The camera's custom developed CMOS Bayer sensor has the same height and width as a 35mm film frame. This has 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 ALEXA sensor, which allows a number of sensor modes only available in ALEXA cameras, including those needed for shooting with anamorphic lenses. The sensor's 3.4K horizontal photosite count delivers unusually 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 HD, 2K, 4K UHD and 4K Cine deliverables.
The Super 35 mm width of ALEXA's sensor results in a cinematically shallow depth of field that works well to separate foreground from background to guide the audience's attention.
In addition, the Super 35 width means that any of the unparalleled range of Super 35 PL mount lenses can be used with ALEXA, be that a standard speed or a high speed prime, a wide or a short zoom, an extreme wide angle lens or the interesting realm of specialty lenses including such specimen as the 'squishy' lens, the 'Hubble' (extreme long telephoto lens) and others.
The 16:9 sensor mode is most often used for TV shows or commercials that need an HD or a 4K UHD deliverable. For the 16:9 HD recording format, 2.8K from the sensor are used and down-sampled. For the 16:9 4K UHD recording format, 3.2K from the sensor are used and up-sampled. The 16:9 2.8K sensor mode can also be used to derive a 16:9 2K Cine recording format.
The 16:9 sensor mode is usually used with spherical lenses and can be recorded in ARRIRAW or ProRes on ALEXA SXT cameras.
The 6:5 sensor mode (formerly 4:3 Cropped) is used most often for feature films and commercials shooting with anamorphic lenses and is crucial for delivering the unique and cinematic widescreen look that can trace its origins back to the CinemaScope films of the 1950s. It is a look that has long been appreciated by cinematographers, directors and the viewing public. Since anamorphic lenses squeeze the 2.39:1 image by a factor of 2, the resulting area used on the sensor has a 1.195:1 aspect ratio, which is 6:5. For convenient viewing of anamorphic images an anamorphic de-squeeze license is included with all ALEXA SXT cameras. The ALEXA sensor is unique in that it has the same height as a 35 mm film frame, and so anamorphic lenses exhibit the same field of view as on 35 mm film.
The 6:5 sensor mode can be recorded in ARRIRAW or ProRes on ALEXA SXT cameras.
The 4:3 sensor mode is as tall but slight wider than 6:5. Shooting 4:3 has become popular for VFX heavy feature films in the 1.85:1 aspect ratio (dotted line). The area above and below the 1.85:1 image is used for placing VFX markers and/or for up/down repositioning.
The 4:3 sensor mode can be recorded in ARRIRAW or ProRes on ALEXA SXT cameras.
Open Gate uses the full sensor area of the ALEXA camera. This format has become popular for those shooting feature films in 2.39:1 or 1.85:1. The extra area above and below the image is used for placing VFX markers or for image repositioning, resizing, rotating or stabilizing. The extra resolution provides the best result when up-sampling to a 4K Cine recording format.
While some use it as their main recording format, others shoot in 16:9 or 4:3 and only certain scenes in Open Gate. The Open Gate sensor mode can be recorded in ARRIRAW or ProRes on ALEXA SXT cameras.
Although the science behind the breakthrough performance of ALEXA's custom designed CMOS 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), 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.