Abstract: A system for determining a dimension of a detail includes an optical scope for producing an image of the detail, an image scaling device for providing a scaled image size, and a processor. The system also includes a video camera which is either within the optical scope or external to the optical scope. The optical scope includes a focusing device for adjusting a focal position of the image of the detail and a device for providing a focus position signal based a position of the focusing device. The processor converts the focus position signal into an object distance signal, or a magnification signal, or both, and determines the dimension of the detail based on the scaled image size and based on the object distance signal, or the magnification signal, or both. The system further comprises means to compensate for changes in temperature of the optical scope to ensure that the measurement accuracy is not compromised by expansion or contraction of the scope.