ALEXA 35 - High Dynamic Range

To demonstrate the enormously high, 17-stop dynamic range of the ALEXA 35, we are using an image from the ALEXA 35 Encounters film “Siren,” shot by James Friend ASC, BSC with an ALEXA 35 pre-production model.



A Rec 709 Standard Dynamic Range (SDR) version of the image does not show very much dynamic range. The highlights in the red emergency light look clipped and the dark parts of the image inside the scuba diver helmet appear crushed. 

This is because an SDR monitor can only display about eight stops of dynamic range. The camera captures much more information than traditional SDR technology can accommodate; it is simply not possible to display all 17 stops captured by ALEXA 35 on an eight-stop SDR monitor.

  

HDR_Image_1

Rec 709 SDR ALEXA 35 output

But when we look at the LogC4 signal (ProRes or ARRIRAW) recorded by ALEXA 35, we can make all 17 stops of captured information visible.


It is possible in postproduction to grade the LogC4 image down, to see what information we have in the highlights. We might end up with an image like this. 

There is no loss of information in the highlights at all. Even the super-bright flashing emergency torch lights are captured without clipping.

HDR_Image_2

Same captured image, but graded for the highlights

To look at the information in dark parts of the image, we can brighten them by grading the LogC4 image up. We could end up with an image like this.

We can see that the camera has beautifully captured everything, even the very dark shadows inside the scuba diver helmet.

HDR_Image_3

Same captured image, but graded for the dark parts


So, when using SDR monitors on set, you will not see everything the camera is actually capturing. To better understand what is being captured, use the “False Color” exposure tool that is built into the camera. The false color image is a flat, black-and-white image (LogC4) with specific exposure levels represented by six colors: two for the highlights, two for the shadows, and two for the midtones.


The red and purple colors are critical, as they indicate regions of your image where there is no more information, either because the image is too bright (just below clipping: red) or too dark (noise floor: purple).

ALEXA_35_False_Color_Index

Here is our image with false color turned on.

There are no areas in the image with purple or red coloring. The yellow and blue regions are close to clipping, but the sensor has captured all the information in these regions.

The false color exposure tool is a very fast and easy method to understand what information is captured.

HDR_Image_5

LogC4 False Color


Please note that most of the time it will be very difficult to clip the image at all, because ALEXA 35 has the highest dynamic range of any camera on the market. While this may be disconcerting when shooting with an ALEXA 35 for the first time, most cinematographers get used (and addicted) to the camera's dynamic range very quickly. 

This unique ability allows colorists and cinematographers the greatest flexibility during color grading, as they can freely decide which brightness levels in the image to reveal or conceal, as everything is captured. It also makes for the very best source material for High Dynamic Range (HDR) deliverables.
 

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