Above is one of the most famous, important and remarkable paintings in history as it was created.

This image is 1.96 times as wide as it is high, just a bit wider than a widescreen TV (which is 1.85 times as wide as high, or 1:1.85 in the shorthand).

Fullscreen is a misnomer, it actually refers to “filling the screen” where the screen is the going-out-of-style squarish TV still found in my living room. It’s an image ratio of 1.33 times as wide as high (or 4:3). I’ve modified the classic painting to reflect how fullscreen, or pan-and-scan, cuts the image to fit onto the differently-shaped screen.

A bit over 40% of the image is cut off to fit the screen shape.
This is the point where fullscreen advocates will tell me that the “important parts” are still there. I’m assuming that Da Vinci was bright enough to put the 9 “important” disciples close enough to centre that they didn’t get cut.

Lastly, and most horrifying to my snooty cinematic ways, is what I see quite often on widescreen sets. Taking a fullscreen image and stretching it to fit a widescreen TV.
What you should see when watching a 4:3 show on a widescreen TV are black bars on the right and left of the screen. On many screens, what I see is this:

I’m not going to tell anyone what they should do with their set-up. Throw a blanket over the screen and pretend it’s a radioplay for all I care. But please, please be aware that you are watching an artistic endevour that has been “modified to fit your set”.