Anamorphic Video sounds complex, but its fairly simple to understand, once you grasp a few key ideas.
The anamorphic format is all about size. The Anamorphic problem is essentially about fitting a cinema sized image (which is wide and narrow. An oblong shape) on to a domestic sized TV screen (which is square and boxy) while maintaining the aspect ratio of the original image. Along with the cutting and cropping that ensues in order to achieve this.
Aspect ratio is the relationship between image width and image height
A little history first. Anamorphic was first introduced in 1955 by 20th Century Fox in an effort to counter the loss of cinema audiences to the new medium of television. People were staying home and watching TV instead of going to movie theatres. To spice things up and entice audiences back to movie theatres, 20th Century Fox introduced something novel CinemaScope, a wide screen format which replaced the existing format of 1.34 to 1.
CinemaScope produced an image that was much wider. Nearly twice as wide as the old studio format. CinemaScope had an aspect ratio of 2.55 to 1 compared to the old standard studio format of 1.34 to 1.
In order to capture a very wide image, when shooting a movie a special anamorphic lens was used. The anamorphic lens distorted, captured and squeezed a very wide image onto standard 35mm film. Using regular 35mm film meant they could keep costs down. No special film needed. The anamorphic process allowed far more data to be caught on 35mm film stock. To project the image in movie theatres, a reverse anamorphic lens was used to correct the distortion and create the wide screen effect.
Here's a dictionary entry for anamorphic:
1. Optics . having or producing unequal magnifications along two axes perpendicular to each other.
2. of, pertaining to, or created by anamorphosis or anamorphism.
The dictionary agrees that the image is unequal along its axis, width v height.
ie its wider than it is high.
An image captured with an Anamorphic lens, its image is almost 2.5 times wider than it is high. That's fine when the image is projected in a movie theatre where the screen's are huge and the dimensions are right for anamorphic film, but when you need to display the same film on a TV screen, which does not have the same correct dimensions, you have a problem.
Anamorphic on a 4x3 TV
Lets look at an anamorphic image on an old CRT TV set with 4x3 ratio. Some people still use these old TV sets. I'll use simple measurements so its easily grasped.
The old standard TV screen had a ratio of 4x3. That's 4 across and 3 down. If the TV was 40 inches across, its vertical edge would be 30 inches high. (See image). That's a 4x3 ratio.
4x3 Standard TV Dimensions
Anamorphic cinema screens have a ratio of 2.39 to 1. If we have a small cinema screen made for anamorphic movies and it was 40 inches wide, its height on the vertical edge would be 17 inches. That's the correct ratio of 2.39 to 1.
Punch it into your calculator and see. Multiply 17 by 2.39. The answer will be just under 40. Thats our correct ratio for watching anamorphic video.
Anamorphic TV Dimensions
As you can see the width for the two screens is the same at 40 inches, but the vertical height differs greatly. The anamorphic is about half the height of the old 4x3 standard TV screen.
Here's an anamorphic image displayed on a 4x3 TV with a standard 4x3 ratio. If your TV screen is 40 inches across with a vertical edge, 30 inches high. It will display an image like this.
4x3 Standard TV Dimensions
The image is vertically stretched.
Here's the same image displayed on an anamorphic screen with a ratio of 2.35:1.
Anamorphic TV Dimensions
The image appears correct, more natural. The 4x3 image appears squished.
The anamorphic image has a 'width' of 40 inches, and a 'height' of 17 inches.
Cinematic projectors have lenses allowing them to produce a wide image to fill the cinema screen. The anamorphic original movie's ratio 2.351 means it is 235 across and 100 down. Its dimensions are very different from those of a standard 4x3 TV.
When the image is transferred to DVD or for TV broadcast, the image has to be squeezed from its original large wide frame format into a much smaller square frame so it fits the TV screen.
If we place the anamorphic image on a standard 4x3 TV, it won't fit. Its just too big. If we make the anamorphic image 30 inches high to fill the vertical edge, it will have a horizontal width of 70.5 inches. (Do the math, 30 x 2.35 = 70.5). Its too wide for the standard 4x3 TV by 30.5 inches.
If we keep the anamorphic horizontal width to 40 inches, the 2.35 ratio will only create an image that's 17 inches high. The image will not fill the TV screen completely. Its too short, way too short. At 17 inches its just over half the height. To get around this problem, engineers place the image centrally on a 30 inch high TV screen, and leave a gap of 13 inches. 6.5 inches at the top and 6.5 inches at the bottom. You get what is known as a letterbox effect, with black bars top and bottom to fill the gap.
Anamorphic Image displayed on 4x3 TV Letterbox Effect
Cut Chop Dice
To fit an anamorphic image on a standard TV it will be 40" wide but only 17" high.
When the anamorphic image is displayed at 40" width, it has a height of 17". This leaves a gap at the top or bottom of the TV screen. The gap is normally filled with black bars.
If we want to display the full height of the anamorphic image on the TV, namely at 30" high, the anamophic image is too wide for the square TV and spills over on both sides.
If we set the width of the 16x9 to 30" high, its width is 50". That's 10" wider than our standard TV.
If we set the width of the anamorphic image to 30" (the height of the TV) the anamorphic image is 66" wide. Thats 26" wider than our standard TV.
Hopefully you now see the problem and understand the solution TV / Video engineers have come up with.
The solutions that have been worked in the past normally include reducing the height of the image so the width fits. This means you get black bars across the top and bottom of your TV screen.
Now lets look at the anamorphic image displayed on one the new generation widescreen TVs.
Anamorphic on a 16x9 TV
Digital HD TV has an aspect ration of 16x9 (1.78:1)
The latest/new generation widescreen TVs have a ratio of 16x9. That's 16 across and 9 down.
To fit a 16x9 TV image into a standard 4x3 40" TV, the image when centered on screen will be 40 " wide and 22.5" high. (40 inches x 1.78 = 22.5 inches).
Here the same anamorphic image displayed on 40 inch widescreen TV 16:9 (ratio 1.78 to 1), with no vertical stretch. But we have the letterbox effect. The image appears more natural with the correct dimensions. The face does not appear distorted.
16x9 Widescreen TV - No Vertical Stretch
As the image is only 22.5 inches high it creates a letterbox effect once more. We end up with a total gap of 7.5 inches. 3.25 inches top and 3.25 inches bottom. Its not as bad as the old 4x3 TVs.
Hopefully you now understand of anamorphic video and why you get the letterbox effect.