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So i think it's pretty clear that if a movie is released today and you watch the normal DVD version side-by-side with the blu-ray version (i'm talking standard blu-ray and not ultra blu ray or whatever the term is) then the blu ray version is just better. That said, my PS4 did a pretty decent job of upscaling my regular DVDs in to a quality that was better than when i was playing the DVDs on a regular old DVD player.

But what about movies that were never originally released on blu-ray because blu-ray wasn't even a thing when that movie was released?
And what about the blu-ray versions of movies where DVDs weren't even a thing.

I don't really know how the industry works but i imagine that the movies are limited in terms of quality by the technology that is available at the time.

So in a nutshell i'm wondering whether it's worth it getting movies like Scarface (1983) as one example in blu-ray as can it really be that much better than the standard DVD version when blu-ray nor DVD were even a thing back in 1983?

MOST OFTEN (but not always), the BD is authored from an original film copy of the movie. Film has much higher resolution than DVD or BD (and usually UHD), and there are things that happen during disk authoring that can be done differently.

For example, DVDs are authored for either 50 (PAL) or 60 (NTSB) Hz displays, while file is shot at 18 or 24 frames per second. This means that frames are added at regular intervals to make up the difference. DVDs are also usually interlaced video, meaning the "even" lines of the frame are sent, then the "odd" lines, and your TV makes it look like one frame.

Blu-ray USUALLY is done at the original film speed (24 FPS), and the player has less work to do. You also don't get the "combing" that comes from half a scene change when the camera is "moving".

If you have a movie on DVD and you're happy with how it looks, why bother? Seeing it in higher definition doesn't improve the content.

If you don't own it on DVD, the cost for BD isn't that much higher (sometimes it's lower!).

To help make a choice, buy something you're interested in in a "combo pack", and compare the video. Larger screens are going to make you say, "BD for sure!", while smaller screens, relative to how far you sit from them, will make you wonder what the fuss is about.

Depends what you call a large screen. Large to some will be small to others. I have a 58" 4K TV, although i don't know how much 4K i actually get on the thing but i'm happy enough with it and i'd consider that to be a large TV although i have some friends with even bigger ones. I sit approx 6-8ft away from it, directly in front, not at any angle.

I'm actually going to acquire a blu-ray copy of Scarface and compare it to my existing DVD copy. I tried to tell the difference between DVD Bad Boys (1) and Blu-Ray Bad Boys but it was difficult, probably due to the upscaling of my PS4.

Since i'm looking at ripping my entire collection and viewing through Plex, i'll see if the difference is more noticeable.