A dual objective optical micrometer for strain measurements of up to 8% is described. In this instrument the images of two reference marks on a specimen are brought to a focus in one field on the reticle scale of a filar micrometer eyepiece. The relative displacement of the images is measured by the eyepiece and the readings are reduced to strain by the use of a calibrated magnification factor. Since only the separation distance of the images is of interest, the exact position of the images in the field is not critical. No external point of reference on the instrument mount is necessary, hence alignment problems are minimized and the instrument may be moved between readings. The described calibration procedure shows that the least count of the instrument with a 2.64‐in. gauge length is 42×10−6 in./in. and measurements can be made with an accuracy of ±13×10−6 in./in.