A server in communication with a first device capable of receiving input from a camera attached to the first device, and a second device capable of displaying output, is provided. The server is configured to receive, from the first device, data relating to characteristics of the camera and characteristics of an image obtained by the camera of a screen of the second device； determine a screen pixel length of the screen； and provide instructions to the second device to display on the screen an object at a desired visual angle in response to the data received from the second device and the screen pixel length, while the first device is positioned at an arbitrary distance from the screen.
