Alexa Log C - Workflow Options

This post is intended to be a work in progress so I'll be adding more information over time as workflows evolve. I'm intentionally leaving some of this open ended because I'd like anyone reading this who can speak to the topic to help me flesh this out. If you have something to add, please shoot me an email.

These past few months, it's been all about the Alexa. Now that there's a healthy amount of them in the rental market, they're popping up all over the place. I can't speak for other people but for me, all of the jobs that would have been RED three months ago are now Alexa. What happened?

First of all, the images are just stunning. In my opinion, for now, the Alexa is the digital option that really stands up to motion picture film. Just like color negative, there's no harsh and sudden clip in the white, instead the highlights gently and elegantly roll off. There's tons of picture information in the shadows and the noise signature looks more like pleasing film grain than crummy video gain. The color response is faithful and natural. And it's unbelievably simple to work with. Arri really has done an excellent job with this camera and commercial production has been quick to embrace it.

There's a lot of discussion out there as to the best way to work with it - Log C, Rec709, and as of a few days ago, Arri Raw. So what's to be gained by shooting in Log C over Rec709? Really the answer to that question very much depends on how much time and money is available for post production. The out of the box Rec709 video on the Alexa actually looks very good and has ample dynamic range so if there's no time/money for a grade, then the WYSIWYG video is certainly a viable option. The fact is though that Log images encoded to a high quality RGB recorder are always going to give you far more flexibility in the online suite. And now with 2k Arri Raw there's even more flexibility and more resolution than the current 1920x1080 raster. As this newest option is only now available and I haven't worked with it yet, I can't speak to it in this blog post. Instead I'll just be listing a few options for live color correction of the camera's Log C video output.

Unless you have a lot of experience with Log space, it can be difficult to evaluate exposure because there is very little contrast in these images. The nice thing about Log though is that it's a conservative format and bad exposures are actually pretty forgiving. Within reason (more on that later). But you still need to ensure an overall baseline for consistency and continuity. To do this, savvy productions would use some form of viewing LUT (Lookup Table) or in other words, a color space transformation of the Log stream to non-destructively put it into a more normal range of contrast and color saturation.

I really like this simple definition of a LUT that my friend Adrian Jebef came up with -

"..a LUT (Lookup Table), in image processing, is used to transform input data into a more desirable output format. Turn X to Y. Grey to Black. Or Grey to White."

And that's it in a nutshell. To really get into the meat and potatoes of color space transformations is way beyond the scope of this post so if you're looking for more information, Steve Shaw ofLight Illusion published some articles on the topic that answer a lot of questions.

Using LUT's

1D vs. 3D LUT's

Linear vs. Log

Technical Papers (the DI guide is particularly useful)

Using a LUT on-set not only makes evaluating exposures easier but it also makes the production feel much better about what they're shooting. Who wouldn't rather look at a colorful, contrasty image than a flat, murky one? To me this is kind of a no brainer - which would you rather the agency see -

this?