Before revision:  Rotation of 2-D images is usually done with the assumption that the magnification of the electron microscope is the same in all directions , a condition that has been found to be untrue with some electron microscopes when used with the settings necessary for cryo-EM with a direct detector device (DDD) camera (Grant and Grigorieff, in preparation).
After revision:  Rotation of 2-D images is usually done with the assumption that the magnification of the electron microscope is the same in all directions . However, due to electron optical aberrations, this condition is not met with some electron microscopes when used with the settings necessary for cryo-EM with a direct detector device (DDD) camera (Grant and Grigorieff, in preparation).