The present invention generally relates to methods and devices for mounting an image capture device to an optical viewing instrument such as a microscope, telescope, or binocular. More specifically, some embodiments of the present invention relate to an apparatus for mounting a smart phone to an observation tube of microscope. The optical viewing instrument may have an observation tube and an ocular attached thereto. The adapter may include an observation tube ring mount configured to be installed on the observation tube. An observation tube mount may be configured to engage with the installed ring mount. An image capture device holder may be configured to couple with the observation tube mount and configured to receive an image capture device. The image capture device may be a common device such as a smart phone. Optionally, a window may be included in the observation tube mount to facilitate viewing of ocular indicia.