The optimum TV screen size is directly related to the available viewing distance - but there are other factors as well that need to be taken into consideration.
Viewing Distance: Is it just a matter of personal preference?
Sit too close to your big screen TV and you will be able to see the image build-up structure - scanning lines or pixels forming the image - thus distracting your attention and spoiling your home theater experience. Yet, sit too far away, and the impact will be lost.
There are differing opinions on the best way to determine the optimum TV viewing distance for a specific screen size. Just go to the movie theater and you will soon realize that it is all a question of personal preference - some would sit at the very back. Others would go straight to the front row, as they prefer the bigger picture and a wider angle of view, while some would simply choose their seat randomly somewhere in between these two extremes.
The truth is that there are no scientific rules her. This does not mean that there aren't any guidelines that you should follow when planning a big screen purchase or a would-be home theater room.
SMPTE Recommendations and the THX Certification standards:
The Society of Motion Pictures and Television Engineers (SMPTE) recommends that the screen size for home theater use should occupy a 30 degrees field of view - in the horizontal plan - for the audience. Alternatively, the ideal TV viewing distance should be such that the screen width occupies an angle of 30 degrees from the viewing position.
This 30-degrees viewing angle seems to have been accepted by many as the standard in home theater and motion picture viewing.
This SMPTE guideline is also in line with the THX certification standards in that these recommend that the back row of seats should have at least a 26 degrees viewing angle and while recommending an optimum viewing angle of 36 degrees.
It is believed that within these viewing angle limits, the viewer will get better immersed into the action movie itself.
Vision System limitations:
There is also the issue of TV viewing distance based on visual acuity. This does not represent the optimum viewing distance - rather, this relates to the maximum viewing distance beyond which some picture detail will be lost.
Technically speaking, visual acuity is a measure of the eye spatial resolving power and indicates the angular size of the smallest detail that a person visual system can resolve. A person with 20/20 (or 6/6 when expressed in meters) normal vision can resolve a spatial pattern separated by a visual angle of one minute of arc angle i.e. 1/60th of a degree, at the eye when viewed at 20 feet away. Expressed differently, a person with normal 20/20 vision is capable of identifying an object with a height of 1.76mm at 20 feet way.
In terms of TV viewing distances, these represent the point beyond which some of the picture detail will no longer be resolved by the viewer vision system.
So How Does All This Translate In Practical Terms?
A few rules-of-thumb can help put in practice the guidelines detailed above. These rules for viewing distance refer to the screen width rather than the screen diagonal and therefore apply to both 4:3 and 16:9 display formats.
A general rule for the TV viewing distance based on the SMPTE and visual acuity guidelines, is that the nearest TV viewing distance between you and your big screen TV should be limited to approximately twice the screen width (more precise 1.87 x screen width for a subtended angle of 30 degrees), while the furthest distance being no more than five times the width of your screen.
This rule of thumb should give you a fairly good approximation for your TV viewing distance. It does not necessarily represent the ideal home theater viewing distance but rather the limits within which your TV viewing distance should theoretically be out of the trouble zone.
In other words, move closer than twice the screen width size, and the picture scanning lines, pixels and any other video artifacts will become too visibly intrusive - leading to distractions that will spoil your movie watching experience. Move further away than 5 times the screen width and your vision system will no longer be able to resolve all the picture detail.
But...
It is also important to realize that these maximum and minimum viewing distances should be seen in the light of the video signal definition.
A fully resolved high definition TV (1080i, 1920x1080) supports a closer viewing distance than standard analog TV. Thus while twice the screen width would be the ideal TV viewing distance for a HDTV display, it would be a bit too close for standard TV; in the later case, a three times the screen width would be a better option.
Similarly, the five times the screen width as the maximum view distance, while more than adequate for a standard analog TV picture, is a bit too far away for a person to see the fine detail supported by a HDTV picture - a three to four times the screen width represents a more practical limit for the maximum viewing distance in the case of HDTV.
These rules-of-thumb work best with big screen TV sizes in the range 42-inches and over.
When it comes to the use of regular-size standard definition analog TVs in the home theater, i.e. up to 36" / 40" diagonal, the optimum viewing distance range is between 8 feet and 12 feet. TV sets smaller than 36-inches aren't big enough to qualify for Home Theater use; their smaller screen size will not provide the desired impact on the viewer.
Vertical Angle of View & Screen Height:
For optimum viewing, the eyes of the viewer should be level with the center of the screen.
Maximum vertical angle of view: In those home theater set-ups where this is not possible, the SMPTE guidelines suggest that the maximum vertical angle measured at the seated eye height from the front row center seat to the top most part of the projected image should not exceed 35 degrees.
This does not represent the optimum viewing angle but rather the limit beyond which the viewer will be subject to an increased neck strain.
This maximum vertical angle limit is always measured from the front row as this represent the extreme angle of view.
Minimum angle of vision: While there do not appear to be any specific SMPTE or THX guidelines in this respect, yet studies have shown that if the screen size occupies less than 15 degrees of the viewer's vertical field of view, than that image appears small.
Practical Considerations:
In a typical home theater set-up, you do not need to really worry about neither the maximum vertical angle of view, nor about the minimum vertical angle of vision for an effective movie theater experience.
If one were to adhere to the recommended TV viewing distance of twice the screen width (as further detailed above based on the SMPTE guideline of 30 degrees horizontal field of vision), you would automatically be complying with the minimum angle of vision. The reason being that there is a fixed relation between screen height and width in accordance to your home theater screen aspect ratio of either 16:9 or 4:3.
Further more, in a typical home theater setup, it would be very difficult to exceed the maximum vertical angle of view beyond which you will be subject to an increased neck strain.
Taking into account that most home theater rooms are approximately 10 feet (3m) high, the resultant vertical viewing angle is normal well within the maximum of 35 degrees detailed in the SMPTE guidelines - all you have to do is just remain within the twice the screen width guideline for your viewing distance.
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