In this paper, we address the problem of online estimation of spherical features in the field of camera for robotics. Specifically, we consider a mobile robot equipped with inertial measurement units (IMUs) -- providing linear acceleration and rotational velocity measurements in the body-fixed frame -- and a pinhole camera that projects 3D points onto the image plane. To tackle this problem, we adopt the parameter estimation-based observer (PEBO) approach on manifolds to design a feature observer. Under a sufficient excitation condition, our design guarantees a globally exponentially convergent estimate of both the radius and the center coordinates of spherical targets. Simulation results validate the theoretical analysis and demonstrate the performance of the proposed feature observer.