High Precision Mechanics for Optical Performance
High Precision Mechanics for Optical Performance
It took until the early 1980s when the now industry-standard for 16 mm and 35 mm lenses was introduced, the PL-mount. PL, short for positive lock, is the most common mount used for lenses in the world of cinematography. Unlike the spring-loaded bayonet mounts, the PL-mount solidly locks the lens in place with a threaded flange. The greater diameter allows for lenses with substantially larger rear optical elements thus opening the door to modern, high speed lens designs.
Multiple Mounts for AMIRA/ALEXA Mini
To position AMIRA in a documentary ecosystem, it needed the possibility to adapt lenses from other worlds as well, such as B4 or EF-mount lenses. The AMIRA’s and ultimately the ALEXA Mini’s frontend was designed with an exchangeable mount: We offer AMIRA/ALEXA Mini lens mounts for the PL, B4, and EF. Third parties offer various other mounts like Panavision, Nikon F, Nikon G, Leica R, and others.
Maxi PL, XPL and LPL
In 1989 a larger PL version has been invented for the ARRIFLEX 765, the Maxi PL which had a 12 mm larger diameter to match 65 mm film.
To further improve the legendary stability of the PL lens mount, ALEXA was equipped with a novel lens and sensor mount design. The stainless-steel lens mount is attached to a stainless-steel tube, which itself is attached to the sensor mount. The whole unit is then attached to the camera body. By creating a fixed link between lens and sensor, a super stable flange focal depth is ensured — even if strong mechanical influences are applied to the lens mount, such as when a heavy zoom lens is not properly supported. Using stainless steel ensures minimum material expansion or contraction during temperature changes.
This principle has been adapted on the second 65 mm-class camera from ARRI — but it demanded its own lens mount: In 2015, the XPL mount was introduced with the ALEXA 65, a mount even larger than Maxi PL (XPL: 72 mm diameter).
In 2018, ARRI introduced the LPL mount alongside with ALEXA LF to offer a proper mount for large format (or “full frame”) camera systems.
The LPL mount (LPL = Large Positive Lock) is the first universal mount designed specifically for digital capture. The PL mount has a number of limitations when working with digital sensors and modern lenses, and the LPL mount has been developed to specifically address those limitations.
The LPL mount combines a proven mechanical design with new dimensions.
A Proven Mechanical Design
The LPL mounts mechanical function is based on the proven design of the ARRI 54 mm PL mount. In many productions, this mount was found to be robust and reliable while guaranteeing a precise flange focal distance. This was one of the main reasons that lead to the wide and universal adaption of the PL mount.
LPL mounts are equipped with blue locking levers, so they can easily be distinguished from the black locking levers of the PL-to-LPL adapter.
Entering New Dimensions
In comparison to the PL mount, The LPL mount has a wider diameter (62 mm vs. 54 mm) and a shorter flange focal distance (44 mm vs. 52 mm).
Unlike 35 mm film, digital sensors come in all different shapes and sizes, and the wider diameter of the LPL mount covers them all, from ALEXA 65 to ALEXA Mini. This gives additional degrees of freedom for the optical design and helps to avoid disturbing effects:
• A small lens mount diameter would naturally lead to low-level corner illumination due to vignetting. Also, light would reach the corners of the sensor at steep angles, something that must be avoided when working with digital image sensors. Those mostly are more sensitive when perpendicularly illuminated. Furthermore, digital cameras usually incorporate a filter package in front of the sensor, which contains an optical low-pass filter as well as spectral filters. Focused light passes the filter package before getting to the sensor. Even if these filters are perfectly made, plane parallel glass plates, placed in the beam of focused light, must be considered during optical design. The influence of these parallel plates becomes even more severe when penetrated at steep angles.
• To guarantee the highest performance across the image even when used with different cameras and filter packages, light should strike the sensor nearly perpendicular. This is called telecentric. A lens containing a rear element that is smaller than the sensor cannot fulfill this requirement. Depending on the layout of the single image sensor pixels, light striking at steep angles may be detected partially only by the sensor or lead to so called crosstalk when absorbed and converted opto-electronically by a neighboring pixel. These effects are avoided by using telecentric optical designs.
• Reduced flange focal distance gives additional degrees of freedom for lens design that can be used for reducing aberrations like distortion, spherical aberration, field curvature or chromatic aberrations while enabling high speed lenses (low T-stop-number). The reduction of weight is also easier to achieve since space for a mechanical shutter does not need to be provided any more. All these features are provided with the LPL mount.
In summary, the new dimensions of the LPL mount can accommodate all currently common digital sensor sizes and allow for the design of smaller, lighter, and faster lenses with enhanced imaging quality properties. Rather than purchasing a lens with a specific lens mount intended for use with a specific camera, any lens can be used on any camera with an industry wide standardized mount, regardless of image size or resolution.
Towards a Universal Standard
ARRI shares LPL specifications openly and free-of-charge across the industry in order to establish the LPL lens mount as the new standard interface between cameras and lenses.
ARRI LPL lens mounts are available for ALEXA LF, ALEXA Classic/XT/SXT, ALEXA Mini LF, ALEXA Mini, AMIRA, and ALEXA 65 cameras. In addition, third parties already offer LPL lens mounts for non-ARRI cameras, director's viewfinders, and lens testing tools, so ARRI Signature Primes and other LPL lenses can be universally used, allowing for maximum flexibility and return on investment.
The PL-to-LPL adapter, which attaches securely to the LPL lens mount without tools, offers backwards compatibility with all PL mount lenses. The PL mount was meant to work with 35 mm image-sizes, whereas the PL-to-LPL adapter enables compatibility with PL lenses on future cameras if respective image sizes are taken into account.
The PL mount with Lens Data System (LDS) contacts premiered in 2000 in our analog cameras such as the ARRICAM Studio or Lite camera and is being used throughout ALEXA, AMIRA and ALEXA Mini.
LDS describes digital lens and motor settings. Lenses with LDS functionality deliver information about its current settings (e.g. focus, iris, zoom) to the camera; the camera interprets these values and feeds the information to its recording module and to any accessories connected to the Lens Control System, for example a Wireless Compact Unit WCU-4 or Master Grips. As a solid companion for visual effects work, it needed to be incorporated into the files our digital camera systems as well.
We offer compatibility with the Cooke /i protocol in ALEXA, ALEXA Mini, and AMIRA cameras.
LPL mount is of course equipped with lens data capability, too: LPL introduced a next-generation Lens Data System, LDS-2. Compared to LDS-1 it eliminates the need for initial calibration through absolute encoders which cuts time on set when changing lenses. The new system uses a 8.7x faster interface that enables lens manufacturers to send more data to the camera than before.
The ARRI Signature Prime lens range is the first set of lenses to use that system. LDS-2 extends the possibilities of lens data and is being licensed to other lens and camera manufacturers.