As a direct and effective non-contact full-field optical measurement method, the digital image correlation method has been widely used in 2D/3D displacement and strain measurements of materials and structures in various fields. Combined with advanced speckle preparation technology, in situ loading experiments are conducted under a microscope, which can realize the displacement and strain measurements of micro-scale digital image correlation. Out-of-plane displacement inevitably occurs in the loading process of specimens at the micro scale. Because of the limitations of the depth of field of optical microscopes, small out-of-plane displacement leads to defocus blurring of speckle images, which generates a corresponding error in deformation measurements. To reduce the error caused by out-of-focus blur, the blind deconvolution method is used to blur the speckle image, and the Gaussian filtering method is used to denoise the speckle image to address noise problems. The effects of image restoration on the digital image correlation measurement accuracy are analyzed quantitatively, and a stretching experiment of an indium tin oxide (ITO) film is conducted. Experimental results show that the error in elastic modulus measurement after image restoration is reduced by 13.91%, and the accuracy and stability of the strain measurement results are higher.