Anamorphic, Aspect Ratio, Resolution
Dear Gary: I am a new subscriber to Widescreen Review. I am hoping to get an education to enable me to make an informed selection of HDTV. I read Video Technical Editor Greg Rogers’ piece in Issue 58 on anamorphic DVD. 1. I’m not sure I yet understand the difference between “formatted for 16:9” and “pan-and-scan 4:3.” Mr. Rogers implies that there is no “squeezing” in the anamorphic format. But as I understand, some of the widescreen photo processes actually “squeeze” a 1.8:1 frame into 1.25:1 on the film. So the resulting picture has a bigger aspect ratio but lower horizontal resolution (or is it vertical resolution?). If a widescreen image is compressed with the same resolution as a 4:3, then it would take, on average, more bits for the widescreen version. Is this the case, or is the horizontal resolution actually less on the widescreen version than on the pan-and-scan? 2. This raises a more general question. There is much discussion of aspect ratio and lines of resolution, but one thing I have never seen discussed is angle of view. If I shoot a picture with a 50mm lens, then the same scene with a 28mm lens, I get more “picture” in the wide-angle view, but with the same total resolution and the same aspect ratio. If I watch a movie in the theatre, the screen nearly fills my range of peripheral vision. But if I watch the same movie on a smaller widescreen TV, the TV image may fill only a small portion of my angle of view, so I don’t get the same sense of being “in the picture.” So it seems there is more to producing the movie theatre effect than aspect ratio and lines of resolution. What if a movie is shot in a process that gives more vertical angle of view, such as Cinerama? Or Imax? Wouldn’t it be better to state measurements in terms of degrees of view and pixels per degree than lines and aspect ratios?
Joseph H Austin, Jr.
Video Technical Editor Greg Rogers Comments:
You’ve raised several good issues so I’ll try to address each of them. First, the anamorphic film process refers to using a special camera lens to compress the width of an image on film (usually by a factor of 2:1) in order to capture a wider aspect ratio (usually 2.35:1 or wider). An anamorphic lens with an inverse characteristic is used during projection to expand the image back to the intended aspect ratio. While this process is important to film, it has virtually nothing to do with DVDs or digital video. In fact, anamorphic films are too often mastered in the 4:3 letterbox (“non-anamorphic”) DVD format (a terrible practice because of the limited vertical resolution of that DVD format), and non-anamorphic (1.85:1) films are usually mastered in the 16:9 (so-called “anamorphic”) DVD format. So using this same term for a photographic process and a DVD format can be just plain confusing. Resolution is another term that can be confusing, particularly when applied to images mastered in different formats. There are two basic DVD frame formats—16:9 and 4:3. As I said in more detail in Issue 58, the only difference between the formats is the shape (aspect ratio) of the picture frame being represented by the MPEG data. In both cases, the picture being encoded is a 720 x 480 pixel array (in NTSC countries), so the amount of MPEG data is the same and the pixel resolution per frame is the same. But is the image resolution the same for the 16:9 format and the 4:3 format? If image resolution refers to the ability to resolve details in the image content, then the answer is no. I believe your questions refer to resolution in that context. A 4:3 pan-and-scan picture is a cropped section of a wider aspect ratio (let’s assume 16:9) picture. Since both the 4:3 and 16:9 picture frames have the same pixel resolution, the 4:3 pan-and-scan picture will have more image resolution, at least horizontally. It may also have more vertical image resolution if the pan-and-scan picture doesn’t include the full height of the 16:9 image. The image width in 4:3 letterboxed pictures is the same as in 16:9 pictures, so the horizontal image resolution is the same. But the image height in the 4:3 letterbox format has only 75 percent of the scan lines of the 16:9 format. So a 16:9 picture has 33 percent more vertical image resolution than the same picture in the 4:3 letterbox format. I agree that field of view is an important factor in establishing the “movie theatre” experience at home. I’ve written on this subject and the Critical Viewing Distance in the past. There is no magic distance to the screen that is right for every viewer, just as not everyone (fortunately) wants to sit in the same row at the cinema. But studies, and my experience, suggest that a 25 to 30 degree viewing angle is preferred in a home theatre. For a 16:9 display, the viewing distance for a 25-degree viewing angle is 4.0 picture heights (PH). The distance for a 30-degree viewing angle is 3.3 PH. That’s 2.26 and 1.87 picture widths respectively. An easier number to remember is two-times the picture width, which produces a viewing angle of 28 degrees. The Critical Viewing Distance (CVD) is the maximum distance at which the smallest picture details can be discerned. This can be computed by knowing that the limit of angular discrimination for normal vision is approximately 1/60 degree. Using that assumption: CVD (in PH) = 3438 / (# of pixels per PH) Therefore, for 1080i HDTV, the Critical Viewing Distance is 3.2 picture heights. Notice that this is just about the same viewing distance that provides a 30-degree field of view. Of course there are very few displays that can resolve the full resolution of 1080i, so sitting a little farther back may not affect your ability to discern the full resolution of the display. Notice that the CVD for a standard-definition picture (480 scan lines or pixels per picture height) is 7.2 picture heights. Although at that distance we can see all the detail the picture has to offer, the field of view is only 10.6 degrees. As you point out, that is nothing like a movie theatre experience. If we move closer to screen to increase the field of view, we begin to see the scan line structure. This is why we need scalers in front-projection [and rear-projection] home theatres where we naturally want to achieve a wider field of view. It is also an argument for 1080i instead of 720p for HDTV. The CVD for 720p is 4.8 PH, which provides a 21-degree viewing angle. When we move closer (3.3 - 4.0 PH) to get a wider field of view, we may see scan lines. Finally, the reason we don’t express the resolution of displays and video sources in terms of “degrees of view and pixels per degree” is because measures of that type depend on the user’s viewing distance and screen size. Obviously those factors vary from user to user. So like the rest of the industry, we specify resolution for the display or source directly, and users can compute other measures specific to their system.
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