Autor: Redaktion

Arri Alexa

Arri's high end camera platform since 2010

The models are divided into Classic, XT, and SXT models. There is also an Alexa Mini, but that will not be covered in this article. Since February 2018 there is also the Alexa LF available. The LF comes with a Large-Format-4K-Version of the Alexa-Sensor. The Alexa LF is covered in a separate article.

All models use the Super35 ALEV III sensor with 3414x2198 pixels. The effective image resolution depends on the Alexa generation used as well as the selected format and aspect ratio.

Arri Alexa

In the Alexa's first generation, the Classic models include the Alexa, Alexa Plus, Alexa Plus 4:3, Alexa M, Alexa Studio, and Alexa HD / Plus HD. The latter two models are designed for use in studio multi-camera systems.

All Classic models use the Apple ProRes codec or the DNxHD (to 2K) on SxS cards. The ARRIRAW format can be recorded externally in  2880 x 1620 12-bit. The camera requires a T-Link certified recorder if capturing in RAW on an external device.

The following devices are T-Link certified:

Codex Onboard (S / M), S.Two OB-1
Astro Design HR-7502-A
cineFlow cineTake
Convergent Design Gemini 4:4:4

All models can be upgraded with the XR (Extended Recording) module. The XR upgrade allows in-camera ARRIRAW recording up to 120fps using Codex XR Drives (with up to 512GB capacity), an XR Adapter for SxS or CFast 2.0 cards. Furthermore, ProRes 4444 (XQ) is possible up to 3.2K.

The Alexa XT (Extended Technology) range were in distribution from 2013 until 2016. The range came equipped with a 4:3 sensor and Open Gate Sensor Mode as standard, which uses the entire sensor area for recording in ARRIRAW. The range also includes ARRIRAW recording to 120fps, ProRes 4444 (XQ) to 3.2K and 8 internal ND filter of ND0.3 to ND2.4 in all models. The Arri XT was  comprised of 4 models: XT, XT Plus, XT M, and XT Studio.

The latest generation of Alexa is the Alexa SXT (Super Extended Technology) range. The SXT models are easily distinguished by their 4K capability and entirely new signal processing. Although the sensor can only achieve 3.2K natively, Arri uses up-sampling to provide output formats of 4K UHD (3840x2160) or 4K Cine (4096x2636). All XT models (except XT-M) can be expanded via an SXR upgrade. The XT-M has been completely replaced by the Alexa Mini.

Working with the Alexa

The Exposure Index (EI) is, strictly speaking, not specified for film light sensitivities, but a portion of the available f-stops are above and below 18% grey. Irrespective of the value, the Alexa always has about 14 stops. An EI of 200 delivers about 5.5 stops above and 8.9 stops below 18% grey, while an EI of 3200 delivers 9.7 stops above and 4.8 stops below 18% grey. Exposure measurements can be performed with the False Color check.

False Color can be used in the viewfinder or on the MON OUT port to measure the signal corresponding to the preset gamma curves (Rec 709 or LOGC). Red and violet give white and black clipping, green gives the level of 18% grey and pink is an ideal measure of the exposure of skin tones.

The Alex range has two available colour processing modes: Rec 709 and LOG C.

Rec 709 is a display-specific signal and corresponds to the HD standard. It can be immediately viewed and further processed in any HD workflow without the use of LUTs.

LOG C is a logarithmic encoding technique and does not denote a colour space as such, but a set of curves based on the characteristics of the film negative. LOG C shows a low-contrast image, which cannot be reproduced correctly on a display. LUTs can take the required "tone mapping" to represent LOG C correctly. LOG C is ideal for recording image information and offers a wide range of options for the colour correction process. The newest Alexa Alexa SXT also supports REC.2020 .

The Alexa range also offer Legal and Extended Range options during recording. In terms of YCbCr video, the difference between Legal and Extended is that an Extended level signal will have picture information from -9-109% whereas with Legal Levels, the signal will be clipped at 0% and 100%. The ProRes and DNxHD codecs are, as a rule, always in Legal Range since this is set in the codec specification. Extended Range is available when the signal is transformed into an RGB colour space.

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