SLOG ISO ratings?

[Mark Kenfield]

Hi guys,

I've just been going over Abelcine's ISO ratings for the F3 natively:


-3 db. = 500 ISO
0 db = 800 ISO
+3 db = 1000 ISO
+6 db = 1600 ISO
+9 db = 2000 ISO
+12 db = 3200 ISO
+18 db = 6400 ISO

And I'm wondering if anyone has confirmed ratings for the camera in SLOG? Is it simply +1 stop to all of the ratings (I.e. 0 DB = 1600 ISO, 18 DB = 12800 ISO)?

Cheers

[Alister Chapman]

Those sensitivity ratings are for interlace only, in progressive the sensitivity is halved.

Base S-Log ISO is 800 when progressive and 1600 when interlaced. You really don't want to use any other gain/ISO settings with S-Log as you will not have the full dynamic range.

[Dennis Hingsberg]

Abelcine also rated the F3 in non-slog 0db as 800ISO (as did I with a gray card and meter) - so there is not change in ISO between slog and non-slog in progressive frame mode?

[Mark Kenfield]

Yeah, watching Andy's video again, I noticed that he mentioned that although the exposure is higher with SLOG, once you 'de-log' the footage and bring the levels back within normal ranges - your exposure ends up the same anyway.

[Alister Chapman]

The F3 when in progressive and not in S-log is ISO 400, not 800, it's only 800 in interlace.

The sensitivity of the camera increases by 1 stop (6db) when you switch to S-Log mode. You must remember that middle grey with S-Log should be between 34-38% while for non s-log with rev-709 it would be around 46%.

When you de-log the footage your middle grey will become brighter. If you wanted to make middle grey brighter without changing anything else you would have to increase the camera ISO, not decrease it, so it is obvious that the camera is clearly more sensitive in S-Log than standard modes.

[Mark Kenfield]

Thanks Alister. How do you usually set exposure with the camera? Point the 'marker' at a grey card and adjust lighting/aperture until it hits 38%?

[Alister Chapman]

Yep, that's pretty much it, quite simple really. I also check that white from a good quality matt white card is so somewhere around 68%. You normally end up about 1.5 stops under conventional exposures.

[Dennis Hingsberg]

Originally Posted by Alister Chapman

The F3 when in progressive and not in S-log is ISO 400, not 800, it's only 800 in interlace.

The sensitivity of the camera increases by 1 stop (6db) when you switch to S-Log mode. You must remember that middle grey with S-Log should be between 34-38% while for non s-log with rev-709 it would be around 46%.

Speaking strictly non-slog I don't agree that progressive is 400. I rated the camera as 800ISO in progressive 24p 0db, non-slog mode using a grey card at middle grey @ 45%-50%. It took about 42 footcandles with taking lens at f4 to light reach exposure for middle grey. My light meter at f4 read 800ISO.

Andy also did this test and measured f4 at ISO400 but the camera lens was at f5.6 with middle grey at 50%. So really at f4 it would be ISO800. Interlace would double it as you've suggested.

Originally Posted by Alister Chapman

Yep, that's pretty much it, quite simple really. I also check that white from a good quality matt white card is so somewhere around 68%. You normally end up about 1.5 stops under conventional exposures.

I agree with getting mid-tones low around 40% but the suggestion of putting white at 68% (only 3 stops over middle grey) makes for very strict light control and just isn't practical in many situations.

[Chris Medico]

Originally Posted by Dennis Hingsberg

I agree with getting mid-tones low around 40% but the suggestion of putting white at 68% (only 3 stops over middle grey) makes for very strict light control and just isn't practical in many situations.

Attached is a snipit from the Sony's white paper on S-LOG. The values for 18% and 100% are shown in the image.

Obviously we all have to work in the real world so we aren't always able to adhere to design criteria but it is good to know what it is so we can understand the risks we take when we exceed it.

You can reference the complete white paper here - http://www.sony.co.uk/res/attachment...7476953066.pdf

[Doug Jensen]

Originally Posted by Dennis Hingsberg

Speaking strictly non-slog I don't agree that progressive is 400. I rated the camera as 800ISO in progressive 24p 0db, non-slog mode using a grey card at middle grey @ 45%-50%.

It depends a lot on your picture profile. Using my DSC Chroma Dumonde chart and Leader waveform monitor, I have rated my camera at ISO 250 in progressive, non-S-LOG. Exposing so no grading will be necessary . . . if I get it right.

[Dennis Hingsberg]

Yes true, since different profile will shift your grey around.

Me and Andy Shipsides were both using Rec 709 where middle grey is around 50%. We measured 800ISO.

It should double in SLOG (I haven't confirmed that) and it should double in interlaced (I know it gets brighter but I never bothered measuring it).

Good stuff here..

[Alister Chapman]

Remember were dealing with stops which are ratios here, not finite values.

When your dealing with a good quality mid grey card (18% reflectance) and a white card (90% reflectance) in S-Log if you put middle grey at 38%, then white will be 68%, if it isn't then you middle grey is incorrect.

The contrast ratio between white and middle grey never changes, no matter how bright or dark the scene the middle grey will reflect 18% of the light and the white 90%, so the brightness ratio between the mid grey and white always remains constant, white will always be 500% brighter than mid grey. So it doesn't matter how bright your scene, with the S-Log curve, if middle grey is 38% then white will be 68%, 500% or 2.5 stops brighter than middle grey, always is, it's a reflectance ratio and the difference between mid grey and white will always be 2.5 stops which on the S-Log curve puts white at 68% when mid grey is 38.

You don't want to over expose with S-Log. It is highly compressed, if you over expose faces etc you risk putting them in the highly compressed part of the log curve and this makes it very hard to recover a decent looking image. You really need to keep faces and skin tones at least a stop under white, so faces need to be down at around 45-50%.

Do consider that when you look at the Sony S-Log graph linked to in this thread that the horizontal scale is stops, so it is a Log scale which is why the plot looks like a straight line. If the horizontal scale was linear the compressed nature of the curve would be obvious.

[Mark Kenfield]

This has been a tremendously helpful and informative thread, thanks so much for your input everyone.

[Bruce Schultz]

Originally Posted by Mark Kenfield

Thanks Alister. How do you usually set exposure with the camera? Point the 'marker' at a grey card and adjust lighting/aperture until it hits 38%?

One thing I've found is a potential "gotcha" would be making adjustments to the LUT through EV or even just turning the LUT on will start to bump up your grey card reading from 38% in the middle exposure box % readout of the LCD. What I do is to set the S-Log exposure at 38%, then tape the iris ring so it doesn't move. Then I can turn on LUT's and do EV increments without being fooled that I've actually opened the iris which you might think has happened if you look at the 38% starting to jump up in value on the LCD. Having a separate S-Log monitor like the Samurai screen is good to make sure you don't move the exposure on this accidentally.

Faces do tend to fall into the 40 - 50% area at MG 38%, but I like to stop down 1/4 stop or so to get them as close to 40% as possible since there is plenty of room to open them up in post.

I just shot Angelina Jolie on Thursday with S-Log on an F3 and I was very happy to have convinced the producers to use S-Log because she showed up in a pure white satin dress and I was able to easily hold the exposure on her well-lit face and the dress with S-Log.

[Dennis Hingsberg]

Bruce, do you recall where the white dress fell on either F3 spot meter or ext. WF if you had one?

Thanks,