Angles Of View
Everybody knows that projection screens come in a huge variety of sizes. We in the business understand, however, that screens don't come in an equally large number of shapes. That's because there are only a limited number of projection formats each of which is in part defined by a numerical relationship between the height and the width of the images it makes. And that relationship has nothing to do with size. The height of our television screen, for example, has always been ž of its width regardless of how big a set we own. And the short edge of a 35mm slide will always be of the long one whether we measure the slide itself or the biggest image we can imagine projecting from it. As we shall see, the variance of these proportions from one format to another can noticeably affect our appreciation of the projected imagery. Because this strict correlation between height and width can always be fully expressed by a single pair of numbers, the fraction appropriate to each format is called its
Aspect RatioIt used to be that the only sensible way to size a projection screen was to give its measurements, its height and its width. Nowadays people are much more inclined to specify a screen by giving just a single number - its diagonal.
Since any rectangle can be defined as two identical right triangles sharing a common hypotenuse (the diagonal "c" in Figure 1), we could theoretically always figure out an actual screen size from just its diagonal as long as we know the aspect ratio. Harkening back to high school trigonometry, we recall that the square of the hypotenuse of any right triangle is equal to the sum of the squares of each of its sides: a2 + b2 = c2. That's the Pythagorean theorem, remember?
Armed with that venerable formula, let's have a quick look at some typical aspect ratios. If it's a screen for slides, we know their aspect ratio is 2:3. Since those numbers seem friendly enough, what would the diagonal be? Well, the square of the diagonal is going to equal the square of one side plus the square of the other, hence 4 + 9 = 13. The diagonal then is going to be the square root of 13. But the square root of 13 is neither a warm nor a friendly number and no one should blame us for deciding not to use it when we talk about slide screens.
How about HDTV? That's the famous 9:16 aspect ratio (of which a good bit more will be said). What about its diagonal? 81 (92) + 256 (162) = 337(?2). The √ 337 exists, of course. To five decimal places it equals 18.35756 which isn't very helpful, either. So we'd better leave HDTV's diagonal alone, too.
Ignoring these disappointments, the world remains full of 67, 84, and 120-inch diagonal screens. How come? Because the aspect ratio for video and TV screens just happens to be 3:4. By a quite remarkable numeric coincidence, the diagonal produced from those numbers turns out to form a perfect integer relationship with them: 3 - 4 - 5. And 5 (with no decimal places, mind you) is a usable number.
For instance, how big is a 100-inch diagonal video screen? Dividing 100 by 5 we get 20. Multiply that by 3 and we get the height, 60". Multiply the same 20 by 4 and we get the width, 80".
Aside from its striking numerical convenience, were there other reasons behind the establishing of 3:4 as the aspect ratio for all original video images? As a matter of fact, there were.
When television was being developed at the beginning of the 1940s, the principal aspect ratio of the motion picture industry was 1.33:1. Where did that ratio come from? It's actually still our familiar 3:4 only in a Hollywood disguise. Film people, you see, like to state aspect ratios as the number by which you need to multiply the image height to get the image width. Hence 4 = 1.33 x 3.
The real origin of the 3:4 aspect ratio had to do with the size of the negative in 35mm movie film after you subtracted for the perforations needed to pull it through a projector.
If television, then, began its life with tubes of the same aspect ratio, movies could be broadcast without any significant reduction in the frame size.
It is also true that when the developers of commercial television decided that its bandwidth couldn't afford to be more than 6 MHz and that its vertical resolution had to be not less than 525 lines, something very close to a 1.33 maximum screen width popped out of the calculations as a mandated default.
Notice that no part of the 3:4 genesis had anything to do with how pleasing images in this aspect ratio actually are visually. And in fact there isn't anything intrinsically appealing about 3:4 pictures. This is why the movie industry, which at first regarded television as a major threat to its revenues, was quick to develop a whole series of wide, panoramic screen shapes which included
CineramaŽ (2.76:1), CinemaScopeŽ (2.35:1), 70mm (2.05:1), and the currently familiar PanavisionŽ (1.85:1) - the prevalent "letterbox" ratio.
Any of these widescreen formats is a better approximation of the human visual field, than the boxy, nearly square shape of a TV screen. After all, our two eyes are set side-by-side and their field-of-view therefore has an aspect ratio a good bit wider than 3:4. Yet TV screens were everywhere and when video projectors appeared on the scene, to what aspect ratio were they obliged to conform? You guessed it, 3:4 again.
Are we doomed to watching video pictures shaped like 50-year old television screens forever? We can hope not. There is, thank goodness, the shape of things to come. Its name is High Definition Television and compared to the video pictures we're used to, HDTV's specifications are certainly impressive.
US television nominally has 525 lines of resolution (the overseas PAL system supports 625). To avoid seeing these raster lines we're supposed to sit 7 screen heights back from an NTSC display. That suggests the proper viewing distance for a 27" diagonal is about 9˝" feet. Also from the "7 screen heights" number we can determine that the image we're watching will occupy only 10o in our horizontal field-of-view.
Now let's look at the HDTV picture (Figure 2). First of all it's aspect ratio has gotten much wider. 3:4 has jumped to 9:16 (or, in the film nomenclature 1.33:1 has become 1.78:1). In addition it has twice as many lines vertically (1050). This statistic is a little subtle because the overall resolution of HDTV is not two times better than NTSC, its more than five times better. Video resolution is established by the total available pixels inside a display. That number is calculated by multiplying the vertical lines times the horizontals. Hence there are just over 350,000 pixels on your screen today; there will be almost 2,000,000 on an HDTV screen.
At that resolution in that aspect ratio how far back should we sit? The answer is 3 screen heights. And at a viewing distance of 3 screen heights the screen fills fully 30o of our horizontal field-of-view.
These numbers are extremely significant because the designers of HDTV appreciated that
a wider aspect ratio coupled with a vastly improved picture would provide the viewer far more involvement with the program. It was determined by exhaustive research and testing that a 30 degree field of vision would not only excite the central portion of the human visual system, but the peripheral vision as well. That gives a very heightened experience of reality to the viewer....1
Other, independently conducted research showed that "the human visual system is clearly divided by two functions - the ability to see detail better in the central area and the ability to see motion in the peripheral." 2 Thus if video was ever going to match the impact of the movies it needed, quite literally, to change its image. Anyone who has seen an HDTV, 9:16 display recognizes instantly its enormous visual superiority over the old 3:4 aspect ratio.
Even though real HDTV isn't generally available yet, advances in projector technology now permit owners of multi-scan projectors to broaden the aspect ratio of the image they're watching at the touch of a button. To enhance this convenience Da-Lite has developed an electric, twin aspect ratio screen series called the Dual Masking Electrol which enables the user to have a screen sized exactly for either letterbox (1.85:1) or HDTV (1.78:1) projection in one configuration and a viewing surface sized precisely for conventional, 3:4 video in the other.
To convert from whichever widescreen format to the standard TV aspect ratio, the Dual Masking Electrol drops a vertical black masking strip down each of its sides which then recedes tautly back against the underlying projection surface. The careful engineering necessary to bring the masking flush back against the face of the screen ensures that no shadowing will be present to distract the viewer.
Effectively, then, the Dual Masking screen works by reducing the screen's visible width. A 9:16 is converted to a 9:12 (3:4) when each descending black masking strip is 2 units wide.
Whether we identify them by their diagonals or by the lengths of their sides, whether for front projection or rear, at Da-Lite the correct aspect ratio for any format is always differentiated.
Whether most of our screens will ever be formatted for HDTV is a question only the networks and the set manufacturers can answer. Because the costs attendant to its installation are so enormous and because the international competition for its configuration remains ferocious, it is difficult to guess how long we may have to wait for this potentially splendid advance in the overall quality of our visual displays.
1 Cripps, Dale, Widescreen Television - The HDTV STORY, Wide-screen Review, July/August 1993, Page 17.
2 ibid., Page 20. The author is indebted to Mr. Cripps and to the editors of Wide-screen Review for their exceptionally informative coverage of this and related issues.