The Focus Mask tool is used to evaluate whether an image was sharp at the time of capture. You can alter the threshold or level at which the Focus Mask will be triggered. To assess sharpness more critically, the threshold should be increased above the default 250 setting (i.e., the slider dragged to the right). The amount will vary by use case, however, the threshold slider can be adjusted while observing the effect in the Viewer. The color (default is lime green) and the opacity of the mask are also adjustable.
Warning! While this is a useful tool to evaluate the sharpness of a capture initially to assess critical sharpness, you should check images in the Viewer or Focus tool at 100% magnification.