Maxing Out Resolution
Getting the best picture resolution remains one of the chief goals of HDTV shoppers. But as I explained in last month's "Tech Talk," human visual acuity limits how much detail you can see in any image, live or onscreen. This month I'm laying it all on the line - or rather, the several trace lines in the accompanying graphs, which relate diagonal screen size for 16:9 widescreen TVs (in inches across the bottom) to seating distance (in feet on the vertical axis). The two graphs are the same except that the one with curved lines uses a logarithmic scale for the vertical axis (I'll explain the advantages of that below).
The traces indicate for various image formats what combinations of screen size and viewing distance will "saturate" your eyes with detail to the point where any more detail in the image would not be visible. They were calculated using only the horizontal pixel count of each format and assuming progressive display of still images. You won't get quite as much detail with real-world video programs and screens.
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If your combination of screen size and seating distance places you below any particular image-format trace, you're sitting too close. That is, a TV of that format and size can't provide all the detail your eye is capable of seeing at that distance, and the picture will look "softer" the closer you get. For example, watching a 60-inch TV at 11 feet puts you below the trace for 720p HDTV, so a high-def program on a 720p HDTV - or a 720p program viewed on a 1080i or 1080p HDTV - might look a little soft.
If your screen-size/distance point puts you above a particular trace, your eyes will be saturated with detail before you reach the resolution limit of an image in that format. Watching a 60-inch screen from 11 feet puts you well above the 1080i/p HDTV trace, meaning that a 1080i program can produce more detail than you can actually make out at that distance. You could even move closer, to around 8 feet, before your ability to see details in the image will max out. That is close to the recommendation of the Society of Motion Picture and Television Engineers (SMPTE) that the width of a screen should span at least 30° of your field of view (anything below the orange trace).
As might be expected, Lucasfilm THX's recommendation for the comparable angle for watching movies in theaters (light purple trace) is much more demanding, namely 36°. Neither a 1080i/p HDTV nor even a 2k Digital Cinema projection is capable of providing full visible resolution for a picture of that width. For a 36° image you'll need to leap to 4k Digital Cinema encoding. Such 4k pictures allow you to sit less than a screen width away, which is what often happens when you arrive late to the theater.
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This graph can be used to help set up your system or to shop for a TV. How you use it depends on what you are able to vary in your viewing room - the space allotted for a screen or the distance from the screen to the main viewing area. If you want to go for a full theaterlike presentation, select among 1080i/p screens and sit at just the right distance for your screen size as indicated on the green trace. Only a 1080 set will produce the minimum SMPTE picture width of 30° without running out of resolution.
If your room layout restricts either your viewing distance or the screen size, you actually have more choices. Say you're limited to a seating distance of around 10 feet and a screen width of 50 inches. In this case buying a 1080i/p set won't get you better resolution than a 50-inch 720p set (the 10-foot/50-inch point lies above the 1080i/p trace). You might be able to save some money by choosing a 720p model. Then again, all screen sizes seem to be switching over to 1080i/p pixel counts, and eventually 720p sets may be hard to find.
When comparing screen size/distance tradeoffs, it's easy to go overboard with the straight-line version of the graph, which can be misleading as to the improvements/degradations in resolution you'll get. Transformation of the vertical axis to logarithmic scaling, as in the curved-line version of the graph, will help prevent this. The logarithmic version contains the same information as the "linear" version, but scaled so that the vertical intervals are more perceptually meaningful. Equal vertical movement on the logarithmic version corresponds to equal changes in perceived or possible resolution. For example, descending along the same vertical line from the DVD trace (orange) to the HDV camcorder line (magenta) corresponds to a doubling of horizontal pixel count (from 720 to 1,440) and is the same distance as between the 2k (dark purple) and 4k (dark blue) Digital Cinema traces, which also involves a doubling of pixel count (from 2,048 to 4,096). From the logarithmic version, you can see that slight changes in viewing distance from the 1080i/p line correspond to larger changes in viewing distance from a 720p screen of the same size. The lower-rez screens are more forgiving of seating-distance variations.