Camera calibration is an indispensable step to get three-dimensional information from images.In this paper constant focal length self-calibration from multiple views is under investigation.Our approach is based on explicit constraints which relate absolute dual quadric to its images.Two constrained polynomial minimization problems with respect to two types of parametrization on absolute dual quadric

whose properties are incorporated into the constraints

are proposed and solved by linear matrix inequality relaxation optimization method

which could avoid the local minimum.The difference with the other self-calibration approaches is that constant focal length is also an optimization variable besides the absolute dual quadric in the objective function.Experiments with simulated data and real images show that our approach works well.