Capturing 2:1 Anamorphic with Alexa LF and S-35 lenses

Capturing 2:1 Anamorphic isn’t quite so straight forward on the Alexa LF and Mini LF, due to the number of photosites on the sensor.

In a nutshell, according to ARRI, shooting on 35mm format anamorphic lenses for a 2:1 aspect ratio inside a square box, requires a resolution of 2,880 x 2,880 pixels. This satisfies the 4K quality threshold by Netflix as an acceptable solution, if you are shooting with Alexa LF and ARRI Mini LF.

The reasons for doing this are easy – 2,880 x 2,880 has the same number of pixels as 3,840 x 2160 – try it and you will see that this is correct. 

Nextflix do not specify the native code, whether RAW Prores 4:4:4 etc, suffice to say that this needs to be a qualifying version of Prores, which to my knowledge is most of them

In case you are interested, Netflix has approved ARRIRAW, ProRes 4444 XQ, ProRes 4444 and ProRes 422 HQ for use with ALEXA LF.

This sounds great, as it means that Alexa LF can work with S-35 anamorphic lenses and record 4K, which is exactly what our clients will want to do and shoot in the hallowed 4K format without compromise and without having to choose large format (or full frame) spherical lenses.

The problem is that this is a non-standard sensor size and not all S-35 lenses cover this fully.

The devil is in the detail, so upon investigation, we established:

Two things.

  1. 2880 pixels height is VERY high and that some S-35 anamorphic lenses do vignette a bit. 
  2. Next, could conventional Alexas with 3.2K open gate achieve this coverage?  

Whilst I prefer exacting language and try to articulate myself carefully, the truth is that the widest lenses may compromise the cinematographer, so I don’t think that this is that useful.  See below.

  1. Whilst the longer lenses will fully cover without issue, wider lenses vignette.
  2. The Alexa 3.4K open gate covers this at the widest point but the height is compromised and ARRI advise me that in their tests, a maximum vertical pixel count of only 2,202 is possible, so 4K on regular Alexa will never be capable of true 4K acquisition.

So in English, this means that whilst 2,880 x 2880 with a suitable set of lenses would satisfy 4K anamorphic capture, this only really works with Full Frame anamorphics.

Unless the cinematographer has assured themselves of the suitability of given lenses at all focal lengths at this image size, be prepared for them to be disappointed, since wider focal lengths are likely to vignette at this image size. 

However, as you can see from the images supplied by ARRI, all Master Anamorphics cover from 28mm onwards, though the 28mm is a bit close.  ARRI have advised me that the Cooke Anamorphic/i lenses have slightly greater coverage, though we have not confirmed this ourselves. 

It is our expectation however, that cinematographers will demand that the desired sensor size is covered adequately and without compromise.

So if you are able to find lenses which keeps the cinematographer happy, then shooting with 2x anamorphics and Alexa LF to achieve 4K capture in a 2:1 aspect ratio which satisfies the exacting standards of Nexflix is difficult.

Meanwhile, we are all waiting for the next sensor which ARRI will bring to the market with the next generation of 4K/6K/8K Alexa Mini…  Though on this, ARRI have not been too forthright about their plans.

Barry Bassett

VMI, July 2018

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