ALEV III Image Sensor

ALEV Sensors

Image Sensors with the DNA of Film

It Started With A Tall Super 35 Sensor

The custom developed ALEV III CMOS Bayer sensor used in the ALEXA and AMIRA series of cameras has the same height and width as a 35 mm film frame. This bears a number of advantages. The Super 35 mm width allows a cinematically shallow depth of field as well as plug and play use of the unparalleled range of Super 35 PL mount lenses. Unique for a digital sensor is the height of the sensor, which allows a number of sensor modes only found in ALEXA cameras, including those needed for shooting with anamorphic lenses. The sensor's 3.4K horizontal photosite count delivers large photosites for an optimal balance between image sharpness on the one hand and high dynamic range, high sensitivity, and a low noise floor on the other, resulting in the best overall image quality for all kinds of deliverables.

The next evolution of our ALEV III came in 2014/2015 and was housed within the ALEXA 65 camera. The ALEV III A3X was reinvented as a larger sized sensor offering a photosite count of 6560x3100 with the same large photosites known from the Super 35 sized sensor. This makes ALEXA 65 the digital camera with the largest digital motion picture sensor on the market. Its image size even surpasses 65 mm film.
Next in line was the ALEXA LF using the A2X version of the ALEV III. Introduced in 2018, this sensor offers a lower photosite count than ALEXA 65 but introduces “Large Format” (also known as 35 mm full frame in still photography) to the market. Covering an area of 36.70 mm x 25.54 mm, this format also introduces a new lens mount, the LPL.

Sensor Modes

ARRI cameras offer a multitude of different recording resolutions. At first glance this looks like a confusing charade but quite the opposite. It started with ALEXA using the 16:9 and 4:3 readout of the sensor to offer the maximum possible resolution for a given situation, e.g. TV productions, spherical, or anamorphic capture for cinematic release. Then the desire to use as much of the sensor as possible emerged and brought about a third recording mode: Open Gate. This allowed filmmakers to use the surround view area for capture.
The more use cases that emerged, the more sensor sizes were created and we maintained compatibility throughout the camera lineup.

The Science Behind the Sensor

Although the science behind the sensor is complex, the use of large photosites and a Dual Gain Architecture are its two main principles.
By employing unusually large photosites (in today's world of tiny cell phone sensors and high megapixel counts), ALEXA's sensor exhibits high dynamic range, high sensitivity, and low crosstalk. The larger a photosite is, the more light it can capture and the lower the noise.

The Dual Gain Architecture simultaneously provides two separate read-out paths from each pixel with different amplification. The first path contains the regular, highly amplified signal. The second path contains a signal with lower amplification to capture the information that is clipped in the first path. Both paths feed into the camera's A/D converters, delivering a 14-bit image for each path. These images are then combined into a single 16-bit high dynamic range image. This method enhances low light performance and prevents the highlights from being clipped, thereby significantly extending the dynamic range of the image.

Optical Low Pass Filter

The low pass filter blocks high image frequencies that would lead to artifacts when captured by the sensor. It is made from a modern, high-performance crystal with optical properties that have been fine-tuned for a perfect match between the transmitted image content and the sensor's photosite structure in the camera. The result is the creation of super sharp yet natural images.

IR and UV filters

ARRI cameras use a high-end infrared (IR) and ultraviolet (UV) filter, which are custom designed to block enough of the spectrum to avoid false color effects (as could otherwise be seen e.g. with dark synthetic materials) but that leaves enough of the near red spectrum to give skin tones a pleasant look. Designing the IR filter is a precarious balancing act, and the IR filters in ARRI cameras are also closely matched to the spectral response of the camera's sensor and the color processing in the camera for best skin tone reproduction.

FSND Filter Mechanism and Behind the Lens Filter

The AMIRA was the first camera to feature ARRI’s motorized FSND filter mechanism which is also used in ALEXA Mini and bigger in size for ALEXA Mini LF. The ARRI FSND filter guarantees an even light reduction over the whole spectrum. Built into small filter frames in a motorized mechanism in front of the sensor, the FSND filter features three strengths that are available to dial in via the camera menu (0.6, 1.2, and 2.1 — plus optical clear on ALEXA Mini LF: 0.6, 1.2, 1.8, and clear ).

The ALEXA camera series inherited the filter type in form of a round “behind the lens filter” that sits magnetically secured in front of the sensor. For “matte box use” the filters are also available as regular 4” x 5.65” and 6” x 6” filters.