The core principle behind anamorphic zoom’ horizontal compression lies in their special cylindrical optical structure (distinct from the spherical lenses of standard lenses). This structure enables asymmetrical refraction of light in two dimensions—*horizontal* and *vertical*—only compressing the horizontal light path while keeping the vertical path unchanged. Ultimately, this "squeezes" a wide-format image into the standard size of a camera sensor.
To understand this process clearly, we can break it down into three key layers: optical principles → core components → full imaging workflow, with analogies to simplify complex concepts.
1. Core Optical Principle: "Unidirectional Refraction" of Cylindrical Lenses
Standard spherical lenses have a spherical shape (similar to the surface of a basketball). Their refraction of light is symmetrical in both the horizontal (left-right) and vertical (up-down) dimensions—regardless of the direction light enters, the refraction angle remains the same. As a result, the aspect ratio of the captured image is not compressed (e.g., a 16:9 sensor captures a 16:9 image).
In contrast, the critical component of anamorphic zoom is the cylindrical lens—shaped like the "side of a water pipe": it has curvature in only one direction (vertical) and is flat in the other (horizontal). This structure dictates unidirectional refraction of light:
- Vertical light: The vertical curvature of the cylindrical lens refracts light normally (like a standard spherical lens), so the vertical proportion and details of the image remain uncompressed.
- Horizontal light: The flat horizontal side of the cylindrical lens barely refracts light—but the lens’ "curved edges" alter the horizontal light path, causing originally "dispersed horizontal light" to converge toward the center. This is equivalent to "squeezing" part of the horizontal image.
Simple analogy: Think of a cylindrical lens as a "horizontal compression spring"—it only squeezes space in the left-right direction, not affecting the height in the up-down direction.
2. Key Components: "Precise Collaboration" of Cylindrical Lens Groups
A single cylindrical lens cannot achieve uniform, distortion-free compression. anamorphic zoom use combinations of multiple cylindrical lenses (typically including "front compression lenses" and "rear correction lenses") to address this, with two core functions:
1. Controlling compression ratio: Cylindrical lenses with different curvatures achieve different horizontal compression ratios (the most common in film is 2.39:1, meaning the horizontal image is compressed to 1/2.39 of its original width). For example, an image originally with a 16:9 aspect ratio, after 2.39:1 compression, has a reduced horizontal width and ultimately appears as a "near 4:3" ratio on the sensor (while vertical height remains unchanged).
2. Correcting optical errors: A single cylindrical lens tends to cause "horizontal distortion" (e.g., horizontal stretching at the image edges) or "chromatic aberration" (color fringing at edges). The rear correction lenses adjust the light path in the opposite direction, ensuring the compressed image is uniform, sharp, and free of obvious distortion.
3. Full Imaging Workflow: From "Capture Compression" to "Playback Reconstruction"
Horizontal compression is not the "final effect" but an "intermediate step." The complete process requires two stages—*capture* and *post-production/playback*—to present the ultra-wide cinema format:
Stage 1: Capture Phase – Horizontally Compressing the Image
When light enters the anamorphic lens, the front cylindrical lens group first converges the "horizontal light" toward the center. For instance, an image that originally needs to "fill the horizontal width of a 16:9 sensor" is compressed to only require "horizontal space equivalent to a 4:3 format," while vertical height remains unchanged. Finally, the camera sensor records a "narrow image with compressed horizontal dimensions and normal vertical dimensions" (e.g., a 16:9 sensor captures an image with an aspect ratio close to 4:3, but vertical details remain intact).
Stage 2: Post-Production/Playback Phase – Horizontally Stretching to Reconstruct
After capture, the "compressed narrow image" is stretched horizontally using post-production software (e.g., DaVinci Resolve, Premiere Pro) or playback devices (e.g., film projectors). By reversing the original compression ratio (e.g., 2.39:1), the previously compressed horizontal image is restored to its normal width. The end result is an ultra-wide cinema aspect ratio (e.g., 2.39:1), with vertical height still unchanged.
Concrete Example
- Suppose the camera sensor has a 16:9 aspect ratio (16 units wide, 9 units tall), and an anamorphic lens with a 2.39:1 compression ratio is used.
- During capture: The horizontal image is compressed to 1/2.39 of its original width. The original 16-unit horizontal width is compressed to only ~6.69 units (16 ÷ 2.39 ≈ 6.69), while vertical height remains 9 units. Thus, the sensor records an image with an aspect ratio of approximately 6.69:9 (close to 4:3).
- During post-production/playback: The horizontal dimension is stretched by 2.39x. 6.69 × 2.39 ≈ 16 units, so the horizontal width is restored to the original 16 units (vertical height remains 9 units). The final aspect ratio becomes ~2.39:1 (16:9 ÷ 1/2.39 ≈ 2.39:1)—the standard ultra-wide format for films.
Key Summary
The horizontal compression of anamorphic zoom essentially relies on the unidirectional convergence of horizontal light by cylindrical lenses. Through an asymmetrical optical design that "only compresses left-right, not up-down," they "pack" an ultra-wide image into a standard camera sensor without losing vertical details. Later, reverse stretching during post-production or playback reconstructs the cinematic ultra-wide format—this is the core technical logic that distinguishes anamorphic zoom from standard spherical lenses.