‘Anamorphic’ is a process by which image quality can be optimised, although how that precisely has worked is different now to what it was 60 years ago, says Stephen Dawson.

Most movies come in one of two different aspect ratios: roughly 1.85:1 or 2.40:1.

What do you see at the movies when one of the latter starts? Why, the curtains at the sides of the screen open a little wider of course! But what do you see at home? You see black bars at the top and bottom of the screen, while the width of the screen remains unchanged.

In the great majority of systems that is. But by using an anamorphic lens and a compatible Blu-ray player, you can enjoy that cinema-style widescreen picture. Such projection systems are called ‘constant image height’.

A Touch of History
Anamorphic lenses first started to be used in a significant way in cinema in the early 1950s. One of the responses to the threat of television was the introduction of widescreen formats to replace the previous ‘Academy ratio’ of 1.37:1. Various shapes, from 1.66:1 up to 2.7:1 began appearing.

But, how to fit these shapes on film? Just shrinking the vertical height of the picture reduced the resolution because the area of the film frames was smaller. Various heroic new techniques were tried, including new 65mm film stock. But a common dodge was an anamorphic lens.

The special lens was attached to the film camera. This would stretch the picture vertically so it use the entire height of the 1.37:1 frame on 35mm film. At the cinema the process would be reversed with another anamorphic lens. This stretched the picture sideways, restoring it to its correct, widescreen, proportions. This had another advantage: the same old cameras could be used. Only the lens had to change.

Keep that in mind because that’s similar to the process we’re talking about.

And then there was the DVD
The term anamorphic is more widely known thanks to the DVD.

Right from the start the DVD was designed to operate in an anamorphic mode. The frame on a typical Australian DVD is 720 pixels wide by 576 pixels tall. If it is carrying video intended to be shown in the ‘full screen’ aspect ratio of 1.33:1 (very slightly narrower than the old Academy ratio) then the picture still had to be stretched sideways slightly, because at 1.33:1 a 576 pixel tall picture implies a 768 pixel wide picture.

Some widescreen DVDs used a ‘letterbox’ format, in which the regular frame was used with black bars at the top and bottom, and the DVD player would apply that same small horizontal stretch. But that wasted so much resolution.

If the movie had an aspect ratio of 2.40:1, as quite a few do, then it would only be using 320 of the 576 pixels of vertical resolution available (the figures were even worse for American DVDs). Even 1.85:1 movies would use only 415 pixels vertically. There was not really enough vertical resolution in DVD to waste any like that.

Anamorphic DVDs, though, were designed to be widened from the 720 pixel wide frame to 1,024 pixels rather than 768 pixels. That ratio, 1,024:576, gives us the 16:9 aspect ratio of widescreen TVs (roughly 1.78:1). With that system a 1.85:1 movie got 554 pixels rather than 415, and a 2.4:1 movie scored 427 rather than 320. That 30% increase in the total number of pixels delivered noticeably more detail and sharpness in the resulting picture.

Blu-ray enters the picture
DVDs were introduced at a time when just about all TVs were still the old squarish sort with 1.33:1 aspect ratios. By the time Blu-ray appeared nearly all TVs, and certainly all high resolution models, had a 16:9 widescreen aspect ratio. Also Blu-ray had resolution to burn – or so they thought – so no provision was made for anamorphic movies.

Rather than the oblong pixels of DVDs, Blu-ray pixels are square. No stretching of the picture was required. With a 1.85:1 movie, the display height within the Blu-ray frame was an impressive 1038 pixels. But with a 2.40:1 movie, that fell to 800 pixels.

For some enthusiasts it seemed a real pity that the rest of the 1,080 vertical pixels of resolution weren’t being used. And so anamorphic came back.

Modern Home Entertainment Anamorphic
The use of anamorphic lenses allows two attractive effects. One I’ve already alluded to: a widescreen picture gets wider rather than shorter. Movie makers generally use 2.40:1 or greater aspect ratios to emphasis the broad sweep of their subjects. Somehow reducing the height of the picture, rather than making it wider, seems stingy and contrary to the filmmakers’ intentions.

The other result is more theoretical: you get to use more pixels in your projector – for this really only works with projectors – for displaying the image. That may give the impression of providing higher resolution. But we’ll discuss that in a while.

There are two vital requirements for such an anamorphic system: a Blu-ray player which supports anamorphic playback and an anamorphic lens.

The function of the Blu-ray player in this system is to stretch the picture vertically by 33.3%. That 2.40:1 movie will now, instead of having to make do with 800 vertical pixels, get 1,067. Only 13 out of 1,080 lines of pixels are wasted.

The tops and bottoms of the original Blu-ray frame are pushed completely off the frame, but that doesn’t matter with such a movie anyway because their content is just the black bars.

This of course distorts the picture so that everything within seems unnaturally tall and skinny. But an anamorphic lens placed in front of the projector’s lens will stretch the picture sideways, by the same percentage, to restore the proper proportions.

This prepares your system for a home cinema experience that mimics the one in the real cinema. Ideally the system will be installed so that selecting vertical stretch will slide the anamorphic lens smoothly into place, and open up the screen curtains to accommodate the wider picture.

Picture Quality
The natural assumption is that by increasing the number of pixels you improve picture quality. That was the logic behind anamorphic film (with grains of pigment rather than pixels, of course) and anamorphic DVDs. However all the vertical stretch process is doing in a constant image height projection system is mapping a fixed number of pixels in each frame onto a new, greater set of pixels. At best this will lead to no picture degradation, but it can in fact yield some slight loss of sharpness.

This is illustrated in the grid pattern printed on these pages (see Figure 1).

Each vertical line represents one pixel width. I’ve stretched the thing vertically by 33.3% and you can see how the horizontal lines have been smudged by comparison.

This is unlikely to be visible with most movies, most of the time. The main thing to remember is that this anamorphic process is mostly about the experience rather than higher picture quality.

But if the Blu-ray player is capable of 4K output and implements quality scaling, and you are using a 4K projector, then the results will be quite different. With double the resolution, that 1080 pixel tall test image can be stretched vertically with just about no loss of quality (see Figure 2).

Of course, had an anamorphic option been designed into Blu-ray in the first place so that 2.40:1 discs were mastered with the vertical stretch, then real resolution improvements would have been possible.

The end
Pursue constant image height home theatre for what it is: an attractive replication of the theatre experience, and a more faithful rendition of the movie makers’ sense of the picture. Just don’t take it for an actual technical improvement.