An optical coherence tomography scanning controller for en face optical coherence tomography imaging during ophthalmic surgery, the optical coherence tomography scanning controller being configured to interface to an OCT scanner used with a surgical microscope and further comprising: a processor and memory media , the processor having access to the memory media storing instructions executable by the processor for: sending a first command for en face viewing of an interior portion of an eye of a patient undergoing ophthalmic surgery to the OCT scanner the first command instructing the optical coherence tomography scanner to generate first scan data of the interior portion of the eye ; receiving the first scan data acquired by the OCT scanner; generating a first en face image from the first scan data; sending a second command instructing the optical coherence tomography scanner to generate second scan data of the interior portion of the eye receiving the second scan data acquired by the OCT scanner; generating a second en face image from the second scan data; digitally subtracting the second en face image from the first en face image to generate a third en face image; based on the third en face image, generating an overlay image indicative of changes to the interior portion of the eye between the first en face image and the second en face image; and a display interface and a shared bus , the display interface connecting the shared bus with an output port for at least one display in order to display the overlay image overlaid onto an optical image of the interior portion of the eye generated by the surgical microscope to a user of the surgical microscope . a processor and memory media , the processor having access to the memory media storing instructions executable by the processor for: sending a first command for en face viewing of an interior portion of an eye of a patient undergoing ophthalmic surgery to the OCT scanner the first command instructing the optical coherence tomography scanner to generate first scan data of the interior portion of the eye ; receiving the first scan data acquired by the OCT scanner; generating a first en face image from the first scan data; sending a second command instructing the optical coherence tomography scanner to generate second scan data of the interior portion of the eye receiving the second scan data acquired by the OCT scanner; generating a second en face image from the second scan data; digitally subtracting the second en face image from the first en face image to generate a third en face image; based on the third en face image, generating an overlay image indicative of changes to the interior portion of the eye between the first en face image and the second en face image; and a display interface and a shared bus , the display interface connecting the shared bus with an output port for at least one display in order to display the overlay image overlaid onto an optical image of the interior portion of the eye generated by the surgical microscope to a user of the surgical microscope . generating a second en face image from the second scan data; digitally subtracting the second en face image from the first en face image to generate a third en face image; based on the third en face image, generating an overlay image indicative of changes to the interior portion of the eye between the first en face image and the second en face image; and a display interface and a shared bus , the display interface connecting the shared bus with an output port for at least one display in order to display the overlay image overlaid onto an optical image of the interior portion of the eye generated by the surgical microscope to a user of the surgical microscope . <EOS>