Flange Focal Distance: What Back Focus Means for Lenses

What flange focal distance actually means

Flange focal distance (FFD) is the distance between the metal or plastic mounting ring on a camera body and the plane of the image sensor. It is also called back focus or register distance. Every interchangeable lens system has a specific FFD, and the lens must be designed to project a sharp image at exactly that distance from its own mount.

The camera body's depth is directly tied to this specification. A shorter FFD allows the sensor to sit closer to the front of the camera, which permits a thinner body. This is one reason mirrorless cameras can be more compact than DSLRs: with no mirror box taking up space, the mount can sit very close to the sensor. Micro Four Thirds is an example of an open standard designed around a compact mount, and its specification is maintained by the Four Thirds consortium.

• Flange focal distance is measured from the lens mounting surface to the sensor plane
• It is fixed for each lens mount standard
• It determines how much space the camera body has between the mount and the sensor

How flange distance shapes lens design

Lens designers must work within the flange distance of the system they are designing for. A shorter FFD gives them more freedom: the rear element can sit closer to the sensor, which often allows for simpler optical formulas and smaller lens barrels. Wide-angle lenses especially benefit, because their design does not need to clear a mirror or other mechanical parts.

A longer FFD, common in SLR and DSLR systems, forces the rear element to sit farther from the sensor. The lens must project light through a longer distance, which often requires more complex optical corrections. This is why many DSLR wide-angle lenses are bulky and use retrofocus designs. Manufacturers of mirrorless systems often highlight this advantage in their own materials. Nikon describes how its Z mount lenses were designed to surpass what was possible with DSLRs, in part because the shorter flange distance gives engineers more freedom.

The trade-off is not that one is better in every way. Longer flange distances remain perfectly workable, as decades of DSLR lenses prove. But for new systems, a shorter FFD is generally seen as an advantage because it lets designers make smaller, lighter lenses for the same optical quality. This matters when you are choosing between mirrorless cameras and DSLRs.

Why flange distance determines adapter compatibility

The most practical consequence of flange focal distance shows up when you use a lens mount adapter. An adapter is essentially a spacer that positions a lens at the correct distance from the sensor. For the adapter to work without optics, the camera body's FFD must be shorter than the lens's FFD. The adapter then fills the gap.

If the camera's FFD is longer than the lens's FFD, a plain mechanical adapter cannot work. The lens would project its image before it reaches the sensor, and you could never focus at infinity. In that case, you need an adapter with corrective optics that effectively change the lens's focal length. These optical adapters are more expensive and can slightly reduce image quality.

In practice, this means you can mount DSLR lenses on mirrorless bodies with simple, reliable mechanical adapters. The opposite direction is not possible without optical correction. If you shoot with a mirrorless camera, you can often adapt older lenses from SLR systems, which is a popular reason to choose a system with a large lens catalog like Sony E or Nikon Z. For more detail, see the guide on lens mount adapters.

Flange distance across camera formats

Flange focal distance is independent of sensor size, but the two are often discussed together. A camera with a full-frame sensor may have a short FFD, just as an APS-C camera might have a long one. However, the sensor size influences how demanding the lens design is: a larger sensor needs a larger image circle, which can make lenses bigger regardless of the FFD.

Some systems combine a relatively short flange distance with a smaller sensor to achieve very compact cameras and lenses. The Micro Four Thirds system is a clear example: its open standard allows multiple manufacturers to build lenses and cameras that interoperate beyond brand boundaries, as stated by the Four Thirds consortium. This makes it a strong option if you want a lightweight kit. On the other end, medium-format cameras often use longer flange distances because of their larger shutters and mechanical requirements, and their lenses tend to be bigger overall. See the medium format guide for more.

When comparing cameras, the flange distance matters most in two situations: choosing a system with good adapter options, and understanding why some bodies are thinner than others. For everyday shooting, you rarely need to know the exact number. You just need to know whether the lenses you already own or plan to buy will mount and focus correctly.

What to pick for your use

Flange focal distance is a specification you use indirectly. You do not pick a camera for its FFD alone, but you do pick a system based on what it enables. If you already own DSLR lenses, a mirrorless camera from the same brand usually lets you keep them with a simple adapter. If you are starting fresh, the lens catalog and body size matter more than the FFD itself.

For compact travel kits, systems with short flange distances are usually the best fit. For maximum lens choice, look at systems with large native lens lines plus good adapter support. If you are interested in vintage glass, almost any mirrorless camera works well, since most old SLR lenses have longer flange distances that adapt easily.

What to pick for your use

Questions

Is flange focal distance the same as back focus?

Yes. Back focus is another name for flange focal distance, defined as the distance from the lens mounting flange to the sensor plane. Some people also use back focus to refer to the distance from the rear element of a lens to the sensor, but the more precise term for the mount specification is flange focal distance.

Can I mount a mirrorless lens on a DSLR?

Generally no. Mirrorless lenses have a short flange focal distance, while DSLRs have a longer one. A DSLR body physically cannot position a mirrorless lens close enough to the sensor, so you would need an optical adapter that changes the image projection. In practice, this is rarely done because image quality suffers and the setup becomes awkward.

Does a shorter flange focal distance mean better image quality?

Not automatically. A shorter flange distance gives lens designers more freedom, which can lead to better or smaller lenses, but image quality depends on the entire optical design. Many excellent lenses exist for systems with longer flange distances. The advantage of a short FFD is more about flexibility and compactness than a guarantee of sharpness.

Why do DSLRs have longer flange distances than mirrorless cameras?

DSLRs have a mirror box between the lens mount and the sensor. The mirror flips up when you take a photo, so the lens must project its image across that extra space. Mirrorless cameras do not need a mirror, so the mount can be placed much closer to the sensor, resulting in a shorter flange focal distance.

Do all camera brands use the same flange focal distance?

No. Each lens mount standard defines its own flange focal distance. Even within the same brand, different mounts can have different values. For example, a brand's DSLR mount and its mirrorless mount usually have different flange distances, which is why adapters are needed when switching between the two.