The Rangefinder mechanism, the attainable accuray, the required precision of engineering and assembly make for fascinating reading and study. Let us first review the simple mechanism in the SLR to get an idea what is involved. Generally we have two separate actions when focusing the lens on the object: 1. setting the distance by physically moving the lens relative to the film plane 2. checking the setting by visual inspection to ensure the movement gives acurate focus.Both acts are indeed separate and do not need to be connected. When we use a fixfocus box camera, we skip both and concentrate on framing. When we set the distance ring of the lens to some estimated hyperfocal distance, we do action 1, but not action 2…….

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