Today, with the prevalence of large single-sensor cameras, the number of lens options for the HD shooter has increased exponentially. Not every lens will work with every camera, however. In a previous DV101 column I discussed crop factors, utilizing lenses designed for larger target (sensor) areas on smaller targets, and the changes in apparent focal length/field of view.
- DV101: Delving Deeper: How to Achieve A ‘Filmic’ Depth of Field, Jay Holben, Digital Video magazine, January 2012
Lens choice gets a little more complicated than that, however, when you start to consider the different types of lens mounts and their register, or flange focal distance.
In the SLR and DLSR world, each camera manufacturer uses its own kind of lens mount. Each mount type is proprietary and does not interchange with other types of mounts without a physical adapter between the lens and the camera. In the world of video cameras, there are some standard types of mounts that nearly all camera manufacturers use, but this isn’t the case when you’re using SLR lenses. Nikon, Canon, Olympus, Pentax—they all use different lens mounts, and not all are interchangeable without an adapter.
In the film world, although there are myriad lens mount formats for myriad camera styles, the ARRI PL (Positive Lock) mount arose as a professional standard for both 16mm and 35mm motion picture cameras. Many extraordinary PL-mount lenses have been manufactured by all of the great lens makers. The second most prevalent mount in the motion picture world is the Panavision (PV) mount.
In order to know if you can use a particular class of lens on your new single-sensor HD camera, you have to know a few things first.
Although we all work in a rectangular world where our images are concerned, lenses project images in a circular pattern. In order for a lens to work with a given format, the diameter of the projected image circle must be equal or greater than the hypotenuse of the rectangular target. If the lens is designed for the format, or very close to it, you will have an excellent match. Many large-sensor HD manufacturers are now using the Super 35mm film aperture size (24.89mm x 13.98mm) as standard. In the stills world, this is classified as the APS-C (Advanced Photo System, Type C) group.
Any lenses designed for use with Super 35mm film cameras will also cover the area of these large HD sensors. This image area, however, is considerably smaller than that of standard still cameras (24mm x 36mm), which means that lenses designed for still cameras can be used on Super 35mm sensor cameras, but there will be a decreased field of view and increased apparent focal length—what is commonly called the “crop factor.” Lenses designed for use with Super 35mm/APS-C targets cannot be used with “full-frame” cameras such as the Canon EOS 5D.
There’s an additional factor to consider when you’re looking at intermixing lenses with different cameras. In a single-target camera such as an SLR or large single-sensor digital cinema camera, there is a fixed distance from the rear of any lens mounted on the camera to the sensor plane. This distance is called the flange focal distance (sometimes it’s called the register). It is measured from the point at which the back of the lens mount connects with the front of the camera’s mount to the target/sensor itself. This distance is different for different camera manufacturers, different types of cameras and different photographic media.
All lenses designed for a particular camera system are designed to work with that system’s fixed flange focal distance. This is how Canon knows that all of its EF lenses will work with any Canon EF body; every Canon body has a fixed flange distance of 44mm. In the PL-mount world, all cameras that have PL mounts have a fixed 52mm flange focal distance. This large distance, which is required to accommodate the rotating shutter in reflex motion picture cameras, has the benefit of making these lenses compatible with a lot of other camera systems.
You can use a lens designed for a larger flange focal distance on a camera with a smaller distance by merely applying a lens adapter that increases the camera’s smaller distance to be equal to that of the lens; however, you cannot really use a lens designed for a smaller distance on a larger camera. In the latter case, the lens will not be able to focus to infinity (or even closer distances), and your lens’ focus witness marks will also be off.
Interestingly, expanding the flange focal distance greater than the lens is designed for will turn the lens into a macro lens, increasing its close-focus ability. This is generally what a borescope tube does to lenses.
In optical design, many lens’ rear elements are actually positioned behind the mounting point—and there is a difference between the flange focal distance and the optical back focus distance, which is the measurement from the edge of the rear optical element of the lens to the target.
It is considerably easier to manufacture a higher-quality lens if the rear element is very close to the target. The greater the distance that the lens has to project the image, the more aberrations that have to be manufactured out.
In HD cameras that don’t require a spinning shutter or a reflex mirror, the lens can be positioned very close to the target, which allows the manufacturer to create inexpensive high-quality lenses. Additionally, these cameras with very small flange focal distances can be easily adapted to work with higher-end lenses created for larger flange focal distances, such as the PL and PV lenses.
The Panasonic AG-AF100, which is a Micro Four Thirds camera (“micro” for the lack of a reflex mirror), has an extraordinarily shallow flange focal distance of 20mm, making it compatible with nearly any lens out there (as long as the physical barrel diameter of the lens does not exceed that of the camera’s mount) that will work with Micro Four Thirds, Four Thirds, APS-C, Super 35mm, APS-H and full-frame cameras.
The chart below lists several camera and lens systems and their respective flange focal distances. Generally speaking, if you have a camera with a small flange focal distance, you can use a greater variety of lenses with it.
Notice that the Canon FD mount, which was the universal standard for Canon SLRs, cannot work with the EOS EF or EF-S, the new standard for DSLRs. The FD has an FFL of 42mm, whereas the EOS has an FFL of 45mm. However, PL lenses, which have an FFL of 52mm, can work on the EOS cameras (requires additional modification beyond an adapter) because they have a larger register than the camera system.
Sincere thanks to Guy McVicker, manager of optics at Panavision, for his generous assistance in wading through the mucky mire of my brain to clear up a few questions for this piece.