Slog 2 and ISO

[Lee Faulkner]

HI

Just read that applying sLog-2 on the FS700 raises the minimum ISO of the camera to ISO 2000.

Maybe it needs the extra gain to raise low level signals a couple of stops?

But has anyone seen increased noise levels in the midrange because of this boost? Particularly after compression. It seems a bit of large increase. I avoid shooting above 800-1000 if I can, but maybe I'm too cautious and this camera can handle it?

Thanks

Lee

[Tim Dashwood]

There is no extra gain. That is just the ISO rating for S-Log2 at 0dB gain. Each gamma curve is rated differently.

[Alister Chapman]

While the ISO number goes up, the optimum exposure level goes down. The ISO number goes up so that if you then use a light meter to expose, the exposure will be darker, to put middle grey and skin tones in the low part of the curve where they need to be.

see: Understanding Log and Exposure Levels (also other gammas). PLEASE READ and understand. | XDCAM-USER.COM

SLog was added to the FS700 as a way to monitor the cameras full dynamic range when shooting raw. It really does not work well when recorded using just 8 bits.

[Sami Sanpakkila]

I've been using the Slog 2 lately with internal codec because the colors and highlights look a lot better to my eye. I do feel it grades well and haven't had any real problems with it. You have to be really careful with exposure though, you need to hit the nail in the head without looking at it. :)

[Alister Chapman]

Highlights might be a be held a little better, but you loose all your mid range contrast and texture. From 60% and up you only have approx 10 to 14 grey shades per stop which is woefully inadequate for skin tones or natural textures, but then they should all be exposed below 55% anyway, white being 59%. Even below 60% you only have around 14 to 20 shades per stop. Compare that to Rec-709 which has a minimum of 24 shades per stop and it's plain to see (both on paper and in practice) that SLog2 and 8bit is very poor in the mid range. Especially considering that you need to expose all your important stuff below 60% and then do a lot of post manipulation.

The Cinegammas hold the midrange at around 22-26 shades per stop and then just progressively compress the highlights (from around 70% and up). If you expose the Cinegammas correctly (Middle grey at 36% or 40%, skin tones at 55-65% and white at 70-80%) you get much better mid range contrast and still get nice highlight roll off. The problem is that most people over expose the Cinegammas, using conventional exposure levels when they should be 1 to 1.5 stops under normal 709 exposure.

[Tom Roper]

I've had my PMW-F55 for 10 days so I can only offer limited observations but from day one, I set zebras on 59% (for white) and observe the slopes of the lower gamma with the histogram display in the LCD viewfinder. If I see skin tones covered by zebra, I will back the exposure down a little bit. I've shot extremes of bright sun and snow, to dark basements illuminated by a single incandescent bulb, and all the S-Log2 XAVC 4k footage has been properly exposed.

I'm aware of situations where a HG could be more appropriate, such as when recording all the bright highlight detail is less important than affording more shades of gray to the lower gammas, but the point is that S-Log2 is able to record more highlight detail and still allow the lower gamma to be raised in post without adding unacceptable noise, if properly exposed. Alister's exposure recommendations for targeting middle gray, skin tone and white point are the proper foundation, and all that's necessary, imo.

Tom

[Alister Chapman]

But the F55 is 10 bit so even above 55/60% you have 40+ shades per stop which is at least double the range that you have below 60% with an 8 bit camera and that alone makes a huge difference to how the footage behaves. An over exposed F55 will be far more forgiving than an overexposed FS700, even though both may have the same dynamic range and gamma curve.

Below 60% with 10 bit between 55 and 86 shades per stop, 8 bit only 16 to 24. It's a huge difference.

[Erik Wittbusch]

Thanks for expalining Alister.

As long as people don't know how to expose the logs and cinegammas
properly (which is important for 8bit footage even more), we will see a lot
of noisy shadow detail and missing skintones.

I build myself some kind of pseudo-cinegamma fpr my FS100 that also
needs some proper exposure. You just can't judge exposure by the display.
It's impossible.

[Wade Wofford]

Just got my swanky new Odyssey 7Q and am trying to master the art of using it with my FS700. Are you guys saying that I can't trust the visual display on the 7Q when shooting in S-Log?

The 7Q has this toggle on/off LUT that turns the viewing mode into Rec.709. When I set my apperture and waveforms with it off (in S-Log), then toggle the LUT on, the image is prone to look overexposed. Are you saying that I should set my apperture with the LUT on, to avoid this over-exposure problem you're mentioning?

[Sami Sanpakkila]

Originally Posted by Wade Wofford

Just got my swanky new Odyssey 7Q and am trying to master the art of using it with my FS700. Are you guys saying that I can't trust the visual display on the 7Q when shooting in S-Log?

The 7Q has this toggle on/off LUT that turns the viewing mode into Rec.709. When I set my apperture and waveforms with it off (in S-Log), then toggle the LUT on, the image is prone to look overexposed. Are you saying that I should set my apperture with the LUT on, to avoid this over-exposure problem you're mentioning?

The S-Log image is what is recorded so I would never set my exposure according to the LUT. As it is now the O7Q LUT is totally unusable to me.

[Alister Chapman]

You can and should expose according to the LUT. If your footage looks over exposed when you activate the LUT, then this means your SLog2 is also over exposed. The LUT performs a very accurate and precise conversion from S-Log range to 709 range. As the monitor is a 709 device you really need to be viewing in 709 range to make any kind of accurate exposure judgement, unless using a light meter or grey card and waveform.

It must be remembered that the exposure levels for S-log2 are very different to the exposure levels for standard gammas. Middle grey for S-Log2 is only 32% compared to the 45% of normal gamma. White for S-Log2 should only be 59% compared to the 85-90% of standard gammas. Faces and skin tones for S-Log2 should be approx 45-50% compared to the 65-70% of standard gammas.

The entire S-Log2 range is recorded at a much lower level to make room for the extended 14 stop range. If you over expose SLog2 then you will find:

1: LUT's designed for SLog2 will make your footage look over exposed (because it is).
2: Grading of skin tones and highlights will be difficult as they are in the wrong part of the log curve.
3: Skin tones may look washed out or lack contrast after grading (unless you apply an exposure correction LUT in post). Or may never look good if you are more than +1 stop over exposed.
4: Highlights will lack contrast, even after grading.
5: After grading the image may look plasticky and not as good as if shot with a conventional gamma.

It is MUCH better to expose SLog2 correctly, your finished results will look much better even though things may look dark when shooting. Invest in a grey card and white card and learn how to use them correctly. You can over expose SLog2 by a small amount (+1 stop max with 8 bit), but even at +1 stop skin tones will still be darker in SLog2 than conventional 709. I believe that the majority of people tend to over expose SLog2 and as a result do not get the best results that they could. Shooting with Slog require a new way of thinking about exposure, especially if you try to judge your exposure by looking at the log image on a conventional monitor. Much better to use a LUT to convert from log space to normal 709 space as then you can simply expose by eye using conventional levels.

[Sami Sanpakkila]

Hi Alister

That does make sense.

Here's examples from my recent shoot. First is ungraded shot with FS700 / O7Q (4K2HD) S-log2. Second is with a LUT by Filmconvert (made for Sony F55 but should be about the same as FS700 no?) with no further adjusting of curves.

Would you say the exposure is ok?

Sami

[Alister Chapman]

I would say your SLog2 is over exposed. Not by much, but definitely over exposed. If you apply the Sony Slog2 to 709(800) LUT to your frame grab it looks over exposed, the background on the verge of clipping and the womans face and flowers excessively bright.

Of course you may have chosen to over expose a little to reduce your noise footprint, but it's definitely exposed above the nominal "correct" exposure levels. The white flowers should be around 59%, yours are closer to 75%.

[Sami Sanpakkila]

No this was purely me not knowing what I was doing in this case. :)

This was shot ND 1/64 and f5.6. I remember thinking that I'm slightly under exposed. I set it to Rec709 LUT on the O7Q and to my eye it looked as if the darks were about ok (but I wanted some room in the darks to be safe), the highs clipped a lot.

Lots of learning to do...

[Alister Chapman]

As I said it's not far off, just a little high. Exposing SLog2 correctly by eye is challenging as the image has low contrast so judging relative exposure levels can be tricky. This is where using a 709 LUT really helps as the normalised contrast makes exposure judgements easier.