This is an age old question when converting 4:3 footage to 16:9. Do you keep the original aspect ratio (OAR) in the converted footage to keep the purists happy (small percentage of audience), or do you fill the frame with as much picture as possible to avoid mattes to keep the average content consumer happy (much larger % of audience)? At the end of the day, the producers will make the decision that they feel will please the largest group of viewers.
I have personally been involved with these types of projects. The purists are the smaller section, but they are much more vocal on the internet. While reading forums and tweets and other soapboxes, these purists will eviscerate you publicly. However, using other metrics (sales figures, independent surveys, etc), the numbers show that the overall product is well received. There are so many people online that believe they know everything, but know nothing; yet these people are the most vocal. I was once accused of uprezing existing video tapes rather than re-scanning the film like the company claimed. Never mind that I was personally carrying the film to the transfer house. We took pictures of the employees with the film, took shots of the film on the shelves in the vaults, and wound up making "making of" type documentaries showing the process of the film on the telecine, interviewing the colorist transferring the film, showing the artists doing the restoration, and the sound department working with the audio from the original optical track, etc. Still, the know-it-alls kept on yelling.
Your choices are to keep the OAR as 4:3 so that you have ~240 pixels of pillar bars on the left and right of the image (for HD), or crop into the image to make it full frame. The average viewer tends to not like mattes of any type. They tend to prefer a full frame image to fill up the pixels on their screen. Because of this, producers will make the decision to crop into the picture to get a full image.
The same decision happens when making a 16:9 (1.78) image from content that was shot in the common 2.35/2.40 aspect for feature films. To get those images to 16:9 so the image is full screen requires cutting of pixels from the sides of the original. Content shot at 1.85 requires even less cropping. Even true 4K content is cropped slightly to fit the 16:9 aspect of UHD. It was even worse when television was only 4:3. Content in 2.35/2.40 would crop so much of the content, a shot of 2 people could remove one of the people from the frame entirely. They would use the Pan&Scan technique to slide the image around in the frame to reveal the 2nd person. This was one of the reasons for the slate at the beginning of a movie saying that the video had been reformatted to fit the screen.
Video containers (or the stream itself) should really have a "center of frame" coordinate for each frame, and let playback devices crop based on that center. Maybe this already exists in some standards and im unaware.
For digital files, ideally, the file only contains active pixels so that the file is the frame size of the image. However, for video formats with a clearly defined standard (TV/Broadcast/DVD/Blu-ray/etc), the video frame size must be what is defined in the spec. This is why the mattes exist to fill in the space to make the content fit the space required.
If some people want to crop video to fit different screen aspect ratios, it would be nice for the frame to advertise its "center" so the playback device can apply the appropriate cropping and matting.
Maybe I have my player set to "3:2 full screen, up to 30% crop.". It's not something I would do personally, but it would be better than pan and scan being something implemented in the encode itself.
> Your choices are to keep the OAR as 4:3 so that you have ~240 pixels of pillar bars on the left and right of the image (for HD), or crop into the image to make it full frame.
That's not true. You could keep the original aspect ratio and simply let the player decide. That way you aren't removing any information, and you aren't encoding redundant black bars into the video.
The YouTube player is perfectly capable of filling all available space it is given.
