I came across many articles where the focal length of a camera lens is defined as the distance between the optical centre of the lens and the sensor. One such article is here:
Quoting from the first page, focal length measures 'the distance, in millimetres, between the optical centre of the lens and the camera’s sensor (or film plane).'
But, according to the lens law
(1/v + 1/u = 1/f), when an object is at infinite distance (very far away), the distance from the lens where the image of that object forms is the focal length.
So, for instance, if an object is quite close to the lens and we change the distance between the lens and the sensor such that the object is focused on the sensor, the said distance does not remain same as f in the lens equation (since u is not large to the extent that 1/u tends to infinity). In that case, the distance between the lens and the film has to be
uf/(u-f), which is different from f.
I am new to these concepts, so am I missing something?
Would much appreciate some insights in this regard.