Arri Alexa

oldboy

New member
I have been reading a lot about the Alexa but since its brand new I haven't found much tests with it or a breakdown of the best ways to use it as you can find on reduser.

I have two questions. Since its base ASA is 800 is that the best to shoot with even outdoors? It seems if you bring it down to a 500 or less your going be overexposing your image. 800 seems a pain with ND but shooting at a 800 outside, inside, etc will give you (in film terms) the thickest "negative."

Also what exactly is Log-C? Why would you use it over Rec709? From my reading its adopted from the Arri Film Scanners to give you more room in post. Though if you want that room don't you already have it in the ArriRAW file?

Thank You
 
Yes, 800 ASA is a pain in bright sunlight... it's a problem with all the newer digital cameras, even the Genesis, you end up using ND1.2 filters just to get an f/8 sometimes. I just finished a movie in Texas shot on film and I've never used so much 50 ASA filmstock before in my career, it was so bright out there.

Sensors respond to light in a linear manner; film stocks respond to light in a Log manner, only the midtones are linear (i.e. an equal amount of silver is formed by an equal amount of exposure), the bright and dark areas start to fall-off in terms of the amount of negative density formed by exposure.

One advantage to Log recording for a digital camera is that more signal is allocated to bright highlights within the 10-bit range -- for example in PanaLog, white on a chip chart is not recorded at 100 IRE but 70 IRE. What this means practically is that you have another 30 IRE or so beyond "white" to capture overexposed detail. Of course, you could do this with a linear RAW recording as well, expose the highlights lower to preserve more overexposure information.

RAW recordings tend to be higher than 10-bit, so in some ways, you can think of them in similar ways, the same linear exposure range captured in 12-bit RAW can be "crammed" easily into a 10-bit Log curve.

But other than the limitations of 10-bit, the other reason for using Log is simply that film is naturally log, in other words, your exposure information in a film scan falls into a Log curve. So digital cameras that use Log for output will go through a post pipeline designed for 10-bit Log DPX files of film scans more easily than cameras that have a linear RAW output. Of course, one solution is to convert 12-bit (or whatever) linear RAW to 10-bit Log before starting the D.I. work, but another solution lately has to been to just work in linear.

The problem with Log conversions is that some Log conversion are better than others, or some post houses at least have better luck with some Log formats than others.

ARRI has developed Log-C to be close to the look of a 10-bit Log film scan, whereas if you look at PanaLog (for the Genesis) it's a much less aggressive Log format in terms of color and gamma, which can make it easier to color-correct in a wider variety of systems, some of which are really optimized for Rec709 video more than for Log.

Rec709 is both a gamma and a color space, both of which are more limited than a Log or RAW recording; Rec709 is really designed for broadcast and monitor viewing, not for film outs. Most Log recordings, for example, can capture at least two more stops of exposure information than a Rec709 recording unless extreme flattening of the signal is used, like with HyperGamma. Rec709 really should only be used as an output format after color-correction, when a broadcast master is needed. I did one season of a TV series though where I had to use Rec709 instead of Log in the camera because the production wanted to save money by making straight tape dubs for dailies and editorial, avoiding any Log-to-Rec709 conversions. But that was a bit silly because it only benefitted the dailies at the expense of the final color-correction and mastering where working with a Log recording would have been better.

But obviously if you are delivering a tape for immediate broadcast, no color-correction, and it has no other purpose, then most people would just shoot in Rec709. Reality shows, news, etc. generally don't need wider-than-Rec709 gamma for their purposes.

Though ARRIRAW is not yet enabled in the Alexa yet, the ARRI digital cameras can give you either a 10-bit Log output or something more directly from the sensor in terms of a RAW output, though it seems that ARRI is formatting/converting the RAW linear output to Log even for 12-bit:

This is from the ARRI site:

http://www.arridigital.com/alexa/faq#5-4

The D-21 ARRIRAW has the white balance baked in, since back in 2002 we felt that it would be closer to the way film negative behaves. The EI setting is not baked in. The 2880 x 2160 pixel images (4:3 aspect ratio) are transported as 12 bit linear data.
The ALEXA ARRIRAW file format has neither the white balance nor the EI setting baked in. However, both white balance and EI that are set in the ALEXA are stored in the ARRIRAW header as metadata. This metadata will also include other information, including the green/magenta shift value (CC) that has been set in the camera. The 2880 x 1620 pixel images (16:9 aspect ratio) are transported as 12 bit log data, since this is a better way to transport ALEXA's wide dynamic range.
Both file formats can be transported via ARRIRAW T-link. The initial ALEXA ARRIRAW file format has been purposefully kept very close to the D-21 file format (both are 12 bit), since that means only minor modifications to the debayering algorithms used by the recorders and post tools. The recorders and post tools will be able to distinguish between D-21 and ALEXA ARRIRAW based on the metadata, so the whole thing is transparent to the user. In the future, we are planning other ARRIRAW formats to further exploit the capabilities of ALEXA.
 
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