm is a tele-foto lens.

If I calculate the angle of view of the camera, can I calculate the focal
length?
If so, how - and do I use the horizontal angle or the vertical plane
(assuming the camera is used in landscape).

Is this calculation an absolute or does it differ for 35mm cameras and
digital cameras?

Thanks

Also, If i add a 0.5x adaptor, does this half the focal length, or does it
half the area (i.e. the square of the increase in focal length)?


"TS" wrote in message
...
I am trying to calculate the focal length of the lens in my digital
camera.

I understand that a smaller number such as 18mm is wide angle and a larger
number such as 200mm is a tele-foto lens.

If I calculate the angle of view of the camera, can I calculate the focal
length?
If so, how - and do I use the horizontal angle or the vertical plane
(assuming the camera is used in landscape).

Is this calculation an absolute or does it differ for 35mm cameras and
digital cameras?

Thanks