To determine the total magnification of an image viewed through a microscope, multiply the power of the eyepiece or ocular lens by the power of the objective lens. If the magnification power of the ocular lens is 10x and that of the objective lens is 4x, total magnification is 40x.
Most compound microscopes have a 10x eyepiece and three objectives, 4x, 10x and 40x. Others also have a 100x magnification power. Therefore, the least total magnification of an image is 40x and the highest is 1000x. In order to view a specimen clearly under the microscope, begin with the lowest power objective (4x) to focus on the specimen, then use higher objective lenses (10x to 100x) to get a closer view of the image.