HDR display technologies for cinemas and private homes are evolving rapidly, bringing vast improvements to image quality and richness. However, technical standards and workflows for creating and delivering HDR content have yet to be established. ARRI has been pushing the boundaries of high dynamic range digital imaging since the development of the ARRILASER and ARRISCAN, and is involved in the ongoing discussions that could see HDR become an important storytelling tool for filmmakers.

With HDR still in its early days, there is much confusion and misinformation across the industry. Amidst this, an increasing number of productions that already create HDR deliverables -- such as the Amazon streaming TV series “The Man in the High Castle”, “Mozart in the Jungle”, and “The Grand Tour” -- are turning to ARRI cameras, which have offered unsurpassed dynamic range since the launch of ALEXA in 2010.

What is the difference between HDR capture and HDR display?

While there are minimum requirements for HDR display, most notably from the UHD Alliance, there are none for HDR capture. Footage from any camera will need to be specially graded for HDR; no camera captures images that can immediately, without processing, be displayed in HDR. The most suitable images for HDR display and distribution will be those with the highest dynamic range and the most natural colorimetry.

How can a camera's dynamic range be measured?

Different camera manufacturers use different measuring methods, so their claims about the absolute number of stops of dynamic range cannot be compared. The most practical way to assess the relative difference between cameras is to shoot a test similar to that used to determine the latitude of film stocks. A scene is set up that contains test charts, specular highlights and real faces, allowing skin tones and color rendition to be assessed; it is lit to a latitude of 10 stops and exposure-bracketed to at least +5/-5 stops. When the over-exposed and under-exposed clips are graded to match the normal, they will at some point show clipping in the highlights and noise masking detail in the blacks.  Such a test will not only reveal the dynamic range of the single photocells, but also of the whole sensor, which may yield a different result due to characteristics such as line noise or fixed pattern noise.

Top cinematographers do not just believe what manufacturers tell them, they do their own tests. Many have told ARRI that dynamic range is the single most important parameter of image quality to them, which is why they choose ALEXA and AMIRA after conducting comparative tests.

Is a higher bit rate the key to HDR capture?

Some manufacturers claim that 16 bit linear is the key to HDR, but it is important to make a distinction between the bit rate used in the camera and that used to store and transport the resultant images. ARRI uses 16 bit linear to capture and process images inside the camera, but then packages them in 12 bit, either as Log C in ProRes files or as a similar log curve in ARRIRAW files. Representing digital images with a logarithmic scale is an efficient allocation of the bits, because a relatively equal amount of code values is assigned to each stop. It also mirrors the incremental way the human eye responds to light, storing the same fractional increments in the highlights as in the shadows.

Packaging images in 16 bit linear results in larger files that bring no added benefit, whereas 12 bit log allows for smaller files that avoid heavier data compression and higher costs without sacrificing image detail, making it the perfect basis for HDR mastering. Just as ARRI has always stated that better pixels are more important than more pixels, when it comes to HDR, better-allocated bits are more important than just more bits.

ARRI is not alone in this attitude. The industry has been aware of the wastefulness of linear encoding for decades, which is why video and computer images, film scans and now HDR distribution formats such as PQ and HLG have always been based on non-linear encoding.

Should I monitor for HDR on set?

Opinion is widely divided because workflows are so new and practical on-set HDR monitors are not yet available. Some see little benefit in monitoring for HDR, while others consider it vital -- ARRI covers both options. ALEXA cameras offer HDR monitoring, as will AMIRA and ALEXA Mini from SUP 5.0. Currently, however, most productions that are graded and distributed in HDR use SDR monitors on set and simply take care to capture a well-exposed image.

The main reason to monitor in HDR on set with ARRI cameras is not to avoid actual technical problems with highlight clipping or shadow losses; it is more to ensure that the creative intent of a scene is appropriately represented on both SDR and HDR displays, without the audience's attention being distracted when bright elements or strong colors within the frame become too prominent in HDR.