@shian, which Panasonic-provided Varicam V-Log LUT do you mean exactly?

Panasonic's V-Log-L tone curve requires exposure calibration in order to precisely match the dynamic range of the footage to the curve of the Varicam V-Log LUT. The reason this is necessary is because V-Log-L is not purely logarithmic throughout its 12-stop dynamic range, the bottom 4 stops have a non-logarithmic rolloff down to 0% reflectance. In addition, the entire V-Log-L curve is offset with a 12.5% pedestal. The DVX200 (and presumeably the GH4R) enforces this LOG-LUT match by scaling the maximum sensor level down to 79 IRE and biasing the black level to 12.5%. Unfortunately, that discards the camera's 80-109 IRE range, leaving you with little more than 7-bit data precision when V-Log-L is encoded into 8-bit H.264 format. What's worse is that the bottom 4 stops of the 12-stop dynamic range are encoded with just 4 bits to cover all 4 stops. Such a crude level of digitization makes the effort to capture 12-stops of dynamic range futile. Bottom line: V-Log-L was designed for recording at 10-bit or better data precision; 8-bit H.264 encoding is not quite good enough to preserve shadow detail.

@Vitaliy_Kiselev

The LOG-LUT workflow provided by the post production tools isn't designed to be as flexible as your mathematical manipulations would require. As you claim, any exposure mismatch between the footage and the LUT could be easily corrected by adjusting the gain of the footage before feeding it into the LUT. But that would require manual calibration of post production levels, and Panasonic's engineers wanted to avoid that complication. They instead enforce the proper V-Log-L exposure level at the time of recording, producing a turnkey workflow that requires no technical understanding of the process on the part of the user.

@Vitaliy_Kiselev

you can just tune things looking at proper tools if you need to change how footage looks.

Yes, so long as you tune things after first processing the log footage through a precalibrated LUT. If you use an uncalibrated LUT or just eyeball it without a LUT, you're basically using the log curve as a subjective flavoring, like any other tone curve.

@lpowell said

there's no such thing as an "extended highlight range"

Sure there is. A typical camera clips at 3.5-4 stops above middle grey. The Varicam 35 clips at 6.5 stops above middle grey. Canon Log clips at 5.3 stops above middle grey. S-Log2 clips at 6.2 stops above middle grey. S-Log3 clips at 7.7 stops above middle grey.

Your claim that "you don't want to devote a lot of bits to encoding noise" is absolutely the wrong approach to preserving shadow detail.

Then please tell Canon, Sony, Panasonic, GoPro, and Arri that they have done it wrong. All of their camera log color spaces use decreasing precision towards black.

this one http://pro-av.panasonic.net/en/varicam/35/dl.html

@shian, you mean VariCam 35 3DLUT V-Log to V-709 (Ver.1.00)? That LUT maps to Rec.709, not to a linear space.

They instead enforce the proper V-Log-L exposure level at the time of recording, producing a turnkey workflow that requires no technical understanding of the process on the part of the user.

@lpowell, Panasonic does not enforce any particular way of working or setting exposure level. They readily provide the definition of the V-Log color space, which can be used to transform it into a log or linear color space, enabling a film-style or VFX-style workflow. What you call "manual calibration of post production levels" is just exposure compensation and color balancing, which are totally normal and standard parts of any film workflow.

The "turnkey" workflow using their V-Log-to-709 LUT is not really a workflow at all, but a quick way of previewing footage that hasn't been graded yet.

With V-Log-L on the DVX200 (and presumeably on the GH4R) "proper" exposure is highly unintuitive. The maximum output level of the sensor is scaled down to 79 IRE, and there's no way to capture anything but white in the 80-109 IRE range.

Have you verified this yourself, or are you relying on the early tests done by other people?

Extended highlight range is not marketing speak. It's a very real property of the camera's behavior, which you need to understand when you're shooting.

Take any camera that records into a display-referenced color space and a camera log color space. Shoot something in the display-referenced color space. Note which objects in the image are below clipping and which are clipped. Now switch to the camera log color space, keeping the same ISO setting. Some of the objects that were clipping before now will not be clipping. The highlight range has been extended. You should understand the highlight range of your camera's output to know how to expose and how to set the ISO setting.

Of course in a camera log mode you may not be able to choose all of the same ISO settings that are available in a display-referenced mode.

@balazer

Note which objects in the image are below clipping and which are clipped. Now switch to the camera log color space, keeping the same ISO setting. Some of the objects that were clipping before now will not be clipping.

That is nothing more than under-the-hood ISO fudging and tone curve manipulation by the firmware. In digital photography, "ISO" is a convenient fiction, what actually matters is the exposure gain of the tone curve. The image sensor responds exactly the same in both cases. So long as you haven't saturated the sensor, whether highlights clip or not is determined simply by the amount of gain applied to the output of the image sensor.

ISO is not fiction. It defines the relationship between the brightness of the exposure on the camera's sensor and the output values.

I do not know that is "brightness of the exposure", it is just raw sensor values.

ISO setting can affect analog gain (part before ADC), digital gain, some processing and some other things (noise reduction parts) In digital world ISO is quite inaccurate thing with firms using various tricks to look better for given number you set in menu on knob.

You have forgotten that before the sensor produces raw sensor values, there is light falling on the sensor. That light can be quantified, and related to the camera's output

Again, it is vague terms, what is "quantified, and related to the camera's output"?

Sensor physics are very good known and all we have is raw sensor values (what happen before is not our task), in processing developers also use some knowledge of sensor lenses and filters properties, as well as how exact pixel position effects catching efficiency. But, for log discussion making simplification we can assume that sensor is almost perfect, has no natural vignetting, and no filters on pixels, and all we have is luma values. Where this does not fit we can use more complex approach.

Note that this is topic about LOG things, not about light physics.

@balazer

ISO is not fiction. It defines the relationship between the brightness of the exposure on the camera's sensor and the output values.

ISO was orginally defined objectively as film speed. Nowadays, it's just a number that camera manufacturers manipulate to make the various camera modes appear to work in a familiar manner to consumers. Different sensors have a wide range of sensel pitches and well capacities, and have widely varying sensitivity to light. But they're all pretty much linear and with decent optics, each can handle bright daylight. In a fixed lens camera, the maximum output level of the sensor is scaled to produce enough overexposure to fully blow out the JPEG or H.264 encoder at whatever number the manufactor selects as the "minimum ISO" (typically 100, 200, or 400). At the opposite end of the ISO range, what counts is how high you can boost the ISO setting before the image gets too noisy, which is specific to each image sensor. At base, it's all relative to the highlight clipping level of the sensor, regardless of how you manipulate the tone curve and its exposure gain. Dynamic range extends downward from that highlight clipping point, until you run out of enough photon sensitivity or bit depth to discriminate anything useful but black.

BTW, that Canon white paper is a prime example of technological marketing-speak. It's intended to wow you with mathematical diagrams in the hope you won't notice how the tonal depth of the C300's 12-stop dynamic range is obliterated by the crude digitization of the internal 8-bit H.264 encoder.

@Aria

Just use proper topic. This one is most deep. :-)

Second is http://www.personal-view.com/talks/discussion/13641/wheres-a-good-explanation-of-v-log

And third it http://www.personal-view.com/talks/discussion/12795/gh4-firmware-2.3-v-log-for-99-epic-panasonic-marketing-fail