When talking about mechanical lens adapters, I mean adapters that are strictly built to physically adapt a lens mount to a camera mount—no optics or electronics involved. These adapters are simply lighttight barrels that get you to the correct flange focal distance. A good example of this is a PL to E-mount adapter.
In the image on the opposite page, you see a Sony a7S with a PL lens attached via an adapter. This acts in very much the same way as changing the mount on the lens itself, but allows the camera to work with a variety of lenses.
By now, you may have concluded that lens adapters can be used in any situation where the flange focal distance of the lens is greater than what the camera mount is designed for. In our example, the flange focal distance of the PL lens is 52mm, but the camera’s E-mount flange focal distance is 18mm. In this case, it works well, but in some situations, we run into problems.
A common assumption is that a PL lens can be adapted to EF-mount. On paper, it looks good because the EF-mount flange focal length is 44mm and PL is 52mm. However, the physical designs of most PL lenses prevent them from being adapted, primarily because a large percentage of PL lenses have rear optics that protrude beyond the lens mount itself. If an adapter were used, these optics would hit the inside of the EF mount, which houses lens mount electronics. So, while you can purchase a PL to EF adapter, often they require physically modifying the camera (removing parts of the EF mount), or they only work with a handful of lenses that don’t have protruding elements.
Another problem we run into when adapting lenses is that many require electronics to adjust iris and power other features. This is where an electronic adapter comes into play. The most popular ones today are, by far, the Canon EF adapters. These adapters perform the mechanical flange focal length adjustment, but they also allow you to power and adjust iris on EF lenses. Popular adapters include the Metabones EF adapters for E-mount and Micro Four Thirds-mount cameras, as well as the OptiTek adapters for Sony FZ-mount cameras. These adapters have enabled many filmmakers to utilize EF-mount lenses on a wide variety of cameras.
While mechanically adapting a lens to a specific camera may be possible, it doesn’t necessarily mean the lens will cover the sensor. By "cover," I mean the image circle that the lens produces may not be big enough to make a full image on the sensor. In my "Sensors & Circles" column (December 2013), I spoke about lens coverage in detail, but a good example of this is putting a Super 16 lens on a Super 35 camera. The Super 16 lens is designed to cover a 15mm image circle, but Super 35 requires a 32mm circle, so the image produced by this combination would have a porthole effect, of sorts.