Depth of Field Scales on Leica rangefinder cameras

To illustrate DOF for a Flickr contact and why the smallest f-stop gives you another few feet in focus, even though it may not be the sharpest stop (sweet spot) for the lens/image overall.

Expand the image to full size so you can read the scale clearly nearest the camera body.

On this dof scale, if aperture is f18, you will see on the left side of the focus point (which is at about 12.25 feet on the camera body ring) that the f16 marker is pointing at infinity. On the right side of the focus point, the f16 marker is pointing at about 6.25 feet.. So at f18, everything would be in focus from infinity to 6.25 feet. Opening up the aperture by one stop to f12.5 would reduce the depth of field by about a foot, from 6.25 feet to 7.25 feet - read off the scale opposite the right hand side f12.5 marker.
So even if the smallest aperture may not be the "sweetest" stop on the lens, it does give you an extra foot or so in focus.