This Range finder device fits onto the hot/cold shoe of the camera and enables precise distance measurement from the camera to the subject to ensure the subject is in focus range finder cameras have these devices built in and they are not required in a TLR or SLR.  The one I have was £5.50 from Ebay so not expensive.

The Watameter-super has an internal distance scale visible through the eyepiece when taking a reading and for close up measurements the readings are taken from the dial

Download instruction sheet pdf Here