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Sunday, February 15, 2015

••◊ Exploring Canon WDR and C-log gamma curves

I was helping Dominique, aka "El Gun Legro" record his Kickstarter video using the Canon C100 with the WDR gamma and looking at the monitor, saying to myself "I sure hope this is right."  The image looked a bit dark, de-saturated, and lacked contrast.  So this weekend I went into the shop and borrowed the use of the camera for a few minutes to gather some test data to see if I was actually using the correct camera settings, or this was another case of a stupid ignorant DP believing he was being "smart."

In the first test I recorded a few seconds of footage using the Gretag Macbeth chart.  This told me about color rending, as well as contrast.  In the screen shots below you can see that the standard setting is pretty mild mannered by default.  The EOS standard gamma is extremely contrast-y and saturated (yuck).  It reminds me of the bad old days of fighting a DSLR to work for filmmaking.  The WDR custom setting shows that the gamma reduces contrast quite a bit (just look at the white chip) and de-saturates the colors.  C-log does the same, but to a much more severe degree.  In each case I chose the corresponding color matrix for each gamma so I wasn't mixing the color setting with the wrong gamma setting.

C100 standard settings

EOS Std. gamma custom setting
 

WDR gamma custom setting

C-log gamma custom setting

Now if we just look at the neutral patches at the bottom of the chart this tells us about the contrast of each setting.  The third patch from the bottom right is just a little darker than middle gray by design of the chart.  So it should show up somewhere less than 55 IRE on the graph.  In the case of the standard gamma and the WDR gamma it does just that, however the highlights are more protected with the WDR gamma.  The EOS Std. gamma raises the almost-middle gray patch to nearly 60 IRE, artificially brightening the image, as well as adding eye popping contrast.  C-log does the opposite and darkens the patch to less than 30 IRE, as well as reduces the contrast.  With an 8-bit camera I would be concerned about the loss of shadow detail given that there aren't a whole lot of 8-bit counts below 30 IRE.  Bringing back shadow detail might be a bit difficult in certain situations.  C-log seems make more sense for a 10-bit camera like the C500; at least to me.  It's the same thing with Sony's S-log - it starts to make sense when you have a 10-bit camera.

Neutral patches with standard settings

Neutral patches with EOS Std gamma

Neutral patches with WDR gamma

Neutral patches with C-log gamma

I took the camera out to the parking lot and used my light meter to determine proper exposure for the sun so I wouldn't bias this real world test.  I also double verified with the C100's exposure meter - which was dead-on when I adjusted the iris.  As before, the standard settings are fairly good and generic by default.  The EOS Std. setting is horribly contrast-y and already over-saturated.  Now if we compare the standard settings to the WDR settings you can see that the highlights are a bit more protected (just look at the sky).  The WDR settings also seem to be lacking a bit of contrast and saturation, which is generally a good thing for protecting any clipped highlights - *IF* you plan on color grading the footage.  If not, then the standard settings seem decent by default.  The C-log image is very dark and needs quite a bit of correction to bring it back to life.

 standard settings
 
EOS Std gamma

WDR gamma

C-log gamma

Here are two quick color grades I did with the WDR and C-log images.  I found that typically the WDR footage needed a boost in saturation of 20-30%.  The C-log footage typically required about a 70-80% boost in saturation, which can be a delicate operation on 8-bit highly compressed AVCHD footage.  It also required a large adjustment with a luma curve to bring the exposure and highlights back to where they should be.  If the production allows for color correction, I would personally still stick with the WDR gamma on the C100 because less post production adjustment is required.  In my opinion this holds with the Ninja Blade recorder as well.  Prores HQ is better than AVCHD, but the camera only outputs 8-bits either way.

WDR gamma, luma adjustment and saturation boost

C-log gamma, luma adjustment and saturation boost

Dominique gave me a small clip of the Kickstarter video.  After adjusting the contrast and increasing the saturation about 25% the footage went from what you see above to what you see below.  So now I can sleep a bit better knowing that everything was OK with the footage.  I just wish I had a monitor that allowed for a programmable viewing LUT so I could know that my final footage would look OK while we're recording the video.



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