Lens Mount Adapters and Related Thingamajigs That Go Between Camera and Lens
The introduction of the Nikon Z mount mirrorless system has given us the FTZ lens mount adapter to enable the use of Nikon F-mount lenses on the newer Z 6 and Z 7. Cool, one more thing we can mount between our cameras and lenses.
Both Nikon's new Z-mount and the similar Canon RF-mount systems have a shorter flange focal distance than their previous SLR systems. If you could mount an older lens on one of these newer mirrorless bodies, it wouldn't focus properly. Both systems significantly revise the electrical interface and contacts between camera and lens making precluding any direct interoperability. And while the RF-mount does have the same diameter as Canon's EF and EF-S systems, the new Nikon Z-mount has a larger diameter than their longstanding F-mount, complicating the use of existing lenses on new mirrorless bodies further. Thankfully, Nikon has solved both issues with their new FTZ adapter.
Existing F-mount lenses are designed to focus an image on a sensor mounted 46.5mm away. I'm sure there must be some reason for that value, but for those of us shooting with the Nikon system, it simply was what it was. However that specific distance was arrived at, all Nikon and Nikon compatible lenses had to follow suit. But the new Z-mount flange distance is a mere 16mm. If it were directly possible to mount an old F-mount lens on a new Nikon Z body, it would focus nearly 30mm behind the sensor. By holding the lens exactly the right distance from the body though, the FTZ positions it so that it now focuses on the sensor where it should instead of behind it. The FTZ contains no glass elements.
If you shoot macro, you may be familiar with extension tubes that work by repositioning a lens further from the camera body and sensor. If that sounds to you at least somewhat like the new Nikon FTZ, you aren't alone. They do share a common physical impact on focus caused by their very presence. Both an extension tube and the FTZ adapter mess with focus, but the difference is that FTZ does so out of necessity, in order to restore the focus distance an F-mount lens was designed for. But in the case of the extension tube, the lens moves further away while the sensor stays put. This has the effect of shifting the entire focal range of the lens by that amount. Doing so lets you focus that much closer, but at the expense of being able to focus to infinity. If you've ever tried focusing on the horizon with a mounted extension tube, you know that it simply can't be done.
An extension tube doesn't have to contend with lens diameter or the arrangement and purpose of electrical contacts when sandwiched in between camera and lens, but it does push the lens further away in much the same fashion as the new FTZ adapter. And as with the FTZ, an extension tube contains no glass. Neither one needs to. The sole purpose of both is to reposition the lens elements, albeit for a different reason.
All lenses focus by moving their elements close or further away from the sensor. As you rotate the focus ring or press the equivalent focus button on the attached camera, the lens elements move. It may seem counterintuitive, but the closest focus is achieved with the lens elements furthest away. By pushing them even further out, you can then focus closer than that lens could on its own, but now there's no way you can move them close enough to the flange and sensor to focus far away. A macro lens is basically just a lens with long enough focusing helicoids that it can do either without help.
An extension tube can make at least some lenses work like a poor man's macro lens at a fraction of the cost. That same tube on a different lense may result in unacceptable vignetting. On other lenses, it may push the focus range so far off that you won't be able to focus at all. Essentially, a bellows is an articulating extension tube with a variable length. I remember being frustrated with my first bellows until I realized that my chosen lens really wasn't a good match. I had shifted the entire focal range so close to the sensor that it fell entirely within the lens and camera body. I couldn't focus on anything in front of the lens at all. Duh.
Long story short though, an FTZ adapter is basically a glorified extension tube, but one with precisely the length necessary to restore focus to where it should be. Since the sensor moved 29.5mm closer to the lens flange, the lens focus distance has to move further away by that same amount.
When you use a teleconverter, it sits between the lens and camera body too. But unlike an extension tube and the new FTZ adapter for Z-mount bodies, a teleconverter does contain glass elements. The focal shift caused by the imposition of teleconverter body is corrected by the added glass elements. The lens itself is still trying to focus the same distance it was designed for, but now the central portion of the resulting image cone gets refocused to the now more distant sensor by the elements in the teleconverter. Some combinations of lens and teleconverter may result in unacceptable vignetting as with an extension tube, but at least they won't cause focus problems. The lens's focal length is increased by just the right amount to achieve focus as intended.
Clearly, extension tubes, teleconverters and the new Nikon FTZ adapter serve different purposes and are not interchangeable. But they are still closely related in how they work. All three rely on the focus shift introduced by their very existence, sitting there as they do between lens and camera.