Image sensors with the DNA of film
The custom-developed Super 35 ALEV3 and ALEV4 CMOS Bayer sensors used in the ALEXA and AMIRA series of cameras have the same height and width as a 35 mm film frame. This bears several advantages. The Super 35 width allows use of the unparalleled range of spherical Super 35 and large-format PL mount lenses. The sensor's height allows for shooting with Super 35 and large-format anamorphic lenses.
A big evolutionary step for our ALEV 3 came in 2014/2015 and was housed within the ALEXA 65 camera. The “ALEV 3 A3X” is a sensor three times as big as the Super 35 ALEV 3, offering a photosite count of 6560x3100 with the same 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 “ALEV 3 A2X” sensor. 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 introduced a new lens mount, the Large Positive Lock mount, or LPL.
In 2022, introduced with the ALEXA 35 camera, the ALEV 4 sensor upped the game: a new Super 35 4.6K sensor with significantly increased dynamic range and higher sensitivity.
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 mode: Open Gate. This allows filmmakers to use the surround view area for capture. The more customers clamored for different modes to increase frame rates, decrease data rates or have time-saving features like in-camera anamorphic de-squeeze, the more recording formats were created.
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 18/16-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.
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.
