This paper proposed a new methodology which converts a full-reference focus quality assessment metric into a no-reference one. The methodology consists of three hypotheses which describe the relationship in focus quality between the original image and its variants. Using the proposed methodology, two no-reference metrics were constructed. The first used Brenner Gradient and the second used a full-reference metric proposed by ourselves. Evaluation was conducted on a public dataset and our own proposed dataset. Comparing with other no-reference metrics, our second one exhibited best performance on both datasets, with calculation time comparable to some fastest metrics considered.