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How to expose logarithmic gamma correctly?

log canon exposure

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#1 Levin Liebig

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Posted 06 November 2017 - 04:46 PM

Hey!

 

Some friends and me tried to expose log with an 18% gray card.

We used a canon c300 mk2 with the c.log 3 cinema gamut picture profile.

 

Canon gives the following IRE values for c.log3: 18% Gray → 34,3% | 90% White → 56.4%

 

But before we tried to use canons values we tried to replicate this workflow to derive the values:

https://www.youtube....h?v=WZb3u220EwU

 

So we set our camera and seconic light meter to 800 ISO and used the spot meter right next to the camera on the 18% Gray chart. We got a value of F 5.68.

Our next step was to set the lens to a F-stop of nearly F 5.68. The waveform gave us a value of 26%.

 

This is not only a big difference to 34,3%, the value of canon, the picture was clearly underexposed. :huh:

Of course, we asked us what our mistake was.

Was it the fault of the video? Is our gray card not standardized? Is our light meter broken? Is a F 5.68 not a F 5.68 on the lens we used, maybe its broken? Is the camera screwing us?

 

Then we stumbled over this article: http://bythom.com/graycards.htm

The author, Thom Hogan, tells us that cameras don't see 18% gray. Instead they see 12% gray.

 

This article confused us a lot. We don't know what to think about light metering. :unsure: What is true and what is a myth? 

 

What do you think about all this? Are you noticing a mistake we made? How is your workflow for exposing log gamma curves? Do you use a gray card, a light meter, ETTR (expose to the right)? Do you use a Bt.709 LUT on your monitor/viewfinder and expose just with that?

 

And do you understand the 12% gray article and do you think it's true/false?

 

Thank you for your help! :)

Levin Liebig
 


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#2 Robin R Probyn

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Posted 06 November 2017 - 08:23 PM

There is alot on the inter web about exposing LOG.. and of course there is more than one way..   dont ETTR.. this is from the stills world and some of those guys think they should expose LOG that way too.. it will be a pain in the grade.. you actually want consistency,ie the opposite.. not every single shot cranked over to the right ..with your levels all over the place.. a uniform 1 or even 2 over exposure is quite often used with Log.. to avoid noise..(you usually have quite a bit of leeway in the high lights )..but Log can get noisy in the shadows due to not much data down there.. depends a bit of your scene.. its a way of lifting the noise floor.. 

 

Personally I dont use a metre for video.. the simplest way to do it is to apply a Rec709 LUT to your EVF and /or monitor .. expose for that at the usually 709 levels.. white 90..etc.. and your underlying LOG will be perfectly exposed.. you can sometimes switch off the LUT if you want to see how the high lights are keeping.. some camera,s have high/low functions to make that easier.. 

 

If you try to work off the log image its very hard.. no one is ever really meant to even see it on set.. very dark and low contrast.. focus is a pain and I have no idea how people judge their lighting this way.. very miss leading and hard to see changes ..

 

Throw in a monitor 709 LUT and shoot.. after a while you will know what the LOG is doing underneath .. mostly its keeping alot of high light detail that you would think is blown out.. I dont know that curve specifically .. but basically there is no shoulder with log curves.. but there is a bit of a toe.. above grey there is about the same amount of data for every stop you go up.. white is pushed way down to fit the high lights into the "bucket".. others will know alot more than me.. but I would suggest using a Rec 709 LUT..as the simplest way ..

 

 

Re the over exposure.. you have to bring those levels down in post of course !  t


Edited by Robin R Probyn, 06 November 2017 - 08:25 PM.

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#3 David Mullen ASC

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Posted 06 November 2017 - 08:27 PM

Wouldnt you have gotten 34% instead of 26% if you lowered the ISO on your meter? Maybe the camera isnt 800 ISO in C-Log, its more like 500 ISO?
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#4 Robin R Probyn

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Posted 06 November 2017 - 10:15 PM

here is a link from an Abel Cine review of Log 3 and the C300MkII

 

https://www.abelcine...it-the-goldilog


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#5 Stuart Brereton

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Posted 06 November 2017 - 10:57 PM

I never refer to Log when setting overall exposure. As Robin says, monitor in whatever variant of REC 709 you are using, and set your exposure from that. If you need to be sure that highlights are not clipping, or that shadows are not crushed, then switch to viewing Log, but otherwise there doesn't seem to be much point in worrying how to 'correctly' expose Log.


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#6 Levin Liebig

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Posted 07 November 2017 - 06:37 AM

Thank you for your replies.

 

 

There is alot on the inter web about exposing LOG.. and of course there is more than one way..   dont ETTR.. this is from the stills world and some of those guys think they should expose LOG that way too.. it will be a pain in the grade.. you actually want consistency,ie the opposite.. not every single shot cranked over to the right ..with your levels all over the place.. a uniform 1 or even 2 over exposure is quite often used with Log.. to avoid noise..(you usually have quite a bit of leeway in the high lights )..but Log can get noisy in the shadows due to not much data down there.. depends a bit of your scene.. its a way of lifting the noise floor.. 

 

Personally I dont use a metre for video.. the simplest way to do it is to apply a Rec709 LUT to your EVF and /or monitor .. expose for that at the usually 709 levels.. white 90..etc.. and your underlying LOG will be perfectly exposed.. you can sometimes switch off the LUT if you want to see how the high lights are keeping.. some camera,s have high/low functions to make that easier.. 

 

Okay I understand that I prevent clipping in the highlights and in the shadows with this approach. But how do I know if I have consistency in the midtones in different lighting situations for example in the skintones?

 

Wouldnt you have gotten 34% instead of 26% if you lowered the ISO on your meter? Maybe the camera isnt 800 ISO in C-Log, its more like 500 ISO?

As far as I know, canon says that the best setting for c.log3 is 800 ISO. That's why we set it like this in camera and on the seconic.


Edited by Levin Liebig, 07 November 2017 - 06:42 AM.

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#7 Robin R Probyn

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Posted 07 November 2017 - 08:29 AM

Use Zebra  or WFM.. all digital camera,s have these.. TBH I usually just go by eye.. but Im not shooting big feature films .. but guys like Deakin's .. do it too.. if your monitor is calibrated and you know the grey /white levels of what ever MLUT your using.. you can really work off it .. or even the EVF if its a good one.. you have to know the levels of your LUTs obviously.. thats why 709 is the easy one.. I guess it also depends how much 709 you have already shot.. 

 

But its not the only way.. I find it easy and I would say its probably the easiest if you have never used LOG and just want to get shooting..there is alot of voodoo about LOG but its not rocket science at all.. its pretty straight forward.. dont be scared of it.. its pretty forgiving actually in the high lights, its the shadow that will give you trouble .. the opposite of standard gammas where the golden rule was to protect high lights... .you can learn all you need to know in an afternoon on the inter web.. 

 

If your used to meters then you should stick with them.. some people do.. thats why camera,s have EI mode and ISO readings I guess too..but 709 LUT ,zebras for 70% skin.. 90 % white .. grey % isnt hugely different really..  its really white that is dragged way down from 90 to the 60,s.. and yes your LOG non LUT footage will definitely look under exposed..its meant to.. thats why I wouldn't really work off it solely .. the hardest thing is often finding a grader who knows how to work with LOG.. you cant just do it on a NLE.. you need something like Resolve.. than your shooting of it !.. alot of editors are being made to grade too.. which was easy enough on standard gamma curves.. but log will behave differently and can end up being butchered .. have a look at Roger Deakin's site he talks about shooting LOG .. 


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#8 Levin Liebig

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Posted 08 November 2017 - 07:56 AM

Use Zebra  or WFM.. all digital camera,s have these.. TBH I usually just go by eye.. but Im not shooting big feature films .. but guys like Deakin's .. do it too.. if your monitor is calibrated and you know the grey /white levels of what ever MLUT your using.. you can really work off it .. or even the EVF if its a good one.. you have to know the levels of your LUTs obviously.. thats why 709 is the easy one.. I guess it also depends how much 709 you have already shot.. 

 

But its not the only way.. I find it easy and I would say its probably the easiest if you have never used LOG and just want to get shooting..there is alot of voodoo about LOG but its not rocket science at all.. its pretty straight forward.. dont be scared of it.. its pretty forgiving actually in the high lights, its the shadow that will give you trouble .. the opposite of standard gammas where the golden rule was to protect high lights... .you can learn all you need to know in an afternoon on the inter web.. 

 

If your used to meters then you should stick with them.. some people do.. thats why camera,s have EI mode and ISO readings I guess too..but 709 LUT ,zebras for 70% skin.. 90 % white .. grey % isnt hugely different really..  its really white that is dragged way down from 90 to the 60,s.. and yes your LOG non LUT footage will definitely look under exposed..its meant to.. thats why I wouldn't really work off it solely .. the hardest thing is often finding a grader who knows how to work with LOG.. you cant just do it on a NLE.. you need something like Resolve.. than your shooting of it !.. alot of editors are being made to grade too.. which was easy enough on standard gamma curves.. but log will behave differently and can end up being butchered .. have a look at Roger Deakin's site he talks about shooting LOG .. 

Yes I know that using a LOG pushes all values on the waveform down in comparison to Rec.709 but I really don't want my LOG footage to be underexposed because, as you know,  this comes together with a lot of unwanted noise in the shadows. And from the test we made I can say the footage was so underexposed that we could not save it in Resolve. Thats why I wanted to find out how to expose correctly. Either ETTR or with a gray card. But the gray card did not answered our questions at all, hence I started this topic.

So you recomend to use a Rec.709 LUT on the monitor/viewfinder together with a waveform (also with the LUT applied.) and expose only using IRE values of 70% skin and 90% white? And does mean 90% white, when shooting regular Rec.709 (no LOG), that there is still some information left or is it already burning out?

 

I'm not sure if it's possible to show a waveform with a LUT applied to it. I have to try it the next time I have access to the camera. But according to you, I can use the LOG waveform on around 60% IRE for white?


Edited by Levin Liebig, 08 November 2017 - 07:58 AM.

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#9 Robin R Probyn

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Posted 08 November 2017 - 07:45 PM

Yes you never want to under expose LOG agreed .. 

 

On my camera the WFM will always read for the LUTed image.. some you can choose .. but to me personally I want the WFM to read for the image Im seeing.. e.g. not be looking at a 709 image but the WFM reading the underlying LOG..  so no I wouldn't myself use the WFM for the log image .. everything is crammed down the bottom anyway .. but its good to know the grey/ white levels of your LOG curves of course.

 

So yes simple way to shoot LOG.. apply LUT.. know the correct levels for that LUT.. expose for those levels.. the LOG footage will be correctly exposed.. a LUT is mapping values from one curve to another.. its not guessing as some think.. if you know the correct levels for each its very precise .. but viewing a 709 image in the EVF parts of your frame may appear burnt out.. (you cant have it both ways).. if you want to check what your LOG is actually seeing .. switch off the LUT .. or on Sony f5/55 use the high /low function to check..  if you dont have time well then after a bit of time.. from experience you will know that that burnt out window will still actually have information ..

 

Rather than ETTR for every shot you shoot.. with the EI mode (exposure index) I think is !.. in the camera you can then change the ISO, from native.. in combination with a LUT in the VF... you can uniformly "over expose" from the native ISO.. (same as you might rate film a different ISO on a light meter).. I wouldn't ever go more than 2 stops.. but alot of people will always over expose their log by 1 stop..to raise the noise floor.. then you are doing it for all your footage at a set uniform level.. not changing it for every single shot which will be a nightmare to grade.. e.g. on the Sony f5.. in Cine EI mode. native ISO 2000.. with MLUT 709 (M means monitoring ).. change ISO to 1000.. VF goes  1 stop darker.. levels go down .. open iris to compensate back to correct levels.. or just by eye.. but your native LOG stays at 2000.. so you are over exposing everything by 1 stop.. you loose one stop in the highlights but with 6/7 stops over grey you can usually afford to do this.. and you have raised the noise floor by 1 stop.. 

 

Some people like to have 709 LUT but WFM reading LOG.. what ever works better.. alot is just down to personal preference .. some people work with the LOG image in the VF.. how Ill never know but they do.. there isnt one best way and anything else is rubbish..people just find what works for them an stick with it..

 

But for sure the easiest is just switch on your camera.. apply a 709 LUT .. most people have already shot alot with 709 or at least know the levels, or just what looks ok in the VF (as long as its set up well).. the image has contrast,you can see focus, lighting and color as close to natural.. and basically its how it will look if its just goes straight out the door. with a 709 LUT plonked on it.. set your zebra for 100% .. press record and your LOG underneath will be totally fine..if you expose correctly for 709.. and you will be very surprised at how much high light detail is retained in post.. so its different in that you dont have to protect high lights at all costs like actual 709 recording.. and as you say the danger is actually in the shadows... I think this is why alot of people have trouble with LOG .. they are so used to protecting the high lights .. where as the opposite is true for LOG..

 

Edit.. for doc work I dont have time to place grey cards .. I dont use them .. so sorry cant really give you any valid info on that front..  I think zebra and WFM and ultimately just your own eye.. too many people are ruled by numbers.. if it looks good on a correctly set up monitor/EVF then the levels are fine !.. this has worked for me from tropics to arctic circle.. I was the same when first started shooting SLOG.. OMG what about all these weird levels ..am I going to screw it up totally.. and if its too dark.. switch to something else .. not everything has to be shot in LOG ..the grader will be happier with 709 than a whole night scene thats full of noise .


Edited by Robin R Probyn, 08 November 2017 - 07:56 PM.

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#10 Ryan Emanuel

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Posted 09 November 2017 - 12:05 PM

Its simple to trouble shoot this, but chances are 800 ISO on the camera is not 800 ISO.  Camera manufacturers definitely lie about ISO so they can say clean 2000! when your really looking at 800 on other camera. Get a second meter if you want to be sure and take incidents and spot readings together to see if yours is off but I doubt it.  Check the camera next, expose with the camera first.  Get your f stop with the waveform and grey card to match 34% and compare the f stop the meter,  adjust the iso on the meter until it matches the camera f stop.  It might end up being 400 or 500 iso, if it is to get the exposure your are after you have to rate the camera lower on your meter.  Usually the camera manufacturers' suggestions are super low for log middle grey to promote high light retention specs, but the darks will be muddy.  I wouldn't trust that either.  Any time you have to lift digital your not in the best place.  For cinema a lot of cinematographers like white skintones at 45-55. If you like it higher and you have to lift your image to get there from 34% grey, and you're loosing some fidelity. You might need to aim for 43% for middle grey in that case.  Trust none of the specs of the camera, trust only the sekonic as a bench mark, you might find that 200 iso is the look your going for.   Best of luck.


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#11 Levin Liebig

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Posted 09 November 2017 - 12:48 PM


Rather than ETTR for every shot you shoot.. with the EI mode (exposure index) I think is !.. in the camera you can then change the ISO, from native.. in combination with a LUT in the VF... you can uniformly "over expose" from the native ISO.. (same as you might rate film a different ISO on a light meter).. I wouldn't ever go more than 2 stops.. but alot of people will always over expose their log by 1 stop..to raise the noise floor.. then you are doing it for all your footage at a set uniform level.. not changing it for every single shot which will be a nightmare to grade.. e.g. on the Sony f5.. in Cine EI mode. native ISO 2000.. with MLUT 709 (M means monitoring ).. change ISO to 1000.. VF goes  1 stop darker.. levels go down .. open iris to compensate back to correct levels.. or just by eye.. but your native LOG stays at 2000.. so you are over exposing everything by 1 stop.. you loose one stop in the highlights but with 6/7 stops over grey you can usually afford to do this.. and you have raised the noise floor by 1 stop.. 

 

That sounds like a good aproach, Robin. Expose LOG with a rec709 LUT and using the ISO for "over-exposing". Thank you.

 

Its simple to trouble shoot this, but chances are 800 ISO on the camera is not 800 ISO.  Camera manufacturers definitely lie about ISO so they can say clean 2000! when your really looking at 800 on other camera. Get a second meter if you want to be sure and take incidents and spot readings together to see if yours is off but I doubt it.  Check the camera next, expose with the camera first.  Get your f stop with the waveform and grey card to match 34% and compare the f stop the meter,  adjust the iso on the meter until it matches the camera f stop.  It might end up being 400 or 500 iso, if it is to get the exposure your are after you have to rate the camera lower on your meter.  Usually the camera manufacturers' suggestions are super low for log middle grey to promote high light retention specs, but the darks will be muddy.  I wouldn't trust that either.  Any time you have to lift digital your not in the best place.  For cinema a lot of cinematographers like white skintones at 45-55. If you like it higher and you have to lift your image to get there from 34% grey, and you're loosing some fidelity. You might need to aim for 43% for middle grey in that case.  Trust none of the specs of the camera, trust only the sekonic as a bench mark, you might find that 200 iso is the look your going for.   Best of luck.

This might explain our results.


Edited by Levin Liebig, 09 November 2017 - 12:50 PM.

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#12 Richard_Swearinger

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Posted 17 March 2018 - 11:28 AM

The 18% gray card is so abused and misused, I feel bad for it....The Kodak instructions that come with the card explain that you have to position the card at 1/3 of the angle between the camera and the main light both horizontally and vertically (by angling it, less light hits the card so that it is not at 18 percent reflectance anymore). Then you open up by 1/2 a stop over that. 

 

If you like olden days film lore, you were supposed to include a few frames of a gray card in all of your shots as a neutral reference for the color timer. Kodak also suggested using a gray card to tell your timer exactly what you had in mind for exposure. If you wanted your film to be printed darker, you were suppose to expose your film as normal, but include a shot of the gray card overexposed, for printing lighter you underexpose the card. That way, when the film was printed, the timer would use the image of the card to set the exposure of the print and your image would look just the way you wanted it (theoretically).  

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Edited by Richard_Swearinger, 17 March 2018 - 11:35 AM.

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#13 Bruce Greene

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Posted 17 March 2018 - 01:05 PM

The 18% level in LOG will depend on the de-LOG transform that you are using.  If you examine LOG-REC709 transforms or LUTs from the camera manufacturers you will find that they don't track standard video REC709 results.  For example the Arri Alexa transforms (in camera transform to video output) makes for a REC709 image with a lot of highlight rolloff and some extras shadow detail than "standard" REC709.  Arri has decided this is a good look for their camera, but each camera maker would create their own "normal" transform, and they will all be different.

 

As it turns out, on an Alexa, it's pretty safe to set the camera to the desired ISO setting and use an incident light meter or even spot meter and expose as you would for film and you will get a very usable image.  Just keep in mind that as you lower the ISO setting, you chop off the same amount of highlight detail that can be used in color grading, but you increase the amount of shadow detail that will be captured.  To view the image that is actually captured just view the LOG image on the display an take a look.

 

If you really want to test your camera, shoot a good test chart at many different ISOs, and set the exposure by your light meter.  Then take the LOG images into color grading software and see what kind of image you get at each ISO when you correct the image.  In this way you'll learn the limits of your camera for the best ISO to use in different situations.  There's really no other way to do this, and googling the answer will just give you someone else's opinion, but it might not fit the way you shoot.


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#14 AJ Young

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Posted 10 April 2018 - 05:28 PM

18% Grey is a unit of measurement that, given the article by Hogan, isn't consistent across cameras. That being said, it is still a tool you can use, in conjunction with your light meter, to measure an exposure value.

 

When I prep for projects, I always sync my meter with the grey card and waveform in the desired final output format(s) (ie, REC709, P3, etc).

 

I adjust my meter (a Sekonic 758C) to read the grey card at 50% IRE on the waveform; choosing this value makes it easy for me to do the math on set. In essence, I'm adjusting the unit of measurement to meet my specific needs as a DP. Doing this ensures that my meter, waveform, and grey card are all "calibrated" and thus say the same thing.

 

---

 

I should also point out a few variables in regards to your test that may have contributed to skewed results:

  • Grey card age?
  • Which brand of grey card did you use?
  • Which brand of grey card did Canon use?
  • How old was Canon's?
  • Has your meter been properly calibrated recently?
  • Angle of reflectance? (pointed out by Richard)
  • Was the light diffused?

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