A shearing interferometer finds a derivative of the wavefront along the direction of the shear parameter. While the general belief was held that the sensitivity of the RSI increases with the shear angle, we find that just the opposite is true. We examine the range of validity of the derivative function for the rotationally shearing interferometer (RSI). We find that all terms of the aberration polynomial are multiplied by the sine of the half-shear angle multiplied by the order of polynomial, l. This factor may be expanded in series for small angles, or for angles close to π and π/2. We propose steps to evaluate the errors in the determination of the magnitude of individual aberrations. We apply findings to the planet-detection problem.