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Exposure compensation the right way!

Because Adobe Camera Raw is really only intended for display referred or print referred workflows and not log or scene referred it has some limitations when it comes to exposure adjustment of log material.

In a typical log DI workflow, exposure is adjusted by offset. This maintains the equal code values per-stop of the log conversion and acts on the whole signal by shifting the entire range up or down relative to the mid point without affecting the log curve.

Linear exposure adjustment works by increasing the signal while keeping the black level at a fixed point. This is the same as Gain in Lift/Gamma/Gain or Slope in ASC CDL.

ACR exposure is restricted by the tone curve and so can only move the mid-point of the raw data under the tone curve which has fixed minimum and maximum points between 0-1 (The Cinelog-C tonecurve has an output of between 0.009157 - 1.00000 for technical reasons). Using exposure offset in ACR is generally ok for very small adjustments when using the Cinelog-C profile but not too useful when a large adjustment or a specific numeric exposure adjustment may be required (i.e. in VFX) to correct the shot as it alters the equally spaced steps-per-stop ratio of the log signal i.e. by pulling exposure down the signal compresses the stops below the mid-point and stretches the stops above the mid point and vice versa and this can reduce the latitude available later when grading.

In Cinelog-C log profiles the log tone curve, mid-point and offset is carefully calculated and fixed to enable the whole raw dynamic range to be transformed to Cineon log within the limitations of ACR. The result produces an image that equates to the same Cineon log levels as you would achieve from a linear to Cineon log transform of the same raw data in apps like DaVinci Resolve and retains values well above 1.0 (in scene linear) which are ordinarily clipped by ACR when using normal DCP profiles.


Adjusting Exposure

To adjust exposure there are a couple of options depending on how you want to work.The easiest method is to use the exposure plugin in After Effects and the plugin's Offset control.

For both the following methods it is important to bypass the plugin's conversion to linear by clicking the checkbox. This disables the internal gamma compensation which is added relative to the workspace ICC profile currently in use but as we can transform the signal to true scene linear values we don't need it.


Method 1

Log exposure adjustment

Here an offset amount of +/- 0.1 is equal to 1 full F-stop of log exposure. You may find the slider too sensitive to adjust accurately so it is usually quicker to enter a value manually.

To better view the exposure adjustment it's recommended to add an instance of the OpenColorIO plugin after the exposure plugin and set as follows:

Input colorspace: Cinelog-C
Output Colorspace: Cinelog-C Rec709
(note: please ignore the v2 shown in the image opposite)

The Exposure and Gamma Correction sliders are not used.

The OpenColorIO plugin can be removed once your exposure has been tweaked.



Method 2

Linear exposure adjustments (gain)

Alternatively, Linear exposure compensation can be used however, as the image is log it must first be converted to linear light before applying the desired exposure amount and then converted back to log (unless you intend to add a final look in which case the second transform can be from Linear to Rec709 or a Cinelog Look

Tip: I recommend creating log masters as the same looks can be applied later but with much less processor overhead.

The Offset and Gamma Correction sliders are not used.

To convert the log image to Linear light (scene linear) add an instance of the OpenColorIO plugin set as shown oposite:

Input Space: Cinelog-C
Output Space: Linear Rec709

Then add the exposure plugin and select bypass linear light conversion, followed by another instance of the OpenColorIO plugin set to the reverse transform i.e.:

Input Space: Linear Rec709
Output Space: Cinelog Rec709
(note: please ignore the v2 shown in the image opposite)

You can now use the exposure plugin to adjust the exposure in the correct linear light environment while viewing the shot in an output referred colorspace (i.e. Rec709 or a Cinelog Look).

An exposure change of +/- 1 is equal to 1 Full F-stop of gain in Linear light.

Once you are happy with your exposure correction simply set the Output colorspace of the second OCIO plugin back to Cinelog-C. You can then render your exposure corrected log masters.

In both methods, gamma adjustments should not be used as these will affect the logarithmic nature of the Cinelog Log curve.



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