Does Viewing Distance Really Matter?

Does it matter how far you sit from you tv set to see all the details in HD picture? Some people would say "probably not" but simple answer to this question is "Yes". Before we dig deeper into technical aspects of this, let's step back and review our own actions when we are in a "viewing mode". We all seem to understand that sitting closer to the actual scene is a better option but when we are talking hundreds of feet, preference for the front seats in understandable.

What about a typical TV room where most people sit any where between 4ft to 10 ft while watching TV? It might not matter if you are watching a cartoon show on your old CRT television because picture doesn't have that many details to start with. But it is a very different story when we start talking about High Definition Televisions(HDTVs). To understand the science behind the distance and viewing pleasure, you need to get a good feel of another optical term known as "Resolution".

Let's take a look a closer look at the images below. As you can tell, all three images show a flower but third image has a resolution that is 5 times of the first image. Higher resolution means more detail in the picture and thus better overall experience.

Now how many more details are contained in a High Definition image compared to a standard definition? To explain this, let's take an example. Our regular NTSC TV picture is scanned using 525 lines, while HDTV uses 1125 lines to scan an image. In term of quality and details, just by doubling the number of lines in HDTV, it gets 5 times more details. If we have same screen size, a resolution of 1920 by 1080 lines will produce lot more pixels than 1280 by 720 lines.

Number of pixels in 1080 resolution = 1920 X 1080 = 2,073,600 = 2 Megapixels (approximate)
Number of pixels in 720 resolution = 1280 X 720 = 921,600

So in summary, 1080 resolution has 2.25 times more pixels than 720 and a far better resolution. Let's think of this scenario: if you own a 42" TV with 720p resolution, and sit 7 feet away from the TV, you'll see a detailed yet smooth picture because the pixels are small enough where you can't perceive them, thus allowing you to focus on picture instead of your TV. However, now imagine looking at a 65" TV with same resolution(720p) from 7 feet. Along with it being overwhelmingly large, you'll be able to pick out pixels on screen. However, if you now increase the resolution to 1080p, and thus made each pixel smaller, then the picture would look smoother and less blocky to your eyes.

To further explain this effect, we will need to refer to some optics rules. In optics, there is a technical term known as "visual acuity". This represent the maximum viewing distance for human eye beyond which some picture detail will be lost. It is a measure of eye spatial resolving power and indicates the smallest angular size of the detail that a person's visual system can resolve.

According to Charles Poynton, a well-respected expert in imaging systems, "it's widely agreed in the vision science community that the acuity of the human visual system for luminance (at 100% contrast " pure black to pure white) is about 1/60 degree, equivalent to 1 minute of arc." Now what does that mean? Well, in simple terms it tells you that a person with 20/20 or perfect vision, is capable of identifying an object with a height of 1.75mm at 20ft away.

Table shown below translates these optics calculations into real-world data when it comes to measuring TV sizes with varying resolutions. It does so by:

1. Calculating vertical pixel pitch
2. Equating pixel pitch to 1/60 degree
3. Deriving ideal viewing distance "D" (i.e. where all detail can be resolved without seeing individual pixels or "screen door" effect) as the radius of a circle
where: D = radius = 360 deg.* 60 * pixel pitch / 2 * Pi.

Based on this data, you can see that in case of a 42" 1080p display, the ideal viewing distance is only 5.5 feet--beyond that, you can't see the pixels and you can't really appreciate the full resolution of the display. In other words, it would be virtually impossible to distinguish between a 42" 720p display and 42" 1080p at distances of about six feet or more. Given that many people view their televisions from 8-10 feet away (if not even more), you would have to have a 65" or larger screen to really notice the difference between 720p and 1080p.

And last but not the least, is your viewing environment. When most people purchase a High Definition set, they just put the new set in place of their old standard TV and leave everything else as it used to be. Moving up from a standard TV to a HD set itself is a big improvement in viewing pleasure, but are you getting the most out of the new HDTV set?

In my opinion, it is good idea that you experiment with the viewing distance and setup. Most of the calculations related to viewing distance are done in "ideal world" environment and your living room or home theater is far from it.

So experiment with the ambient light setup in the room, sound system (if you have one) and finally the viewing distance between your TV set and your seat. Critical viewing distance data as shown above could be great help in deciding the starting point for your experiment and allowing you to experience the best viewing pleasure from your investment.