ARRI ALEXA LF & Anamorphic Lenses white paper

ARRI announced the ALEXA LF at BSC Expo back in February this year. Fast forward 6 months and ARRI has now shipped hundreds of ALEXA LF cameras. ALEXA LF cameras are currently on feature films like Terminator, Lyrebird and Point Blank; TV series like Umbrella Academy, She’s Gotta Have It, and His Dark Materials; and has been used for commercials for companies such as Chanel, Ford, Heineken, Land Rover, Toyota, Victoria’s Secret, and Volvo.

The ARRI ALEXA LF Large Format Camera features a 36.7mm x 25.54mm sensor that is slightly larger than full frame. It’s 1.5x bigger than the ALEXA SXT, and the ALEXA 65 is 1.5x larger than the ALEXA LF. To put the sensor size is perspective, the Sony VENICE uses a 36.2 x 24.1mm sensor. The MONSTRO 8K VV sensor that can be found in the RED WEAPON and just announced Panavision Millennium DXL2 is 40.96mm x 21.60mm, and ARRI’s own ALEXA 65 has a whopping 54.12mm x 25.58mm (active image area) sensor. Just like the Sony VENICE’s aspect ratio-agnostic sensor, the ALEXA LF has the ability to be used in three different sensor modes:

LF Open Gate – 36.70 x 25.54 mm with (4448 x 3096) @ 44.71mm image circle
Delivers the ALEXA family’s optimal pixel size for highest overall image quality results in a 4448 x 3096 picture

LF 16:9 – 31.68 x 1782 mm with (3840 x 2160) @ 36.35mm image circle
Maximizes lens options while meeting 4K deliverable standards

LF 2.39:1 – 36.70 x 15.31 mm with (4448 x 1856 photo sites) @ 39.76 mm image circle
Combines a cinematic widescreen image with high frame rates up to 150fps for slow motion

 

With these varying sensor modes available, ARRI and Netflix have collaborated to come up with a white paper that explains the use of anamorphic lenses with the ALEXA LF cameras. Amongst other topics, the white paper shows in detail how to shoot with ALEXA LF and 35 format 2x anamorphic lenses for a 4K UHD mandate.

Here are some of the key points listed in the white paper:

  • When shooting with 35 format 2x anamorphic lenses for a 2:1 aspect ratio result, Netflix accepts the use of a 2880 x 2880 area on the ALEXA LF sensor. All Master Anamorphic lenses cover this area.
  • When shooting with 35 format 2x anamorphic lenses for a 2.39:1 aspect ratio result, Netflix accepts the use of a 3148 x 2636 area on the ALEXA LF sensor. Master Anamorphic lenses from 40 mm on cover this area, and the ARRI Anamorphic Ultra Wide Zoom AUWZ 19-36 covers this area from 21 mm on for wide angle shots.

ALEXA LF & 2x Anamorphic Lenses with a 4K UHD Mandate

Shooting with ALEXA LF and 2x anamorphic lenses for a 4K UHD mandate for achieving aspect ratios between 2:1 and 2.39:1 using anamorphic lenses, as specified by Netflix for instance, is a common requirement.

Photosite Math

For a 2.39:1 aspect ratio end result (after de-squeezing in post) you need at least 3148 x 2636 photosites on the ALEXA LF sensor (3148 x 2636 = 8,298,128 = 8.30 Megapixel). Set the ALEXA LF to LF Open Gate sensor mode, as that is the only sensor mode that is tall enough for the 2636 height.

Which Anamorphic Lenses cover 3148 x 2636?

Full Frame Anamorphic Lenses

Any anamorphic lens designed for full frame sensors (36 x 24 mm) will cover 3148 x 2636. Currently, these are the P+S Technik Technovision Classic 1.5x anamorphic primes and zooms as well as the Servicevision Scorpiolens Anamorphic 2x primes.

Master Anamorphic Lenses

While the ARRI Master Anamorphic lenses were originally designed to cover the 35 format, they have an illumination area large enough to cover 3148 x 2636 from 40 mm on.

Shooting for a 2:1 Deliverable with a 4K UHD Mandate

For finding the best sensor area for shooting ALEXA LF and 2x anamorphic lenses for a 4K UHD mandate when the intended deliverable has a 2:1 aspect ratio (This is an aspect ratio that is increasingly popular with television series), we had four requirements:

  1. Describe an area on the LF sensor that is as small as possible, so the maximum number of 35 format anamorphic lenses will cover this area.
  2. Use an aspect ratio of 1:1 on the sensor. A 2:1 target aspect ratio gets halved in width by a 2x anamorphic lens.
  3. Have at least 8.29 Megapixel in that area (horizontal photosites x vertical photosites, mathematically rounded). 2880 x 2880 = 8,294,400 = 8.29 Megapixel.
  4. Use only even numbers, since post software has an easier time with even numbers.

2880 x 2880 fulfils all those requirements.

Shooting ALEXA LF to match 35 Format ALEXAs

When shooting with 2x anamorphic lenses on 35 format ALEXA cameras, you should use the 6:5 sensor mode. On the ALEXA LF, the LF 16:9 sensor mode (the yellow rectangle in the drawing below) has exactly the same height as the ALEXA 35 format 6:5 sensor mode (the blue rectangle): 2160 photosites. So, if you want to match what a 35 format ALEXA does when shooting in 6:5 sensor mode, set ALEXA LF to LF 16:9 sensor mode, shoot and crop the left and right sides in post.

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