Dilution of Precision (DOP) is a common measure of the quality of measurement geometry in the Global Positioning System. Measurement references that are clustered together in space provide nearly identical measurements and consequently little additional information relative to each other. To have a good measurement geometry, the measurements need to be unique combinations of the navigation states, and DOP is a measure for this. This paper takes this concept of DOP and applies it to optical measurements from a vision navigation system. The DOP for vision measurements is developed by relating noise in measurements of reference location in an image to both position and orientation states. The relationship is demonstrated with a numerical example.