I think you are confused with the terms:
Focus is just one point
There is no starting and end points, or long or short focus depth, or F-stop limits.
It seems to me you are asking about depth of field. Depth of field refers to the "circles of confusion" (as pointed out by @lbalazscs) The range acceptable focus depends on the focus distance, the length of the lens and the F-stop (or iris diameter) of the lens (and the magnification of the image in projection or display). Blender tries to follow the calculations for cameras in the real world. There are plenty of charts on the internet where you can get a reference for depth of field.
In blender there is no tool to show you a range of "acceptable focus", where the image is sufficiently sharp.
If you are using cycles however, you can enable depth of field preview for the 3Dviewport in the camera section's Depth of field and get a very good idea of how focus is working in real time.
Note that the viewport settings and the camera settings for rendering can be set separately.