The state of the art in the area of optical-fiber refractometric sensors includes a large number of different proposals about sensor designs and applications. However, there are few works that compare the transducer response to the refractive index of refractometric sensors with different geometrical shape. This makes it difficult to design the refractometric sensors optimized for particular applications. In addition, the existing knowledge relates mainly to the refractometric sensors with perfectly clean surfaces, while the surface of real sensors is frequently contaminated. In this work, we analyzed the response of optical-fiber refractometric sensors of ellipsoidal shape and the effect of the residual film of liquid (water, oil, etc.) on the transducer's surface on the transducer's response. Also, we determined the sensibility of the response to small variations of the geometrical parameters, in turn, allowed us to determine the tolerance margin in sensor fabrication. We obtained data on the effect of the liquid film of different thickness on the response of the optical detection element as well. These data allow one to make an optimal choice between different possible shapes of detection elements.