Cable-Driven Parallel Robots (CDPRs) offer numerous advantages due to their lightweight design and large workspaces. However, the inherent flexibility of cables significantly impacts their stiffness. This paper investigates the stiffness modeling of a planar CDPR. We present a theoretical model to predict the stiffness of the robot and introduce a method to experimentally measure the active stiffness of the CDPR, isolating it from passive stiffness influences. We validate this model experimentally using a small-scale CDPR, named OSCAR. A vision-based system is utilized for precise displacement estimation during these experiments. The paper includes details on the control law and Tension Distribution Algorithm (TDA) employed for OSCAR, followed by a comprehensive analysis of the experimental stiffness measurements. This research contributes to a deeper understanding of the factors influencing stiffness in CDPRs and provides valuable insights for future design and control strategies.