SLOG ISO ratings?

[Dennis Hingsberg]

Originally Posted by Chris Medico

And remember that mid in LOG isn't 50%, its 38%. ;)

Yep SLOG middle grey is 38% and definitely remember that your middle grey points shift depending on the gamma curve you will use.

Here is a PDF with all the cine gammas curves and their middle grey points for reference which you can easily store on your iPhone or other smart phone:

http://www.starcentral.ca/Sony%20PMW.pdf

[Chris Medico]

Originally Posted by Dennis Hingsberg

Yep SLOG middle grey is 38% and definitely remember that your middle grey points shift depending on the gamma curve you will use.

Here is a PDF with all the cine gammas curves and their middle grey points for reference which you can easily store on your iPhone or other smart phone:

http://www.starcentral.ca/Sony%20PMW.pdf

I would like to add something that I've not read in this or other discussions regarding exposure.

If anyone looks at those gamma curves it is important to understand one of the things those curves are telling you. Where you see more vertical distance between the blocks it is telling you that there is more digital luma steps being recorded there. This translates into less compression being applied which equals more SUBTLE detail being recorded.

Lets step out of the camera for a second and talk about our own visual perception. We are acutely aware of how people are supposed to look when we observe each other with our own eyes. With that in mind it should become instantly apparent where skintones should fall on the exposure curve right? They should fall into the LEAST compressed part of those curves meaning the parts where the spaces between the stops are the greatest. When you need to bias skintone exposure you want to have highlights compressed less than shadows. Our visual system is less forgiving when it comes to highlights and how the subtle translucence of skin appears to us and more forgiving when looking towards the shadows. This assumes you are looking to capture the most accurate representation of a person which may not be your goal from a creative standpoint.

This is where it can be tricky to use a light meter alone to set exposure with a video camera that uses a gamma curve different from film. This is why I choose to use a waveform monitor for judging exposure whenever possible. It allows me to more accurately place the most important parts of the image in the range where the least compression is being applied to those elements. I acknowledge that may cause the overall exposure to not be as equal from shot to shot. My priority is to give the most information to the most important elements in the frame and the final luminance range for presentation will be set during finishing.

Obviously I'm not shooting ENG. :)

Anyway, if you understand what those curves are telling you it becomes possible to make better decisions on your exposure. I've offered my thoughts only as a single point of reference and not as an indication of the pinnacle of exposure theory.

[Dennis Hingsberg]

Chris is makes complete sense what you have described, there is much more data information in the mid and even low-end tones, and highlights are compressed heavily similar to how "gentle" they will appear when shooting on film.

I've therefore been debating on other forums some of the potential benefits of shooting linear Raw based formats like RED, and now 4k Raw found on the new F5/F55.

The other thing to really note is that with Raw you are getting 16-bit resolution, or 65,000 steps of data quantization divided by your camera dynamic range in stops. With 10-bit you only get 1000 steps. I see value in both linear and higher bit depth for high end projects but its not easy to argue or convince others of this, especially when feature films go through post in 10-bit dpx format - but that's AFTER it was shot in 12-bit Raw and converted.

[Alister Chapman]

+1 for what Chris said. It is vital with log that your mid range is in the correct part of the curve and in fact the lower down the curve you are the less compressed the image is. This is the biggest mistake made by some users of S-Log and similar gammas. They forget that this is a log compressed system where each stop of additional exposure is in effect allocated only half of the data of the previous stop. If you place mid grey at 38% as designed, even simple 90% reflectance white (which should be hitting about 68%) is being compressed 9 times more than mid grey. Your highlights are 42x more compressed than mid grey. Over exposing skin tones and mid tones makes it very difficult for the colourist to extract a meaningful amount of data and thus reconstruct the subtle tones and textures that make faces etc look pleasing to the viewer. That's why the EI values only go up and not down, so if exposing correctly you will only ever under expose with EI which pushes mids down the curve. This is of course where linear raw excels as there is no log compression so you can place your mid tones wherever you want within the cameras dynamic range.

[Dennis Hingsberg]

Yes Alister I've done that on a few projects and learned the hard way. It was for that reason I felt that shooting SLOG needs a little more attention when shooting than that of linear RAW.

With LOG your highlghts are way more "baked in" as opposed to linear where you can shift your gamma without compromising data throughout the tonal range. I'm not saying its some fail safe, clipped highlights will still be clipped but linear will allow you more highlight "flexibility" if you will.

Would you sort of agree to that?

[David C. Williams]

You guys all have it totally inside out regarding S-Log.

The entire point of an S-Log gamma curve is to effectively linearize the amount of bits per stop. That is why it looks flat as the brightness values are fairly evenly spread. That is why it's so easily gradable and highlights are easily recoverable if not clipped, giving it a more film like highlight response.

Regular non log gamma curves become more compressed with half the bits per stop, not log.

All you have to do is plot the code values against stops to see this is true. There is even a nice graph out there somewhere Sony released in the S-Log whitepaper.

[Dennis Hingsberg]

Originally Posted by David C. Williams

You guys all have it totally inside out regarding S-Log.

The entire point of an S-Log gamma curve is to effectively linearize the amount of bits per stop. That is why it looks flat as the brightness values are fairly evenly spread. That is why it's so easily gradable and highlights are easily recoverable if not clipped, giving it a more film like highlight response.

Regular non log gamma curves become more compressed with half the bits per stop, not log.

All you have to do is plot the code values against stops to see this is true. There is even a nice graph out there somewhere Sony released in the S-Log whitepaper.

You mean this one?

[David C. Williams]

That's a marketing slide that might be vaguely representative of the dynamic of S-Log range squashed into a rec709 display after grading. It's pretty meaningless.

This is from the Sony whitepaper. It's the F35, not F3 curve, but the concept is the same.

[Chris Medico]

Stops are not a liner measurement so even if you are allocating equal luma steps to each stop you are actually compressing the info as the exposure goes up.

[David C. Williams]

I've said nothing about stops being linear.

Various people have been asserting S-Log compresses highlights greater than regular gamma curves.

It's actually the reverse. S-Log gives you more room through relatively even numbers of bits allocated rather than reducing bits per stop as you near clipping.

[Chris Medico]

Yes, I understand that. It is correct that relative to other gamma curves s-log is a smoother roll off in the highlights.

I should have been more descriptive. I was talking about the difference between the chart Dennis posted and the one you posted. They are both fundamentally correct. The difference is one is showing the info in stops and the other is showing the info as linear.

[David C. Williams]

While that slide is roughly analogous to S-Log displayed on a REC709 monitor, it's totally unrepresentative of the data actually recorded is S-Log, and easily misinterpreted, as shown in this thread.

Exposure isn't more critical in S-Log than regular gammas, it's actually more forgiving, and it doesn't compress highlights more than regular gammas, it compresses significantly less.

Regular gammas have a fat part of the bandwidth where skin tones work best, simply because skin has naturally very fine colour and luminance graduations. The fatter part of the gamma data curve gives a more realistic representation of skin. It doesn't look flat when exposed there, hence the old skin tone exposure rules.

S-Log has relatively even bandwidth spread over all stops, so skin can look natural almost anywhere in the spectrum. Now you can expose scenes as they look naturally to your eye if that's your aesthetic, and you don't end up with a flat dull grey faces in the dark or bright monotone under hard light.

[Dennis Hingsberg]

So this entire SLOG video presented on behalf of Sony is incorrect?

VideON | F3 S-Log Stage Presentation | Home Page Featured Category

[David C. Williams]

Haven't watched it. Pick a point to discuss.

[Dennis Hingsberg]

It talks about compression of the highlights, and that you have less bits of data for it, but as I understand it you are saying the opposite is true.