A negative records exactly what the lens projected, and at wide apertures the margin for focus error is measured in fractions of a millimeter at the film plane. Whether a camera lands within that margin depends on how its focusing system works. The coincident-image rangefinder and the through-the-lens single-lens reflex solve the same problem with opposite geometries, and each is precise where the other is weak.

The Coincident-Image Mechanism

A rangefinder triangulates. Two windows separated by a fixed distance, the mechanical base length, view the subject from slightly different angles. A cam coupled to the lens helix rotates a beamsplitter so that a second, superimposed image in a central patch shifts horizontally. When the lens is focused at the subject distance, the two images coincide. The geometry is a long, narrow triangle: the wider the base and the farther the subject, the smaller the angular change per unit of distance, so accuracy is set by the base length and the distance being measured.

Crucially, that triangulation is independent of the taking lens. A rangefinder focuses a 21mm and a 90mm with identical mechanical precision, because the patch knows nothing about the lens mounted in front of it. This is why rangefinders excel with wide and normal focal lengths in dim light, where an SLR screen goes dark.

Effective Base Length

Raw base length understates the picture, because the patch is viewed through a magnifying eyepiece. The figure that governs real accuracy is the effective base length: mechanical base length multiplied by viewfinder magnification. The standard Leica M body, per Leica’s published specifications, has a mechanical base length of 69.25mm and a 0.72x finder, giving an effective base length of roughly 49.9mm. Higher-magnification finders (0.85x) lengthen the effective base and tighten focus; lower magnifications shorten it.

The consequence is a hard ceiling at long focal lengths and wide apertures. A given effective base length yields a fixed depth-of-focus tolerance at the film plane. When the lens’s own depth of focus at full aperture becomes narrower than that tolerance, the rangefinder can no longer guarantee a sharp negative. This is why fast lenses beyond about 90mm are at or past the practical limit of a 35mm rangefinder, and why no rangefinder offers a true long telephoto.

Through-the-Lens SLR Focusing and Its Failure Modes

An SLR removes the geometry problem by focusing on a ground-glass screen that sits at the optical equivalent of the film plane. What the screen shows is what the film records, so focus accuracy scales with the lens: a faster, longer lens projects a steeper cone of light and shows defocus more dramatically, exactly the regime where rangefinders fail. Parallax does not exist, because viewing and taking share one optical path, and framing is exact at any distance, including macro.

Its failure modes are the inverse. A plain matte screen demands judging the moment of peak sharpness, hardest with slow lenses and in dim light. Split-image and microprism aids sharpen that judgment but black out when the lens aperture is smaller than the prism’s design angle, often beyond f/5.6, leaving the user back on the matte field. Mirror and screen alignment must also be held to film-plane tolerances; any error there shifts focus invisibly. The rangefinder’s weakness is its fixed base; the SLR’s is its dependence on a bright, accurately positioned screen.