CAMERA



Autor: Uli Mors


Working with the S-Log colorspace (3 / 4)

How are the characteristics of other LOG- profiles?

For this comparison I look at the web project of the American cameraman Ben Turley, who finds time to be both a programmer and mathematician away from the camera. Beside his camera work he has written a program around gamma curves and LUTs and published it on www.lutcalc.net.
 
In addition he laboriously compiled the formulas of the manufacturers, made them available and even visualized them. Let's take a closer look at individual log variants:

SONY

S-Log (so to speak S-Log "1") was Sony's first real log curve for cameras with "only" 12 f-stops of dynamic range  in cameras such as the F35 and F3. Since newer cameras have an even larger dynamic range, S-Log was further developed and simply numbered. An idea replicated by other manufacturers.



S-Log2, on the other hand, is designed for 14 apertures and is connected in the cine cameras (F55/F5/FS7, FS5 via Picture Profile) to a color space called S-Gamut ("Sony" color space from the S-Log1 period). 
S-Log2 concentrates mainly on mids and lights. Therefore S-Log2 is also conditionally 8-bit capable.



The newer S-Log3 is permanently connected to the color spaces S-Gamut3.cine or S-Gamut3 in the cine cameras and is based on the Cineon gamma curve. Since S-Log3 can represent even the deepest shadows, the noise of the sensor is clearly visible on a Rec709 monitor. It’s recommened to use targeted overexposure (1-1.5 f-stops) to help even with simple color corrections.



PANASONIC

Panasonics Varicams offer the V-Log ("V "aricam). For some time Panasonic users have been able to pay to upgrade the GH4 with V-Log. But again the question arises - as with S-Log3 in the Sony Alpha 7s MKII - whether such a potent logarithmic gamma curve makes sense at all with 8-bit recording? Wne  the next generation (V- Log2) is coming.



CANON

For the C300 MKI, Canon developed a high-contrast "C-Log" (Canon-Log) curve at the time, which is most comparable to S-Log2, but is less competent when dealing with shadows.



So the 8 bit (the C300 MKII has only 8 bit recording and 8 bit outputs) is better used for mids and lights. Because C-Log doesn't look "superflat" in Rec709, C-Log is and was less problematic for many editors With the C300 MKII and the recording with 10 bit (in HD/2K 12 bit) Canon has extended the log curve accordingly - similar to Sony with S-Log3.





GOPRO

The GoPros record with 8 bit and are more noisy than large format sensors. Accordingly, GoPro sets the log curve "Protune" steeply.



PHANTOM 3

The same challenge meets the drone manufacturer DJI, which offers a logarithmic gamma curve in some models.

8 BIT VS. 10 BIT

Even with "normal" video compression the quality difference between 8 bit and 10 bit is quite complex. Historically Rec709 was evaluated by the EBU as suitable for production and broadcasting, as long as no complex grading was necessary. Without different compression methods, the danger of banding remains with intensive post-processing of 8-bit recordings: Colour and brightness gradients can then no longer be reproduced evenly and show clear steps.



With logarithmic recording, depending on the dynamic range of the sensor and the log formula, significantly more contrast is recorded than before, so more brightness gradients are available. Banding can occur even faster. If an extremely flat log curve - for example S-Log3 - hits only 8 bit, the 14 f-stops, for example with Sony, have to share 255 grey values - often even less. 

The great temptation is  to simply always shoot in Log Gamma. With logarithmic recording, manufacturers promise possibilities that were previously only possible in RAW. But log is not the same as RAW. And even if Log really offered the same possibilities, would you want to shoot every project in RAW/Log?

Summary

Logarithmic recording makes it possible to take the dynamic range of the sensor completely into the recording and make any manipulations later, instead of doing it during shooting.

8 bit is not enough for real work with Log Gamma, unless the sensor has only a few f-stops of dynamic range. 8 bit recording is always an inferior compromise, but the demands are certainly very different. Log Gamma should always be recorded with at least 10 bit.

Think about why do you want to rotate logarithmically at all? Is there a real interest in grading behind the shooting or is "Log" a synonym for "great picture quality with all possibilities" for the client?
Include targeted test recordings ahead of any shoot to evaluate your own demands or as test files for post-production. This is the only way Log Gamma will feel "right" at some point and not cause any surprises during shooting or post-production.

Clarify the workflow before shooting. Long before shooting. For fast production, the slightly flatter cine, film or hypergammas with a clear contrast in the mids (skin) are often sufficient. These bring quite "cinematic" highlights with drawing, but can not be graded as extensively.

Choose your gamma curve according to the motive contrast. This is especially true for TV production. Even whole films were shot in 8 bit according to this principle.  Think about Oceans, where Phillipe Ros used special gamma curves to fully exploit the 8 bit of the HDCAMS.

The next and last part deals with the use of LUTs (Look Up Tables) for shooting and post-production.

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