Where 4:2:0 breaks
One of the defining differences between the C100 and the C300 is the internal codec each camera uses. While the C300 records to the broadcast approved Canon XF codec (4:2:2) at 50 Mbit/s the C100 records to the familiar AVCHD (4:2:0) at 24 Mbit/s.
A common mistake I see people make, is looking at the bit rates of the two codecs and then they assume that AVCHD’s 24 Mbit/s is only half as strong as the XF codec’s 50 Mbit/s. However, since the underlying technology is different, you can’t compare them directly like that.
Instead, the real difference is that XF codec is a 4:2:2 codec, while AVCHD is 4:2:0. But what does that mean exactly?
Look at the video above and see if there is something that stands out. Then continue reading.
Chroma subsamplingDue to storage and transmission limitations it has always been the desire to compress image data. Taking advantage of the fact that humans are less sensitive to changes in color than brightness, methods have been developed where images are encoded with less chroma (color) precision.
Take your time and study the image below carefully. It illustrates the theory behind reduced color precision, or chroma subsampling. The top row represents the Luma channel and Chroma channels combined, while the two lower rows breaks the channels apart so that you can study the precision in the respective channels.
When you see the amount of data dropped in 4:2:0 encoding, you might be amazed that it actually works as well as it does. While it isn’t the optimum way to treat images from a strict quality standpoint, it is a proven method that has worked well for many years.
This type of image compression also allows for long recording times to inexpensive media. It’s up to the user to decide what tradeoffs are reasonable in every situation.
4:2:0 limitations and vulnerabilitiesI have stated on various forums on numerous occasions that I think that the internal recording capability of the C100 (AVCHD 4:2:0) is fine for almost every situation. I still stand by that. Would I rather have 4:2:2? Of course I would.
But I bought the C100 half expecting that for any serious work I’d have to hook it up to an Atomos Ninja 2 in order to get uncompressed 4:2:2 via HDMI out and record that to ProRes at 220 Mbit/s. After looking at footage shared by others on Vimeo (by downloading the original files) I’m struggling to justify the Ninja 2. The internally captured footage simply looks good enough in almost every situation. There are times though, when you should be especially on your guard. If you know what to look for you can check and see if you are in trouble and/or if you need to take steps to correct an issue that might otherwise show up.
Look at this frame grab from the video above. I’ve sized it up to 200% in order to show the problem more clearly. To see it in context, please watch the video above again. You can download the original file in ProRes format.
Since 4:2:0 subsampling only offers half vertical and half horizontal resolution, sharp edges defined by the red or the blue channel will be prone to be jagged. It almost looks like a ‘field’ from interlaced footage before de-interlacing.
Did you notice it while watching the video before the page break? Was it disturbing? Only you can decide how much of a real problem this is to you.
Not suited for green screen workUpdate: I’ve heard from several users that do frequent green screen work that the internal AVCHD codec actually holds up pretty well. That’s good news! I’m sure the generally clean and sharp image from the C100 helps a lot. Also, since green screen work means keying edges from the green color channel (where we have full resolution), depending on what you have in front of the green screen, results can be quite good.
For reasons that should be pretty obvious, any 4:2:0 codec is a poor choice when shooting against a green screen or blue screen. When aiming to key out footage you want clean edges with good contrast and minimal color spill. For that I’d go with 4:2:2 ProRes, if I were to shoot it with my C100.
Since I do very little green screen work, I guess the sane choice it to simply rent a Ninja 2 for those rare occasions. I will continue to evaluate the internal codec, but for now I’m very happy with it.