In this paper we deal with the problem of calibrating a rotating and zoomin
g camera, without 3D pattern, whose internal calibration parameters change
frame by frame. First, we theoretically show the existence of the calibrati
on parameters up to an orthogonal transformation under the assumption that
the skew of the camera is zero. Auto-calibration becomes possible by analyz
ing inter-image homographies which can be obtained from the matches in imag
es of the same scene, or through direct nonlinear iteration. In general, at
least four homographies are needed for auto-calibration. When we further a
ssume that the aspect ratio is known and the principal point is fixed durin
g the sequence then one homography yields camera parameters, and when the a
spect ratio is assumed to be unknown with fixed principal point then two ho
mographies are enough. In the case of a fixed principal point, we suggest a
method for obtaining the calibration parameters by searching the space of
the principal point. If this is not the case, then nonlinear iteration is a
pplied. The algorithm is implemented and validated on several sets of synth
etic data. Also experimental results for real images are given.