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Abdul Rahman Jamous

About the Gamma curves in VARICAM LT

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The graph below is taken from the operating guide of the camera, and frankly I don't get it. 

 

 

also may you guys explain to me what is so special about the 18% input? I always see this percentage when people are testing light meters, filters or may be lenses. It seems to me the 18% is a standard for something but I never understood that standard. 

Capture.PNG

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https://en.wikipedia.org/wiki/Middle_gray

18% reflectance equaling middle grey in photography is a different thing than the first 18% of video signal in this graph. 18% photographic grey is around 46% in video signal terms.

In this graph, it seems that Panasonic has decided for Varicam-Look to slightly flatten (lower contrast) and straighten the response to light in the darkest areas and compensate by slightly increasing contrast in the midtones, and then flattening out the contrast again in the bright highlights.

 

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I will just comment here on the LOG curve, the first in the list.

The idea behind recording in LOG is really a compression scheme for digital capture.  Basically, to minimize the amount of data recorded.  In other words think of the amount of data recorded as a bucket and the bigger the bucket, the larger the file size necessary to store the data. The LOG curve helps to fit the entire range of tones that the sensor can see, and store them in a smaller bucket, by... deleting a lot of the original data.

Digital sensors record light in a mathematically linear curve or a straight line in the graph.  .  And it turns out, that if one squished all the sensor data, in it's straight line into this bucket, this would only be possible by deleting data evenly along the exposure curve.  And this creates gaps in the tonal scale of the image that are quite visible, especially on smooth gradients.

And it turns out that the "meat of the image, the dark to light tones that appear to make up most of an image from black to almost white, would only cover from 0% to maybe 25% of the graph.  Leaving "almost white" to the "brightest tone" using 75% of the data.

It turns out that we have difficulty perceiving the smoothness of gradients in the near whites to the brightest whites, while we are very sensitive to gradients in the dark to almost white tones.  So what they've done here is to stretch darker and mid tones up the scale, so that there are smaller steps between adjacent tones.  They then flatten the highlights so that there is a much wider gap between adjacent tones in the part of the scale that we don't notice these gaps.  So, in the end,  a huge amount of data is deleted from the the lightest tones and much less data is deleted in the darker and mid tones of the image.  And this makes it possible to record a large dynamic range of tones in a small package.

Of course, when you see this LOG curve image displayed on your monitor, it looks quite low contrast and washed out.  In post production color correction, where there is a very large data bucket available, this curve is corrected to create an image that looks like what we would generally call "photographic", but without deleting any additional data.  On the set, there is a setting in the camera to output this correction to your monitor so that you can see a "normal" looking image, while recording the LOG image containing the entire dynamic range that the sensor/camera is capable of seeing.

As David mentioned above, this has the effect of increasing the contrast in the darker tones, which allows much smaller gaps between adjacent tones on the scale.

The other Varicam "look" curves in your example are Panasonic's attempt to capture a larger dynamic rage, while needing less color correction in post production.

The "video" curve is to match traditional broadcast camera look that is standard in live broadcast productions and that's why you'll see so many blown out highlights when you watch your favorite sports on TV.

I hope this makes some sense out of all this for you!

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is it safe to say that this graph is kinda like a waveform monitor, and when the output reaches 110% that means the wave is clipping and it is a total white without any details, and when we shoot in [V-Log] the output will never reaches to the clipping point?  

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Well, if your camera was pointed at a grey scale that started at black on the left and white on the right, then the waveform would resemble that graph.  Remember a camera log gamma is not a display gamma, the log image is meant to be converted and color-corrected into a display gamma.

It is possible that the log formats capture beyond 110% but that's just where the graph ends.  But they all clip at some point. Either that or they simply aren't allowed to hit 110% so at the clip point, the signal is grey-ish.

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6 hours ago, Abdul Rahman Jamous said:

is it safe to say that this graph is kinda like a waveform monitor, and when the output reaches 110% that means the wave is clipping and it is a total white without any details, and when we shoot in [V-Log] the output will never reaches to the clipping point?  

the 110% is the encoded value for the recording and does represent the brightest value that can be saved.  The reason it is a % is that this was the standard for analog video recording and so they've kept to that scale.  In reality, an 8 bit recording can hold 256 values (0 to 255) and a 10 bit recording can hold 1024 values (0 to 1023).  Higher bit rate recordings can hold even more values.  So, today you will come across waveform monitors with values from 0 to 1023 for example, rather than 0 to 110%.  This is what my Divinci Resolve waveform lists as values on it's waveform.

The V-log does also clip, but at a point beyond that which is pictured in this graph.  So, imagine that this graph continues to the right further than is pictured here.

It is also possible that the Varicam LT, pictured here, clips the v-log at roughly 75%, and the values from 76% to 110% are left empty.  And this may be because the V-Log curve is designed to replicate the response of color negative film, which can see values higher (brighter) than the Varicam LT sensor.

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"It is possible that the log formats capture beyond 110% but that's just where the graph ends.  But they all clip at some point. Either that or they simply aren't allowed to hit 110% so at the clip point, the signal is grey-ish."

 

"The V-log does also clip, but at a point beyond that which is pictured in this graph.  So, imagine that this graph continues to the right further than is pictured here."

 

Wow, I must admit that was a very clever commentary.  I believe answering at what percentage does the [V-Log] clips requires some testing, but unfortunately I neither have the camera nor the money to buy it.   So may be I i'll do these tests later. (hopefully)

 

 

Thank you very much for answering my question.

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Off topic.... I tested a Varicam LT for a shoot a few months ago.  And I found that the internal recording and compression produced some unwanted colored artifacts.  Like a grey T shirt becoming a rainbow when in perfect focus.  When out of focus, it looked grey. It seems to avoid this problem, one needs to use an off camera RAW recorder.  I don't think this issue exists in the full size Varicam though.

We shot the movie with an Arri Alexa mini 🙂

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19 hours ago, Phil Rhodes said:

Sounds like a moire problem more than anything else.

It looks like a moire problem, but it's caused by the image processing in the camera.  I don't think it's visible on the live output from the camera, at least I didn't notice it until I brought the footage into Resolve later.

After I discovered the issue, I googled about it and found it is a common concern with this camera.  I've shot ProRes LOG on the bigger Varicam 35 without issue in the past.

I think the Varicam 35 can record 4k ProRes 444 in camera, but the LT only 422 and I think this is where the issue is manifested.

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