Real ?'s About S-Log

[Peter Moretti]

Okay, I've read and seen some impressive examples of S-Log. But digging deeper into the S-Log descriptions and explanations, a lot is remains unclear. I've read Sony's camera formats document that describes S-Log, but even that is too general.

Dynamic Range
Sony states that S-Log allows the recording system to capture the "full dynamic range" of the sensor. They claim this results in a DR increase of 800% over typical XDCAM (EX) recording. This implies additional three stops of latitude. (100% * 2 * 2 * 2 = 800%.)

So by implication, Sony is saying that XDCAM (EX) does not record the full DR of the sensor. My ? is how is the three stops discarded? Is clipping in XDCAM is below where the sensor actually clips? (I've never heard of a camera's recording system clipping below where the sensor clips.) Or does XDCAM (EX) squash the three stops of highlights into something like one stop. This latter approach would allow you to recover some of the squashed highlights in post by adjusting gamma, gain and curves.

Posting S-Log
FWIU, there is no NLE out there right now that has an S-Log LUT. Not Avid MC, CS 5.5 or FCP. Now I know you can make a CC effect that can approximate an S-Log LUT. But that's pretty imprecise and not consistently replicate-able across platforms.

So to accurately work w/ S-Log in post, do you need to use more of a grading/finishing platform?

I'm sure I have a third ?, but it's escaping me at the moment, so I'll leave it at two, LOL.

Thanks much for taking the time to answer the ?'s! I'm especially looking forward to Alister chiming in ;).

[Nate Weaver]

Originally Posted by Peter Moretti

My ? is how is the three stops discarded? Is clipping in XDCAM is below where the sensor actually clips? (I've never heard of a camera's recording system clipping below where the sensor clips.) Or does XDCAM (EX) squash the three stops of highlights into something like one stop. This latter approach would allow you to recover some of the squashed highlights in post by adjusting gamma, gain and curves.

There is documentation somewhere, and I think I read it first from Andy Shipsides, that the Cinegammas take "460%" of the F3s sensor and stuff it into 8 bits. I really hate that method of dynamic range nomenclature, but there it is.

Since we still see a noise floor in Cinegamma footage, I assume it clips the top, not the bottom.

Your comment about being able to get those stops back would be correct, but you run into the practical realities of those 256 levels representing 11 or 12 stops. At some point trying to stretch that contrast range out, it starts to posterize and defeat the point. Those extra 2 bits in 10 bit make it possible.

Originally Posted by Peter Moretti

Posting S-Log
FWIU, there is no NLE out there right now that has an S-Log LUT. Not Avid MC, CS 5.5 or FCP. Now I know you can make a CC effect that can approximate an S-Log LUT. But that's pretty imprecise and not consistently replicate-able across platforms.

So to accurately work w/ S-Log in post, do you need to use more of a grading/finishing platform?

No. There is already a FCP plugin to deal with LogC footage in FCP. I'm sure an SLog version will show up soon enough.

[Alister Chapman]

A lot of the issues with any camera and the dynamic range it can record are not due to limitations of the cameras hardware but to retain compatibility with existing display technologies, in particular the good old fashioned TV set that has been around for half a century. The issue being that in order for all TV owners to see a picture that looks "natural" there has to be a common standard for the signal sent to the TV's that will work with all sets from the very oldest to the most recent. As even the most recent TV's and monitors often struggle to display a contrast range greater than 7 stops there is no point in attempting to feed them with more, Taking 12 stops and simply squashing it into 7 stops will create a disappointing, flat and dull looking image.
So for productions where extensive grading is not taking place, it is not desirable to record information beyond that which the existing broadcast system can handle. This is why the vast majority of modern camcorders with the knee off and using a standard gamma curve all exhibit very similar dynamic ranges (7 to 8 stops typically), because the limitation is generally not that of the sensor, but that of the gamma curves used in broadcast television.
Adding a bit of highlight compression through a cameras knee circuit we can stretch out the dynamic range a bit as our visual system is most acute to inaccuracies in the the mid ranges of an image where faces, people and natural subjects normally appear so we don't tend to notice strong compression occurring in highlights such as the sky or reflections. A well designed knee circuit can help gain an extra 2 or 3 stops by compressing the hell out of highlights but as most of us are probably aware it can create it's own issues with the near complete loss of real detail in clouds and the sky as well as color saturation issues on skin highlights, this is gamma compression rearing it's ugly head.
Moving on, we come to cinegammas, hypergammas and other similar extended range gammas. One of the issues with a traditional aggressive knee circuit is that it is either on or off, compressing or not compressing, there is no middle ground and this makes grading problematic as it is all but impossible to extract any meaningful data from very highly compressed highlights. Cinegammas etc address this by slowly increasing the amount of compression used as image brightness increases. In addition the gamma curve compression starts much earlier, long before you get to what would traditionally be regarded as "highlights". This slow and gentle onset of compression grades in a more pleasing manner than a conventional knee. If you don't grade the added mid-to-highlight compression results in a picture that looks a little flat and lacks "punch", but is not overly objectionable to view.
There is however a limit to just how much data you can cram into a compressed codec or recording system. Cinegammas and Hypergammas are tailored to give optimum performance with existing 8 bit and 10 bit high compression systems and workflows so the design engineers chose to only record a range of about 11 stops as trying to extract more than this from systems essentially designed to only record 7 to 8 stops will lead to visible compression artefacts. Technologies have continued to advance and now it's remarkably easy (compared to just a couple of years ago) to record 10 bits of 4:2:2 or 4:4:4 data without compression or with only minimal compression. By eliminating or at least significantly reducing the compression artefacts it's now possible to extract more meaningful data from a compressed gamma curve than was possible previously. S-Log is in effect nothing more than a heavily modified gamma curve, taking cinegammas and hypergammas to the next level. S-Log needs 10 bit recording to work as the curve compression starts much lower in the curve, so when grading those crucial skin tones and natural objects will need to be un-compressed to look natural and 8 bits of data just would not give enough range. As the image brightness increases the amount of gamma curve compression is increased logarithmically. If you look at the data being recorded this means that the majority of the 10 bit data is allocated to shadow areas then mid tones with less and less data being used to record highlights.

So to answer Peter's questions, most modern cameras, not just the XDCAM's simply ignore highlight information beyond what can be recorded, this results in the image getting clipped at a given off sensor brightness point depending on the gamma curve being used.
Interestingly using negative gain on a camcorder can act as a low end clip as very small brightness changes will be reduced by the negative gain, possibly to the point where they are no longer visible. This normally results in a reduction in dynamic range (as well as noise). I suspect this is why the F3 has less noise using standard gammas because the sensor has excess dynamic range for theses curves and good sensitivity, so Sony can afford to set the arbitrary 0db point in negative space without impacting the recorded DR (recording further into highlights to compensate) but giving a low noise floor benefit. For S-Log however it's possible to record a greater dynamic range so 0db is returned to true zero and as a result the noise floor increases a little.

LUT's are just a reverse gamma curve applied to the S-Log curve to restore the curve to one that approximates a standard gamma, normally REC-709. They are there for convenience to provide an approximation of what the finished image might look like. However applying an off the shelf LUT will impact the dynamic range as an assumption has to be made as to which parts of the image to keep and which to discard as we are back to squeezing 12 bits into 7 bits. As every project, possibly every shot will have differing requirements you would need an infinite number of LUT's to be able to simply hit an "add LUT" button to restore your footage to something sensible. Instead it is more usual for the colorist or grader to generate their own curves to apply to the footage. Most NLE's already have the filters to do this, it's simply a case of using a curves filter or gamma curve correction to generate your own curves that can be applied to your clips in lieu of a LUT.

[Aaron Newsome]

Peter, you're thinking too hard about this. The most important thing to think about when recording S-log is that you're coming as close to capturing the sensor's raw data as possible. That's it.

If you apply a bunch of processing to the incoming sensor data, you're burning in the on-set decisions into the recorded data, period. You can't reverse it.

S-log allows you to keep everything the sensor saw and that's what you record.

This is all very normal to me, since it's exactly like recording FilmStream with my Viper.

[Andy Shipsides]

Well said Alister

[Leonard Levy]

Alister,
Thanks - that's a very clear explication of how all of these issues work and I think why its not a terrible thing to shoot in 709 if you nail your exposure and don't plan to grade a great deal for Production reasons.

I had a couple of questions

1. It wasn't clear to me why the
"majority of the 10 bit data is allocated to shadow areas then mid tones with less and less data being used to record highlights."

2. Won't there be "off the shelf" LUT's in the $3K upgrade that provides S-Log & 4:4:4.

3. Will S-Log be useful simply in 10 bit 4:2:2 or only in uncompressed 4:4:4?

4. In an NLE like Final Cut would simply using the gamma correction filter function effectively as an S-Log LUT or is that too simple?"

Thanks

Lenny Levy

[Peter Moretti]

I can answer number 1.

Our vision is essentially logarithmic. So it is very difficult for us to perceive small intensity changes in bright light. On the other hand, we can readily perceive that same intensity change in dim light.

For this reason, allocating a lot of data bits to the highlights is wasteful. Log format best matches the data bits to how we actually perceive the light.

[Alister Chapman]

The F3 will have a built in LUT. This is for monitoring and off line editing. Using an off-the-shelf LUT essentially negates the benefit of shooting S-Log. The whole point is that S-Log gives you the ability to adjust the image in post on a shot by shot basis. If you just apply a single LUT to your entire project, you may as well just shoot using a carefully adjusted cinegamma or standard gamma.

S-Log will be great with both 4:2:2 and 4:4:4, provided the 10 bit codec is clean enough.

[Leonard Levy]

Can I ask some basic questions about exposure, bits, slog an hyper gammas from a non engineering type?

I was under the impression that with normal gammas - or just in general there are more bits of information in the image as exposure increases. (Until overexposure of course). This was explained to me by a colorist as a reason why he said a pretty brightly exposed shot from a Canon DSLR had much more color correction ability than a darker scene. (again as long as it wasn't overexposed.)

Is that true or do I have it backwards?

However in the posts above Alister states
"majority of the 10 bit data is allocated to shadow areas then mid tones with less and less data being used to record highlights."

is that specific only to SLog or or am I completely confused.

Lenny Levy

[Chris Medico]

Originally Posted by Leonard Levy

Can I ask some basic questions about exposure, bits, slog an hyper gammas from a non engineering type?

I was under the impression that with normal gammas - or just in general there are more bits of information in the image as exposure increases. (Until overexposure of course). This was explained to me by a colorist as a reason why he said a pretty brightly exposed shot from a Canon DSLR had much more color correction ability than a darker scene. (again as long as it wasn't overexposed.)

Is that true or do I have it backwards?

However in the posts above Alister states
"majority of the 10 bit data is allocated to shadow areas then mid tones with less and less data being used to record highlights."

is that specific only to SLog or or am I completely confused.

Lenny Levy

He is talking about S-Log regarding the allocation of data.

This whitepaper isn't for the F3 but describes how you work with S-Log in Sonys other cameras. It might answer some questions for you.

http://www.sony.co.uk/res/attachment...7476953066.pdf