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Sunday, February 22, 2015

••◊ C-log and S-log: Why On 8-bit Cameras?

A lot of the people I congregate with in my small film maker world would easily be described as "low end."  They can afford 8-bit beginner or mid-range cameras and dream of shoots with the Alexa.  One issue I constantly run into is that the camera manufacturers put log gamma curves into their cameras - on 8-bit cameras - and low end folks I work with think this makes their camera just as good as the high end cameras.  Reality is a little different and I'll explain with some simple math.

My two most recent examples are C-log on the Canon C100 and S-log on the Sony A7S.  Let's start with Canon.


In the best case C-log has 5.3 stops above middle gray and 6.7 stops below middle gray, as shown on the picture above.  I stole the picture above from Canon's white paper site on C-log.  From my previous article I measured middle gray on C-log somewhere close to 32IRE.

Now this is going to involve some basic math - and I know videographers generally don't like math.  Just stay with me.  An 8-bit camera produces 256 levels of gray, measured as 0-255.  That same 8-bit level can be measured as somewhere between 0-109IRE.  Doing the algebra here, 32 IRE equals 76 8-bit counts.  That means that the 6.7 stops below middle gray in C-log is represented by 76 counts (i.e. 0-75).  The 5.3 stops above middle gray is represented by the remaining 180 counts.


Doesn't that seem backward that the majority of the dynamic range is represented by 76 counts, while the minority of the dynamic range is represented by 180 counts?  This only gets worse when you decrease the ISO.  For instance, looking at the picture above you can see that at ISO 320 or ISO 400 it's nearly 2/3rd's of the dynamic range that is represented by 76 counts, whereas the remaining third is represented by 180 8-bit counts!

The same is true of S-log on the Sony 8-bit cameras, such as the A7S.  In their case they have Slog-2, which places middle gray at 32 IRE again, with 6 stops above middle gray and 8 stops below middle gray.  So the shadows get 76 counts to represent 8 stops - that's almost the entire dynamic range of a Canon DSLR!

The thing is that these log curves were really designed to work on 10-bit cameras.  In that case you have 300 counts (i.e. greater than an 8-bit range) below 32 IRE.  Stretching the shadow range in post doesn't seem like such a big concern when you start with 10-bits (0-1023).  It's the camera manufacturers' marketing folks that listened to all the low end folks and probably thought they would sell more cameras if they had a box spec that the camera supported a high end feature, like log gamma curves.

The other two issues I usually warn people about is that you need a monitor that supports a LUT in order to see what the final image might look like after color grading.  That's usually not cheap ($2k+).  Plus you need someone that knows how to properly expose (experienced DP) and color correct (experienced DIT) footage shot with a log gamma curve.  If you're eye glaze over while reading about IRE levels, you shouldn't use log gamma curves!  Don't understand vectorscopes or luminance waveforms?...DO NOT use log gamma curves!...Don't use a light meter or know how to measure contrast while lighting - DO NOT USE LOG GAMMA CURVES.  There's a reason log gamma curves are for higher end professionals. 

That said, it's nice to believe for a little while that a low-end 8-bit camera has the same features as a higher end professional camera.  It's almost like those kids who put fancy wheels and a loud muffler on their Honda Civic car and think it's now a sports car.

I color graded some footage off the Blackmagic Pocket Camera yesterday.  Let me tell you; 12-bits makes a huge difference!  What we were doing with that footage would have destroyed an 8-bit capture - and it looked great in the end.  More on that later.

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