Grading Low Light DSLR Footage
Thursday, April 28, 2011 at 9:19AM Yeah, I know this has been done a million times before but I just got my Canon EOS 7D and wanted to test the low-light performance myself. I aslo wanted to see how well the footage could be graded. and while there are myriad examples of this sort of thing on the internet, when I went looking I didn't find one that showed me this in a way I wanted to see it. So who knows, maybe this will one day help someone out.
A very important factor in evaluating the footage of any camera or codec is the ease of use. As much as I love getting down and dirty with color grading, I don't want to spend forever. The gold standard is film. You shoot film, it looks great. For decades the way to control the color of film was by adjusting printer lights to affect the transmission of 3 colors. Simple. The more you need to massage a digital image to get an acceptable image out of it, the more that digital system can be considered a failure. Yes, I know - cheap, fast, good; choose any two - if you're on a budget you can expect to spend more time achieving that good image. But you should still evaluate a system by how painful that sacrifice is.
So I kept this test simple. At my friend (artist, writer, and director) Sketkh William's birthday party, I grabbed some footage on the 7D. Barcade in Brooklyn is lit primarily by these novel little bag lights on the table. They have what can't be more than 20W yellowish tungsten bulbs - kind of like Christmas lights - stuffed inside brown paper lunch bags. Honestly it looks like there's a candle inside and you have to look for yourself because it's easy to feel as though there's a fire hazard. There is also dim light from the arcade game screens (it's really a cool place if you like classic games and/or craft brews) and very dim very sparse lights on the wall. Overall it's a pretty dark place. I took some shots at 1250, 2500, and 5000 ISO. The camera really can see in the dark, but you could see the noise in every image as well. I like noise when it's pleasant though, as long as it doesn't deviate too far from the look of the rest of whatever you're shooting.
For post, I converted the images to ProRes4444 using 5DtoRGB. The footage from the camera has 4:2:0 chroma subsampling and although converting it obviously won't add any color depth, any color grading application will have an easier time handling, and be able to perform more nuanced operations on, an extracted RGB image. To keep things really simple, I graded the images by eye in After Effects using only Colorista. Normally you'd want to use scopes and get precise, but I'll be doing a more in depth grading test next week with the new Technicolor color profile. For the time being, I just wanted a simple test with low-light footage. So first, here are some screen grabs from the original footage:
And here is the graded footage:
Overall I'm pretty happy. I wish I could show the actual quicktime file I output but I had to transcode it for Vimeo. In the actual file you can see the grain structure very well, but the above video will give you a pretty good idea. Here are some basic observations:
- 1250 was a nice setting but underexposed. I really like how the colors graded, but I had to boost the gamma and that created noise.
- 2500 had a surprisingly acceptable noise profile off the camera and I didn't have to grade it very agressively. The noise is mostly noise at the sensor level which is more pleasant and while the 1250 footage started off cleaner, this is the better graded image because the noise is more natural.
- 5000 was overexposed. You can see the bag lights blow out. Yes, the faces are better lit here, but the overall image is brighter than it should be. In the graded image, I tried to supress the gain as much as reasonable to handle some of this.The noise, like with the footage shot at 2500 is camera noise and overall not horribly objectionable but still much more visible than I'd like. This setting fell into the typical pitfalls of overexposing a digital image.
2500 ISO was the best setting here and really the right setting for the environment. At 5000 the faces of are better lit and are less red in the native image, so while 5000 is great in those terms it creates more grain than I want and requires more grading. If I had any control over the environment I'd agument the bag lights and bring the exposure on people's faces up to shoot at a lower ISO, but the point here was to grade an image from a poorly lit evironment. I think 2500 produced the best overall image (I took 3 shots at each setting and the ones not being displayed here suggest this as well).
This shows the importance of getting things right in the camera. These cameras don't have a whole lot of dynamic range to begin with so, much like reversal film, a little over or under can kill you. Under normal circumstances I wouldn't want to shoot at any of these ISO settings, but sometimes you have to. When this is the case, you have to accept that you're going to get some noise; but you have to understand that the best way to combat noise and a limited latitude is to expose properly. This will make grading easier and minimize the visual sacrifice that you're making. If you're lucky, instead of ruining your picture you'll add to its character.
