A file format suitable for storing a full-color high dynamic range image should allow for keeping image information with an accuracy exceeding color visual threshold. Plane use of a floatingpoint encoding results in too large files, while the Extended Range format produces more compact files. However, there is still an opportunity for a better result.This paper suggests file format that not only provides files smaller than Extended Range format and meets the accuracy requirement, but also allows for a trade off between data precision and file size. The format design is based on a coordinate system convenient for brightness, contrast and dynamic range editing which preserves chromatic coordinates.