Lens Mounts | Technology

ARRI's LPL Mount

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, like for example a given distance to the sensor plane which had to take a spinning mirror into account. The LPL mount has been developed to specifically address those limitations and enable a larger imager real estate.

The LPL mount combines a proven mechanical design with new dimensions.

A Proven Mechanical Design
The LPL mounts mechanical function is based on the design of ARRI's 54 mm PL mount. On many movie sets, this mount was found to be robust and reliable while guaranteeing a precise flange focal distance. This is the main reasons that lead to 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 mount as well as 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. 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 only partially 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 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 35, ALEXA Mini LF, ALEXA Classic/XT/SXT, ALEXA LF, ALEXA Mini, AMIRA, and ALEXA 65 cameras. In addition, third parties offer LPL lens mounts for non-ARRI cameras, director's viewfinders, and lens testing tools, so ARRI Ensō and Signature Primes as well as 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.

Lens Data System

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.

Compatibility
We offer compatibility with the Cooke /i protocol in ALEXA, ALEXA Mini, and AMIRA cameras.

LDS-2
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 system uses a 8.7x faster interface that enables lens manufacturers to send more data to the camera than before.

The ARRI Ensō and Signature Prime lens range are the first family of lenses to use that system. LDS-2 extends the possibilities of lens data and is being licensed to other lens and camera manufacturers.

ALEXA 35, ALEXA Mini LF, and ALEXA LF support LDS-1 and LDS-2.

Lens Mount History

It wasn't the first KINARRI camera but the first large scale production camera, the ARRIFLEX 35, which utilized ARRI's first lens mount. A lens mount serves one key purpose: to enable the director of photography to shape images with more than one focal length or type of lens. ARRIFLEX cameras equipped with a three-lens revolver introduced ARRI’s first mount, the “ARRI Standard”. Even though having a selection of three lenses, ARRI never built revolver cameras with fixed lenses.

ARRI Standard could be found on 35mm (ARRIFLEX 35) and 16mm (ARRIFLEX 16/16 ST) cameras serving both formats. In 1965 was replaced by the “ARRI Bayonet” which uses a different locking mechanism combined with a stainless steel housing for a more accurate fit of the lenses.

PL-Mount
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 ALEXA 35, ALEXA Mini LF, AMIRA, and 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.

This clever lens mount design allows for quick adaption in the field if needed and is being used since.

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: ALEX 65 — but it demanded its own lens mount: In 2015, the XPL mount was introduced; 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 nowadays large format (or “full frame”) camera systems.