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More on Log vs Lin.


So this alexa thingy is really gaining ground, and I have been bombarded by questions about Log vs Lin and how the signals actually look on screen and on a scope, so I did test shoot with my wife working away on her glass jewelry.

So lets look at arri's LogC vs Rec709 of the same content.

This image is showing the visual differences between the logC version on the Right and the Rec709 Linear example on the Left.

We all can see that the LogC image is much flatter, gray and muddy compared to the same image on the Right which has been processed in hardware to match the Rec 709 color space that is used as the default for HD signal and is used on all production monitors.

LOG was developed as a process that would allow a longer tonal range in post to be extracted from film negatives, as film negatives have a base layer that includes color masking that has to be extracted prior to making a positive image or Reversal Print.

This masking was done to offset the impurity of the dyes being used in the silver matrix to record the images. It is hard to make all of the colors respond the same way when colors of light operate at differing wavelengths. So when Arri Started making the ArriScan to convert film to digital, they offered the users a digital file that acted more like a film negative rather than giving these highend post house an image that looked and acted like video.
The first film scanners only created digital from film in a frame by frame process when capturing the files, they were usually converted to DPX ( digital picture exchange) frames that were designed to handle the longer film style latitude and lower contrast range that finishing on FILM required.

This is the same image- only here I am just showing the difference between Log and Rec709 on FCP's waveform display.

You can clearly see that while the REC 709 Image fills the waveform with a longer tonal range, there is not any loss associated with conversion of log gamma to linear that the REC 709 space entails.

The Rec 709 on the left clearly is clamping the whites at 100 IRE and have extended the blacks all the way down to 0 IRE.

Where the LogC image- which is the recorded format of original content for BOTH IMAGES has all of the luma weighted in the middle of the IRE scale, allowing the most amount of "correction" to extract information out of the shadows.

I cannot stress to everyone enough that the Rec 709 file is output of LogC master. These are the same image, I am just using the power of the logarithmic acquisition to offer me a much greater range of control in post.

Posted by: gary adcock on Jul 27, 2010 at 9:42:58 am ARRI Alexa, LogC

New Tools, but an old way to look.

I have said many times that changing cameras often time forces one to change everything in their production lifestyle, the movement to tapeless has been wrought with issues and concepts that had to change with the times. Arri has brought back an oldy but a goody, and chances are most of the readers here do not even know about a post color space called LogC.

LogC is Arri's versioning of the Kodak Cineon Format, developed to be able to allow film scanners to more accurately represent the original camera negative when going into DI. Arri has used this as the matrix for the D21 for 5 years now and on their film scanners for over 10 years, so while it might be new to all of you, people on the film side have seen it all before.

Here is a 3 Sec Clip out the window of the van captured as ProRes 4444, Asa 800 with a Cooke 28mm i4 lens this is straight out of the camera and compressed into the default HQ Iphone compression for delivery here

Here is the same clip,using a default LogC conversion matrix for the D21 via Iridas Speedgrade, with the LUT applied and output from Apple Color. ( An Amazing COLOR trick for the LogC format -I got almost exactly the same result by choosing the "auto balance" button in the Primary room, then running the saturation slider up to about 1.4 or so.)

The only lights in these shots are the streetlights, nothing more beyond the practicals that you see in the shot, there is not any additional lighting or processing of the content.

So not only is this camera a new level, it carries with it the best of the legacy formatting. RED required us to all change to fit their acquisition model, Arri is choosing to stick to one that has worked, at the high level, for a fairly long time, It is not required to re-invent the wheel, but changing the materials to get what you want is not such a bad thing

Posted by: gary adcock on Jul 21, 2010 at 12:20:35 pm ARRI Alexa, LogC

Join industry geek Gary Adcock as he explores the world and technology as it relates to Film and Video production. I am also a Glass artist, so I spend my free time creating art from silica and fire when I am not cooking.


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