ARRI Forum

The ALEXA 35 sensor has a 3:2 aspect ratio, just like the sensors of almost all our other digital cameras. Yes, it is true, the ALEXA Classic, XT, SXT, Mini, LF, Mini LF and AMIRA all have a 3:2 aspect ratio sensor; the only exception is the ALEXA 65 with its 2.1:1 aspect ratio sensor.

Why 3:2? Because it is a very good compromise between a very wide sensor, which you would want for shooting with spherical lenses for a 2.39:1 deliverable, and a very tall sensor, which you would want for shooting with 2x anamorphic lenses for a 2.39:1 deliverable. Those are the two most extreme shooting formats for sensor width and height.

So what is 4:3? 4:3 is a left over term from the film days, defined originally in 1917 by the SMPE (before they had the “T” in their name) as the aspect ratio of the film negative's Silent Aperture. Later, this would also be called Super 35. In 1932, the AMPAS defined the smaller Academy Aperture, which also has a 4:3 aspect ratio, known afterwards as Academy Regular 35. When the first ALEXA became available with a sensor taller than 16:9, we marketed it as a “Cinematic 4:3” sensor, even though it really had a 3:2 aspect ratio, in order to differentiate it from the 16:9 competition. But, really, there is nothing 4:3 about it, nor is there any modern deliverable that requires 4:3.

So what sensor area do you need for 2x anamorphic lenses, if not 4:3? For anamorphic 2x lenses you need a 6:5 aspect ratio area on the sensor. The target deliverable aspect ratio for 2x anamorphic lenses is 2.39:1 (don't get me started with that 2.35:1 stuff; it has been 2.39:1 since 1970!). Since a 2x anamorphic lens will squeeze the 2.39:1 image by half, you end up needing a 1.195:1 area on the sensor. For simplicity's sake, we rounded that up to 1.2:1, which is the same as 6:5, to stay with full integers.