Originally Posted by
Manfred M
This is the easy part, if you have the distance measured correctly.
Set the camera to single point focus and focus on something that is 19m away using autofocus. Keep the shutter release pressed half-way down (or use back-button focus), re-frame the shot and press the shutter release all the way to take the picture. This is probably more accurate than using the distance scale on the lens (only a couple of my lenses have the distance markings on the lens barrel).
The only problem with the method is that by pressing the shutter release button half way down, the default setting on most cameras locks in both the focus and the exposure. Using manual exposure fixes this issue, if the lighting is different on the target spot versus the subject.