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Flange Focal DistanceEdit

Flange focal distance (FFD) is a fundamental specification in optics and imaging that defines the precise distance from the mounting flange of a lens to the focal plane (sensor or film) when the lens is focused at infinity. Also known as register distance in some circles, FFD is a key determinant of compatibility between lenses and camera bodies, and it plays a central role in how lens designs are engineered and how systems interoperate. Because the distance governs where the image is formed, it affects optical path, back focus, and the feasibility of adapting lenses across different mounts.

For many readers, the concept may seem abstract until seen in practical terms: if you mount a lens designed for one camera system onto another, the difference between the two FFDs determines whether you can achieve infinity focus with a purely mechanical adapter, or whether optical additives would be required. In short, FFD is the distance that the design assumes between the lens and the sensor; mismatches between the lens’s native FFD and a camera’s FFD drive what is possible with adapters and what remains impractical.

Definition and terminology

  • Flange focal distance (FFD) is the distance from the outer surface of a lens mount (the flange) to the focal plane (the sensor or film plane) when the lens is focused at infinity.
  • The term register distance is used in some contexts to describe the same concept, particularly in broader optical and hardware standards.
  • The focal plane is often called the sensor plane in digital cameras, where the image is recorded.
  • A lens mount is the mechanical interface that holds a lens on a camera body; on this page, the relationship between the lens mount and the sensor is the focus of FFD discussion.

Common flange focal distances and their mounts

Flange focal distance varies across camera systems and lens mounts. The numbers below are widely cited approximations for common systems and are useful for understanding adaptation logic, though exact tolerances can vary by model.

  • Classic DSLR mounts with longer FFDs: about 40–50 mm range. Examples include the Nikon F-mount (around 46.5 mm) and the Canon EF-mount (around 44 mm). These longer distances historically supported optical viewfinders and certain lens designs that put less emphasis on minimal back focus.
  • Modern mirrorless mounts with shorter FFDs: around 16–21 mm range. These shorter distances are a major reason for the attractiveness of mirrorless systems and for the broad potential of cross-system adaptation.

These values illustrate a central fact: a shorter camera FFD generally makes it easier to adapt legacy lenses from longer FFD mounts, often with simple mechanical spacers, while a longer FFD on the camera than the lens expects can complicate or prevent infinity focus without optical correction.

Adapters, infinity focus, and optical considerations

The practical upshot of FFD differences is most visible when adapting lenses across mounts:

  • Passive mechanical adapters: If the lens’s FFD is longer than the camera’s FFD, you can usually achieve infinity focus by adding a spacer (an adapter) of thickness equal to the difference between the lens FFD and the camera FFD. The formula is t = lens_FFD − camera_FFD, where t is the required adapter thickness. When t is positive, the adaptation is mechanical and preserves the optical design, but it may alter ergonomics and close focus performance.
  • When the lens’s FFD is shorter than the camera’s FFD, achieving infinity focus with a purely mechanical adapter is not possible. In this case, an optical adapter (one that adds corrective elements) would be required, which can degrade image quality, increase complexity, and defeat some of the benefits of the original lens.
  • Optical considerations and image quality: Even when infinity focus is achievable mechanically, the adapter can introduce aberrations, vignetting, or softness, particularly at wide apertures or on wide-angle designs. High-quality adapters minimize these effects but cannot always match native performance.
  • Telecentric and optical paths: The FFD interacts with the optical path length and the angular distribution of chief rays as they exit the lens. In some specialized cinematography contexts, designers optimize FFD and mount geometry to preserve consistent focus behavior across focus shifts and to maintain predictable behavior when using additional elements such as matte boxes, follow focus attachments, and other accessories.
  • Cinema vs. still photography considerations: In motion picture work, flange focal distance and mount rigidity affect lens breathing, focus pull accuracy, and the ability to use cinema lenses across different camera bodies. The same principles apply when evaluating adaptation of still lenses for video work.

Applications, design choices, and debates

FFD influences the direction of lens design and camera ecosystem strategy. Shorter FFDs in modern mirrorless systems have driven a wave of cross-system compatibility, enabling more flexible use of legacy glass with inexpensive or no-optics adapters. This shift has been a factor in consumer choice and in the way manufacturers design future mounts.

There are industry debates about openness and interoperability versus proprietary ecosystems. Advocates of standardized or widely compatible mounts argue that shorter FFDs and common adapter ecosystems empower photographers and filmmakers to choose among gear from different brands without sacrificing performance. Critics of widespread standardization sometimes emphasize the benefits of brand-specific optimizations and the ability to protect intellectual property, claiming that tight control over mounts can drive innovation within a given ecosystem. In practice, the market tends to reward a balance: strong native performance for each mount, plus high-quality third-party adapters and a reasonable degree of cross-compatibility for users who value flexibility.

In cinema and high-end photography, precise tolerances in FFD, flange alignment, and mount registration are essential. Even small deviations can yield misfocus at critical distances, so manufacturers invest in tight manufacturing controls, calibration tools, and test methods to ensure consistency across production workflows. For readers and practitioners, understanding FFD helps in planning gear purchases, evaluating cross-brand compatibility, and assessing the potential trade-offs of adapter use.

Historical context and standardization

The shift from film-era mounts to digital systems brought a concerted effort to reexamine flange focal distances and mount standards. The move toward shorter FFDs with many modern mirrorless systems has facilitated cross-brand adaptation and has influenced lens designers to optimize optical formulas for direct mounting on different bodies. In some cases, new mounts purposefully maintain a short FFD while offering rugged mechanical interfaces to support a wide range of lenses. The ongoing evolution of mount standards continues to shape how photographers and filmmakers source and deploy lenses across platforms.

See also