Systems and methods for determining a volume of the posterior segment of an eye during a fluid-to-gas exchange are described. The determined volume of the posterior segment may be used to determine the volume of pure tamponade gas to be introduced into the posterior segment resulting in a desired tamponade gas to other gas (e.g., air) ratio, such as during an ophthalmic surgical procedure. Particularly, the systems and methods may provide for improved gas utilization, less waste, and reduced surgery time, for example, by accurately measuring the volume of the posterior segment of the eye and only using the amount of pure gas required to produce the desired ratio. Further, in some implementations, mixing of a tamponade gas is accomplished intrinsically within the posterior segment of the eye, thereby avoiding manual mixing and reducing surgery time.
