Iowa Public Television

Friday, December 7, 2007

How Big A Screen Do I Need?

Some of the first questions I get asked regarding the digital television conversion come from people who are considering the purchase of new DTV televisions. They generally want to know whether they should buy plasma or LCD. I tell people before they even go to a store to look at the television they should first layout the room where the TV is going to be. Figure out where the TV will sit or hang and where the primary viewing location will be. Then use a tape measure to determine the distance from the front of the screen to the where the eyes of the viewer will be and don’t worry too much about being exact, if you’re within a foot or so of the actual measurement you’ll be fine.

So how do you begin to plan your purchase? Using the SMPTE EG-18-1994 which was originally used for movie theater design and has now transitioned to home theater design you want to try and maintain a minimum viewing angle of 30°. This means that if you are seated in the optimum viewing location and look directly at the center of the screen, by rotating your head (or moving your eyes) 15° to the left or right you would be looking at left or right edge of the screen. There is a lot science research that has gone into this, much of it based on visual acuity and the portion of the human eye called the fovea. This area of the retina has the greatest density of photoreceptors in the eye and is responsible for the amount of detail that we see. It is the area that allows us to read, drive and yes, watch television. So by locating the viewing position at a location that meets this minimum viewing angle you essentially fill the fovea region of the eye and have the perfect viewing experience.

Unfortunately, there is also some reality that enters into the calculations. If you adhere strictly to the calculation you realize that in any environment there is only one good seat and this certainly didn’t make sense in the design of movie theaters and doesn’t make any more sense when planning at home, unless you live alone and will never have friends over to watch television with you. Let say that you make the viewing distance measurement and discover that you’re planning on sitting 13 feet back from your television screen. Doing the math, you should have a screen with about a 96 inch diagonal measurement!!! Now this is not impossible, there are front screen projector systems that can easily do this and there are even 100 inch plus plasma and LCD displays available but that is a seven foot wide, four foot tall television, it will weigh in at 500 pounds or more and will set you back somewhere in the neighborhood of $70,000.00. So clearly there has to be an acceptable compromise.

What has become the generally accepted practice is to specify viewing distance in terms of screen height or width. I have heard people expound that you should measure the height of the screen and sit three times that distance back for the best experience which from the example above would indicate that you either need to move your chair about a foot closer to the screen or try to find a bigger display which I personally believe approaches insanity. If you want to reference the screen, you are better off going with somewhere between three and six times the picture width. In my example with the 13 foot distance I can pretty quickly figure out that I want a screen that is at least 37” diagonal. That puts me pretty close to the furthest distance I would want to be from the set so if I can afford to go larger I will.

I have attached a grid with some of the most common screen sizes and their corresponding optimum and maximum viewing distances. The grid includes both standard (4x3) televisions and wide screen (16x9) televisions to show how the shape of the screen also impacts the viewing distance. Once you figure out how large a screen you want, then you can start looking at what technology you want to buy and find the best match between acceptable screen size and the cost of the set.