Interferometric image-analyzing comparator for gauge blocks of 1-100 mm length is reported. Sophisticated image processing software, that supports 0.1 nm resolution in central length and 1 nm in 3D surface measurements, has been developed. Resolution of the comparator reaches the value of 1 X 10-9 for 100 mm blocks. Inclination error is reduced to the value of 8 X 10-9. The relative uncertainty for inclination correction can be statistically diminished to about 2 X 10-9. The instrument and a new method have been used to study the deformations of steel plates, caused by wringing forces. In these experiments, 3D surface profiles of plates and blocks are being recorded while gradually diminishing the wringing forces by the oil, that slowly penetrates into the wringing contact. Shorter blocks are shown to be significantly deformed by the steel plate, while the deformations of the plate are quite negligible. This allows to realize reproducible wringing, and to perform precise correction on interferometer optics and curvature of the base plate. Blocks longer than 25 mm are shown not to be deformed, practically, by the plate.