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Discussion Starter · #1 ·
This was triggered by another thread, elsewhere, but I didn't want to derail that, so here we are....


This might sound odd, but I've always wondered why we don't talk about this issue in terms of how close to sit versus screen height instead of versus screen width. I'm guessing there is a reason for the discussion always in terms of width, but in my personal experience, height is the issue. I've never once been in a theater where I felt I was too close, based on width, but several times based on height.


If I had a 10 foot wide 4:3, sitting 10 feet away (just using 1:1 for discussion) I think I would get eye strain looking up and down, but much less at a 10 foot wide 16:9, and close to none at a 10 foot wide 2.35:1.


So, why do we not discuss the distance to the screen in terms of screen height and the viewing angle comfort level "up and down"? Ideas?
 

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Based strictly on a THX cinema with 36 degree HFOV ( Horizontal field of view) for the rearmost seats and presuming seating at the 2/3 point (which the audio set up is calibrated for) we then have:

HFOV = 44 degrees

VFOV = 19.5 degrees

this all presuming a 2.35:1 screen.


The anthropometrics for this are a bit difficult to unearth and one of the better compilations was probably the SMPTE Engineering Guideline Design of Effective Cine Theaters, EG 18-1994 which sadly is no longer available.


When the vertical angle to the top of the screen gets to 30 degrees we are uncomfortable. Neck strain.


The trade off that we are forced to deal with has to do with the source and projector resolution combined with the immersive fields of view.


EDIT: changed 45 to 44 degrees
 

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Discussion Starter · #5 ·
Interesting, thanks.


VFOV = 19.5 degrees

this all presuming a 2.35:1 screen.


This is kind of where I was going with this, if the height is 20 degrees for a 2.35:1, shouldn't is also be 20 degrees for 16:9 or 4:3?


It is more of a theoretical question, why/how did we come to the common 1.5xwidth formula instead of coming up with a v.height formula? Anyone know the history there?
 

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IIRC, the 1.5x width guideline was based on 480P material, digital PJ's, and not wanting to see pixel structure or digital artifacts.
 

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Screen width is normally what we deal with when calculating horizontal viewing angle, which is often used to determine how much of our field of vision the screen will occupy, and it is one variable that effects how immersive the experience is. The THX minimum requirement of 26 degrees (and the recommended minimum of at least 36 degrees) is based on horizontal viewing angle.


Because human beings have much more peripheral vision than up/down vision, it is the horizontal measurement that we are often interested in maximizing.


On the other hand, the screen height does become important as a limiting measurement because of the discomfort associated with a vertical viewing angle that is too high. At greater than 30 degrees vertical viewing angle, physical discomfort sets in. Most home theaters do not allow us to approach the 30 degree limit at reasonable seating distances, but a decent guideline is to keep the vertical viewing angle under 20 degrees.


Of course, none of this speaks directly to why we use screen width as the measurement in so many of our calculations. I expect it's because width deals with peripheral vision which may be one of the concerns addressed by the calculations. I'm not sure, since I don't use any "screen width" rules, but instead calculate the horizontal and vertical viewing angles, generally using online calculators, but I have also sketched it out in a program like Visio. The right screen size for you will depend upon many things, including source quality/resolution, projector artifacts, and personal preference. None of these screen width rules apply to all situations.


For example, I have found that I prefer a horizontal viewing angle of 40+ degrees when watching HD DVDs on a 1080p screen, but this seating distance is too close for DVDs, where 26 degrees is much more agreeable to me.
 

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Not considering the effect of source material resolution limitations, a method that can be used is evaluating the pixel structure with respect to visual acuity. Just as DKaps points out, minimizing SDE can be used as the constraint.


For a while PJ's were stuck at XGA rez and a 3/2 viewing distance allowed the 36ish degree inclusion angle in the horizontal plane but made the trade off to about 27 pixels per degree.


Nominal acuity being agreed at 1/60 degree makes most of the Home Cinema setups beg for 1920 x 1080 resolution in order to accomodate both seeing all there is, having an inclusion angle that promotes immersion ( roughly 30 degrees HFOV) and NOT being overwhelmed by artifacting.


Prior to 1920 x 1080 rez, most setups could allow the 4/3 viewing distance with 720p rez setting a HFOV of 41 degrees and an acuity bleeding ~ 30 pixels per degree. Seems most would rather go for a wider inclusion angle than not.


As always YMMV.
 

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On a related note, I posted a spreadsheet in August - actually a combination of several spreadsheets made by others with a few calcs of my own thrown in in this post that calcs view angles and a bunch of other stuff:

http://www.avsforum.com/avs-vb/showt...&post8171419


That whole thread has some stuff that you folks might find interesting.
 
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