The aspect ratio of a screen image are its dimensions. Typically this is done by measuring the width times the height of the frame.
As an example. If a frame is 2:1, it would measure in width twice its own height.
Frame dimensions over the years have changed, but the 35mm ‘Academy’ format had been established for many years, especially its use within the TV industry and what has become known as the 4:3 ratio (older ratio term 1.33:1).
An anamorphic image is compacted horizontally by a special lens. When the image is projected onto a screen using a similar lens it is uncompressed and appears approximately twice as wide as the original frame size. Because the image was compressed / squeezed, making everything within the frame squashed, when it is unsqueezed, everything within the frame looks normal, but the image/frame area is wider.
(1) The compressed film frame image. (2) The uncompressed projected image.
1.85:1 almost contemporary TV widescreen format of 16:9. 1.85:1 is also referred to as the academy movie format. Widescreen TV and HD (High Definition) is actually 1.78:1 aspect ratio.
The following examples include some established formats in terms of film, but with digital movie making, many variations are being introduced.