Image sharpness metric

Is there some robust metric of image sharpness or bluriness? I have various set of images with different parameters of saturation and captured from different optical systems, and i heed to show user something like "quality" of focusing. For getting most focused image i use metric getted with Sobel-Tenengrad operator(summ of high-contrast pixels), but the problem is that for different objects are quite different range of metric(depends on unknown parameters of image intensity, optical system ) - needed some metric where possible to say that image has bad focus whithout comparing with reference image, like this is "bad" or "good" focused image.