Autofocus-fixed focal length

[Pranav]

Hi,
I have logitech Quickcam Sphere AF camera,which has 3.7mm focal length.
I am not clear with:-If it has auto-focus then how it can have fixed focal length?

[krispix]

I think you're getting confused. Focal length relates to the image size projected onto the photo-sensor. Imagine the sensor as a square (it's actually a rectangle, but for the purposes of defining focal length it is a square). Now, a 'full frame' DSLR is 24mm x 36mm so we need to imagine the projected image over a 36mm x 36mm square. If you calculate the diagonal across that it come to 50mm, therefore a 'standard' lens for a Full-Frame camera is 50mm. As you come down the scale of cameras the sensor gets smaller (and cheaper) so the 'standard' focal length gets correspondingly shorter. i.e. on a DX Pro-sumer DSLR it will be in the region of 35mm. On your webcam (which tend towards wide angle anyway) it will have a very small sensor (probably about 3mm x 3mm), but that's fine for the work you want it to do.
Regardless of the size of the sensor and the corresponding focal length of the lens, it still needs to be focused on the sensor; the further away your subject the closer the lens will be to the sensor. It makes no difference what the focal length is.
With modern webcams they use a system called 'follow me' where the camera picks up your face and will track the focus very quickly when you move back and forth. This is the auto-focus you refer to and it doesn't change the 3.7mm focal length of the lens.
Hope this helps.

[Pranav]

Hi,
Thanks for your response,i convinced myself that we can have 'autofocus & fixed focal length' in camera.
Because as per lens equation for thin lenses:-
1/s1+1/s2=1/f;here s1bject distance,s2:image distance(to get sharpest image),f:focal length.
So an autofocus camera will vary s2 to accomodate for various s1's so as to maintain constant 'f' and hence focal length has no role in focusing process(other than just a constant).