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Where's a good explanation of v-log?
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  • Vitaliy_KiselevVitaliy_Kiselev

    To capture more highlights, the sensor gain needs to be turned down. It's sort of like shooting at a lower ISO and then brightening the image in post: the shadows will be noisier. The best exposure for a standard photo style won't be the best exposure in V-Log L. For the same ISO setting, you need to boost the exposure in V-Log L to achieve comparable noise in the shadows and midtones.

    This make no sense. V-Log has nothing to do with sensor, gain or exposure values of the sensor. It is how raw sensor values are transformed into recorded image.

  • balazerbalazer

    1. It's a pseudo-logarithmic RGB scale. The luma scale takes properties of the RGB scale, since luma is derived from the RGB values.

    2. V-Log L extends the highlight range compared to a standard photo style at the same ISO setting. You can capture brighter highlights without clipping. But just how much it extends and where it clips depends on your ISO setting.

    3. I'm not sure what you mean. But capturing an extended range of highlights doesn't mean the midtones and shadows are the same. To capture more highlights, the sensor gain needs to be turned down. It's sort of like shooting at a lower ISO and then brightening the image in post: the shadows will be noisier. The best exposure for a standard photo style won't be the best exposure in V-Log L. For the same ISO setting, you need to boost the exposure in V-Log L to achieve comparable noise in the shadows and midtones. (or, for the same exposure, you need to boost the ISO setting)

  • Brian_SianoBrian_Siano

    Thanks for all the replies. If I'm understanding this correctly, V-log does the following:

    1. It uses a logarithmic luminance scale, thus enabling color correction tools previously restricted to those who worked with actual film.

    2. This scale reduces a slightly wider range of luminances into the workable range, mostly in the higher values, effectively giving the mode two extra stops of exposure. Basically, it "clips" the brightest elements two stops higher than it would otherwise.

    3. If we expose a stop or two more brightly than we normally do, those extra stops shift down and enable us to gain more detail in the midrange and lower ranges.

    Is that more or less right?

  • balazerbalazer

    My little prediction about Barry Green's analysis of Matthew Allard's test of V-Log L on the DVX200: the analysis was not correct and their expectations about clipping are wrong. The ISO was set too low to use the entire V-Log L range. Just increase the ISO setting, and the clipping level goes up closer to where you'd expect it to be.

    I don't believe Panasonic would be so ridiculous as to make V-Log L always clips at 79%.

  • Vitaliy_KiselevVitaliy_Kiselev

    @LPowell

    Here it is same, only members can see this attachment.

    And chart is present in PDF in post above yours.

  • LPowellLPowell

    @Vitaliy_Kiselev The V-Log chart was missing in the other topic. That's the key piece of evidence.

  • Vitaliy_KiselevVitaliy_Kiselev

    @LPowell

    IN other topic we already have same.

  • LPowellLPowell

    Here's a link to Barry Green's explanation of how the GH4's V-Log-L differs from the Varicam's V-Log tone curve:

    The chart shows 16 stops from -8 to +8, with zero calibrated to 18% gray reflectance. Note how the flat log segment of the curve extends down to about -4, with a rolloff knee down to 0% reflectance in the bottom 4 stops of the shadow range. This is the part of the dynamic range that must be accurately exposed in order to match the non-log segment of the V-log LUT you select.

    The important issue, however, is that green rectangle Barry drew on the chart at stop 4. That's where the GH4's 12-stop dynamic range ends with highlight clipping, 4 stops short of the 16-stop range of the Varicam's V-Log tone curve. In order to match the V-Log curve, he claims you should set exposure for highlight clipping at 79 IRE. That would appear to sacrifice the camera's 80-109 IRE range, for the sake of allowing you to use Varicam V-Log LUT's interchangeably with GH4 V-Log-L footage.

    The vertical calibration on the chart is a 10-bit scale from 0-1024. On a camera with an 8-bit codec like the GH4, you would have a scale from 0-256. That means highlight clipping would be recorded at around 180, with no image details recorded in the range 181-255. At the other end of the scale, the bottom 4 stops of shadow detail would be recorded in the range 32-48, just 4 bits of data to cover 4 stops of dynamic range.

    In short, 8-bit V-Log-L appears to squeeze out more dynamic range by sacrificing tonal range in both highlight and shadow regions.

  • driftwooddriftwood

    ... ultimately it's actually much easier to get from A to B with V Log. Else why bother.

  • balazerbalazer

    If you want a purely technical description: http://pro-av.panasonic.net/en/varicam/common/pdf/VARICAM_V-Log_V-Gamut.pdf

    But if you want some context, basically V-Log and V-Log L are the same idea as Canon Log or Sony S-Log, though with specific differences. These documents explain it better:

    Standard cameras record into a display-referred color space: the signals are meant to be sent to a monitor for display, without any manipulation. The images are rendered for a certain type of display, and the look is baked in.

    Camera log spaces such as S-Log and V-Log are scene referred. They represent the image in terms of the way the camera's sensor captured it. The look is not baked in. Contrast, saturation, and highlight handling are up to you and your workflow, instead of being determined by the camera and how it was set during shooting. Scene-referred images aren't meant to be displayed directly. A specific transformation is required to convert them to the color space of a display. For most camera log spaces that transformation involves a matrix or a 3D LUT. Standard color correction tools are insufficient to convert those camera log spaces to a display-referred color space, though many people still try to use them, making the job very difficult and producing inaccurate colors.

  • TronTron

    Log expands the highlight & shadow occupancy of the luminance range (compressing mids). As a result clipping is less harsh and allows for more filmic grading in post.

  • cantsincantsin

    It's very simple. A camera sensor has a certain dynamic range, i.e. can "see" things from shadows to highlights. The way the sensor sees the world differs from the human eye. To make the sensor data match human vision, a curve/picture profile is applied to the data it has recorded. Standard video curves/profiles, such as Rec709 for HD video, tend to crush the shadows and the highlights. Flat/log profiles reproduce the sensor data so that nothing gets crushed or clipped, typically resulting in a greyish/washed out-image to the human eye. This gives you more information for post-production color/exposure correction which wouldn't have been there with the crushed shadows and highlights of a standard color profile.

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