This paper experimentally investigates the effect of the resolution of the optical system on the measurement results of digital image correlation. The resolution is varied by changing the aperture of the lens. The actual values of the displacement are estimated by measuring the rigid body rotation, and the bias and the variance in the measured value are estimated from the measurement results. The effect of the Gaussian pre-filtering and the defocus is also evaluated. Results show that the aperture value affects the image, and as a result, it affects the measurement result. From the results, it is found that the F-number of f/8 or f/11 is suitable for obtaining images. In addition, the application of the Gaussian pre-filtering should be careful because the variance is increased by applying this filer. Furthermore, the defocus should be avoided because the both bias and variance are increased by defocusing.