Nikon made a design change that is perhaps the smartest thing they’ve done since 1959. Their new Z6 and Z7 cameras sport their new lens Z-mount with a 55mm throat diameter and 16mm “flange-focal” distance.
This design change wasn’t possible for Nikon until they made a camera that got rid of its flipping mirror. On a full-frame camera, the mirror rotates through an arc with a radius of about 35mm, and their F-mount lenses had to have a long flange-back distance to keep clear of this flipping mirror.
The F-mount has a 46.5mm flange-focal distance, while its throat diameter (lens rear diameter) is 44mm.
F mount versus Z mount
Many lenses designed for other lens mounts could have an adapter to fit the new Z mount, if they have a sufficient image circle for full-frame coverage. The main key here is that their flange distance must be longer than 16mm, which most lenses have. These adapters don’t need any optics in them, so the resolution won’t get messed up.
The new design means much faster optics, given a larger mount throat diameter and fewer lens elements needed for the shorter flange distance. Fewer lens elements mean less light loss from absorption and reflection. The larger throat diameter means lens elements with wider diameters and less severe surface curvature is required. The larger rear lens element diameters and aperture mean a physically larger area to capture more photons.
It’s much easier to design a wide-angle lens when you can avoid the severe retro-focus required for the long flange distance. Retro-focus means the lens focal length is smaller than the distance to the sensor from the rear of the lens. Now, lenses down to about 16mm don’t need a complicated, expensive, resolution-killing retro-focus design. Imagine some of the cool wide-angle lenses we'll see in the future.
You get less chromatic aberration, due to the large throat diameter permitting the less severe surface curvature in the lens elements.
More light rays hit the sensor edges at a less steep angle from the larger diameter rear lens element, so conversion of the photons into an image is more efficient. Vignetting can be reduced.
Being closer to the sensor and a wider rear diameter, the light rays have a shorter travel distance to the sensor edges. This directly leads to higher resolution throughout, but particularly in the corners.
The simpler wide-angle designs will lead to sharper, cheaper, brighter optics with many fewer lens elements. Telephoto lenses benefit less from the wider throat diameter, but you still can make brighter optics via the larger lens elements at the rear of the lens.
Of course, the camera body thickness can also be reduced by about (46.5 – 16) mm or 30.5mm.
The larger physical rear lens diameter means more room to provide a larger and more powerful focus motor.
Except for losing the auto-focus capability of the Nikkor “AF-D” lenses, their FT-Z adapter still lets you use your existing F-mount lenses.
Way to go, Nikon.