What we soon discovered is that the BMPC doesn't have a very complete feature set when it comes to exposure metering. It has zebras and a histogram, but that's it. No IRE waveform, no exposure meter...none of the usual fare of a professional video camera. This made for a difficult situation when filming outdoors with my variable ND filter. The image from the camera is "flat" in either video and especially film mode. It also lacks saturation, which makes approximating exposure from the back LCD rather difficult. (Blackmagic - if you read this PLEASE update your firmware to include a viewing LUT!)
In all fairness, I do own a very nice Sekonic exposure meter. However most variable ND filters are not marked in stops, much less third of stops. You get generic 1,2,3...markings that are completely without measure. So when you're outdoors at ISO 800 (recommended sensitivity) then you're forced to use ND to get a reasonable stop on the lens. I don't know anyone who carries around a set of screw on fixed ND filters. I have a nice of set ND's that go in a matte box, but the matte box is easily ten times the size of the camera plus lens!
So my best estimate was to get something that looked kind of close on the back LCD and use a GretagMacbeth color checker chart to set the final exposure in post. I used the histogram on the back LCD to make sure that my exposure wasn't clipping black or white and the distribution was approximately centered. This is the same way that you would exposure a Red camera when shooting in raw. Then I made a final tweak to the variable ND filter to bias the exposure to something that looked a little more correct to my eye. I knew this would get me in the ballpark for adjustment in post.
One really great thing about this camera is that it includes two very robust codecs, 12-bit Prores HQ and CinemaDNG raw. Unlike 8-bit highly compressed footage from a DLSR, adjustments to exposure and color in post are very easy and robust on the BMPC. These same adjustments would likely destroy footage from my 5D mark II.
So we're starting with something like this...
If you look left to right on the chart, the fourth neutral patch (bottom row, just below yellow) is just just less than middle gray. I know this because I recently tested it with the Canon gammas in my last blog post. So adjusting the mid tones to have this particular patch just below middle gray should get my exposure approximately corrected.
The first thing I'm going to do is put a four point garbage matte around the neutral patches and look at the luminance waveform in Premiere.
If the picture was properly exposed we would see the fourth patch just below 50 IRE. In the graph above we see it around 45 IRE, so this tells me the shot was a bit underexposed. That's pretty easy to fix by bringing up the mid tones with a luma curve.
For this particular test shot we were using the "video" gamma on the BMPC, which requires a bit less correction than the "film" gamma. To get a more finalized color I used the fast color corrector effect in Premiere. The first step is to use the eye dropper in the effect and choose the middle gray patch so that we can establish a good white balance. Note that I chose 5600K white balance on the camera when recording this shot, so it was already pretty close...just not perfect. This step also removes the green bias from the variable ND filter. Then I added about 40% more saturation to the image to get the skin tones where they look natural to me. Depending on your taste, this may be anywhere from 30-60%. In "film" gamma it was more like 210-230%, depending on the look you're after.
The great thing is that with such robust codecs it's not a destructive adjustment. It's like Blackmagic Design, developers of Davinci Resolve color grading software, want you to do color grading. Hmmmm... The downside of this is that you MUST do color correction on all your footage to get it to look decent. There is no easy-to-use baked-in "look" on this camera. David is an engineer by training, so he probably likes that characteristic of his toy.
One final step I do is take a look at the highlights and shadows and make small adjustments to them using the luma curve. This is just for personal taste and not a step that's required. Often these are very, very minor adjustments. In this case I want to back off the highlights just a bit to give the pictures a more "filmic" highlight roll off. The only place you'll probably see the difference here is in the sky reflection in the window. The shadows look normal to me so there's no need to correct them.
Before I conclude this post I just want to point out that what I was trying to do here was color correct the footage, NOT color grade. That's a whole advanced topic for which I'm not an expert. I think of color correction as more of a technical process and color grading as an intentional artistic process that requires experience and technical know-how.
If you have any questions feel free to leave a comment.