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What is the math behind perspective to screen size?

Discussion in 'Home Theater Lounge' started by Matt Johnson, May 8, 2007.

  1. Matt Johnson

    Matt Johnson New Member

    There's got to be math behind this, wondering if anyone knows it. I'm an engineer by schooling, but it's been 10 years and I'm a manager...so I don't know how to actually do anything now. :lol:

    What I want to know is why is there a perceived difference between a big screen farther away versus a small screen closer to you.

    Theoretically, you could hold a small tv close to you and cover the same square footage of your vision that a big tv would be farther away.

    But it's different somehow. Assume everything is 1080p so we're not talking about resolution...talking about how your brain works.
     
  2. Tom R S 4

    Tom R S 4 New Member War Zone Member

    I don't know the answer, but I do know one thing that is different: The focal point for your eyes. Your brain knows when it's focusing up close vs. far away.
     
  3. Matt Johnson

    Matt Johnson New Member

    If I remember correctly, once you pass your focal point, you can focus fine. So let's assume the small tv is past your focal point...say for example a 37" tv 5' away vs. a 72" tv 10' away.
     
  4. Michael

    Michael Junior Geek

    I thought it was all about screen size and seating distance.

    Something along the lines of seating distance = 1.5 x screen width.
     
  5. Alfer

    Alfer New Member

    I think the THX site has formula's that will help you out.
     
  6. Steve Tillman

    Steve Tillman New Member

  7. Bill Stratton

    Bill Stratton New Member

    I think what Matt is asking is: "why is it cooler" to sit half way back in the theater with a 50 foot wide screen than it is sitting 10 feet back from your 4 foot wide screen at home?

    Assuming the angle from your eyes to the outside of the screens is identical, why does the perception seem different?

    Good question, and one I've wondered about too. Maybe the overall size of the space gives you a different feel? Other ideas?
     
  8. Tom R S 4

    Tom R S 4 New Member War Zone Member

    My idea about the focal point of the eyes was all I've come up with so far, and I think it got shot down.
     
  9. Matt Johnson

    Matt Johnson New Member

    I'm heading towards what Bill said...no way I'm going to hold a 13" screen 2 feet from my face vs. watching 130" screen sitting 15 feet away...might be similar in size theoretically but produces a huge difference in effect.
     
  10. chad

    chad Active Member War Zone Member

    I sit about 12' from a 119" screen. I don't know if it's "right" I don't really care either. The picture typically looks good ( dependent on source of course) and other than rare occasion I don't see any screen door. I had an issue with VB but i took it a wee bit out of focus and I was good to go. I say find the size you like the most, then buy the next size up. This is the first screen size I have been totally happy with and I have had 92", 106" and 110".
     
  11. Chris White

    Chris White New Member

    Great question Matt! This is something I've wondered about and I want to learn more about this topic.

    The article Steve provided is informative and provides a partial explanation based on the resolution capability of the display and the human eye. However, this answer only seems applicable in situations where there is a disparity in display resolution. In other words, the visual experience sitting 8 feet from a 72" display may be different from sitting 4 feet from a 36" display if the display resolutions are different.

    Tom's point that focal distance matters also provides an explanation, but it seems to me that it applies primarily when the near-focus distance is really close. For example, 2 feet from a 24" display versus 20 feet from a 240" display. But would difference in focal distance still matter if we were comparing 5 feet from a 120" display versus 10 feet from a 240" display?

    Here's an idea - perhaps the difference in visual impact might be the extent to which visual elements are proximal in displayed size to actual size. As long as both displays have equal resolution capability and the distance to screen size proportion is maintained, there is an equivalence in field of vision, ability to resolve detail, etc., but the bigger the display, the more life-like the visual elements. No matter the distance I sit from my laptop, a person on the screen can only be 6" tall. If I watch the same movie on a 120" screen, that person can be almost 60" tall - much more realistic in actual size.
     
  12. Dustin B

    Dustin B Active Member War Zone Member

    I'd also postulate (expanding on Chris's last comment) that the elements in the the room besides the screen have a large effect. In the theater you see loads of seats and great big walls. On a Laptop you see this little laptop. Your brain processes the difference.

    If you could have the screen be the only thing visible (and I mean the only thing visible, can't see any of your own body either), then I don't think you could tell the difference between a tiny screen up close or a huge screen far away (assuming same resolution stuff).

    My screen seemed bigger when I masked it off in my old theater as it removed some of the room size effect. And I expect once I get the masking in my new theater setup and the front screen wall all blacked out I'll get the effect again and even more pronounced this time as my whole room is so dark now.
     
  13. Chris White

    Chris White New Member

    I've always thought the main benefit of duvatyne masks was to make the 2.35:1 black bars truly black, thus creating greater contrast for whatever is on the screen. What you're saying leads me to believe that the real enhancement comes from reducing the awareness of the tv itself. Does that make sense?
     
  14. Speaking of focal points - I did my senior year project(many, many years ago) on "dark focus" and all I was able to prove was that there was no predictor of where one's dark focus might fall and that it changed for each individual based on their stress level at the time of testing. We used laser optometers and our failed practical goal was to design fighter aircraft instrumentation for night missions. Got an "A" for it though... :)
     
  15. Dustin B

    Dustin B Active Member War Zone Member

    That's what it felt like to me. It does help contrast, just as having dark non reflective walls in the room does. Both alone are definitely reasons for adding the masking. But before my masking went up, I always noticed the screen on the wall. With the masking up all I ever noticed was the movie.

    I don't have masking up in my new theater yet and I sure miss it. Although it's not as bad as my old room as all the walls are a dark flat paint in the new one.
     
  16. Chris White

    Chris White New Member

    I haven't made masks for my new Samsung yet. The black bars on it are quite a bit blacker than they were on the Toshiba so I haven't really felt the need.
     
  17. Matt Johnson

    Matt Johnson New Member

    I can buy the idea that your brain "knows" the difference...because I saw a difference when I put a border around my screen when it was the same projector, same screen material etc, just gave my eyes a boundary to the screen. If it was pitch black and you held the tv close to you I could understand your brain not being able to tell a difference between that and a big screen farther away if the resolution works out.
     
  18. Matt Johnson

    Matt Johnson New Member

    I can buy the idea that your brain "knows" the difference...because I saw a difference when I put a border around my screen when it was the same projector, same screen material etc, just gave my eyes a boundary to the screen. If it was pitch black and you held the tv close to you I could understand your brain not being able to tell a difference between that and a big screen farther away if the resolution works out.
     
  19. Saurav

    Saurav Active Member War Zone Member

    What Matt said.

    What if you had blinders on and the room was dark enough that the only thing lit was the screen (so no reflections off any other surface), and the screen was the appropriate distance from your eye to subtend the required angle, and it was of adequate resolution....

    Basically, I think you know the image is 6" tall because you can see stuff around the image. If you didn't have an external frame of reference, I wonder if the brain could tell the difference.
     
  20. Saurav

    Saurav Active Member War Zone Member

    What Matt said.

    What if you had blinders on and the room was dark enough that the only thing lit was the screen (so no reflections off any other surface), and the screen was the appropriate distance from your eye to subtend the required angle, and it was of adequate resolution....

    Basically, I think you know the image is 6" tall because you can see stuff around the image. If you didn't have an external frame of reference, I wonder if the brain could tell the difference.
     

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