What Is Equivalent Focal Length?-Knowledge Encyclopedia-Shenzhen Kai Mo Rui Electronic Technology Co. LTD

What Is Equivalent Focal Length?

Source：Shenzhen Kai Mo Rui Electronic Technology Co. LTD2026-07-04

With technological advances, digital cameras equipped with digital image sensors have largely replaced traditional 35mm film cameras. This makes field of view (FOV) a critical optical parameter to consider when selecting camera lenses. Field of view is closely correlated with focal length and image sensor size. However, digital image sensors do not follow a unified dimension standard like 35mm film, resulting in inconsistent correspondence between focal length and field of view. To redefine the relationship between focal length and field of view, the photographic term "equivalent focal length" was introduced.

The original definition of equivalent focal length refers to 35mm equivalent focal length, meaning the focal length that delivers the identical field of view on a 35mm film camera.Mainstream optical sensor formats available today, sorted from largest to smallest, include Full Frame (FF, 36mm × 24mm), APS-H (28.1mm × 18.7mm), APS-C (23.5mm × 15.6mm), and M4/3 (17mm × 13mm).

Here is an example to illustrate the application of equivalent focal length across these formats. A 50mm lens mounted on a camera with a Canon APS-C sensor delivers a field of view of approximately 30°, while the same lens on a Full Frame camera yields a wider FOV of around 45°. To achieve a 30° field of view on a Full Frame camera, an 80mm lens is required. In other words, a 50mm lens used on an APS-C camera equals an 80mm equivalent focal length on a Full Frame camera.

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Why Do We Need Equivalent Focal Length?

Due to varying sensor sizes, surveillance cameras produce images of different dimensions to fully meet diverse monitoring requirements. Take a 3mm focal length as an example: even lenses with identical focal lengths will generate different fields of view and magnification ratios when paired with sensors of different sizes.

How to Calculate Equivalent Focal Length

Full Frame: Crop factor = 1.0 (no conversion needed)
APS-C (Nikon / Sony / Pentax / Fujifilm): Crop factor = 1.5
Canon EOS APS-C DSLR: Crop factor = 1.6
M4/3 format: Crop factor = 2.0
1-inch format: Crop factor = 2.7

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补充：基本术语解释

Supplementary Glossary of Basic Terms

Sensor Size

Literally, sensor size refers to the dimension of the effective imaging area of the sensor, stated in inches. Confusingly, this inch rating does not represent the sensor’s actual physical dimensions.

For instance, a 1-inch sensor has an effective imaging area measuring 12.8 mm wide, 9.6 mm tall, with a diagonal of 16 mm. When marking lens and sensor specifications, the "sensor size" designation primarily indicates hardware compatibility rather than real physical measurements.

Focal Length

When a camera captures an object, light reflected from the subject is collected by the lens and focused onto the focal plane — the position where the image sensor sits. When the shooting subject is at an infinite distance, the distance between the lens reference point and the focal plane is defined as focal length.

Typical optical lenses consist of multiple lens elements or groups with complex structures. For simplified explanation, we take a single convex lens as an example: its reference point is the lens center, and focal length is the distance from the lens center to the focal plane.

Field of View (FOV)

Field of view describes the angular range of a scene that can be captured by the camera’s image sensor. It is divided into diagonal FOV (ӨD), horizontal FOV (ӨH) and vertical FOV (ӨV). The theoretical FOV can be calculated easily based on lens focal length and sensor size.

Distortion

Distortion is an indicator for measuring optical aberration generated by camera lenses. Classified by positive or negative calculated values, it is split into pincushion distortion (positive value) and barrel distortion (negative value).

Within an optical system, distortion creates discrepancies between actual measured FOV and theoretical calculated FOV: barrel distortion widens the real FOV versus the theoretical value, while pincushion distortion narrows it.

Calculation methods for distortion vary depending on the application of optical components. For cameras, the two most common standards are SMIA-Distortion and TV-Distortion, where the SMIA-Distortion reading is twice the value of TV-Distortion.

Zoom Magnification

Zoom magnification is a key specification for zoom lenses. It is defined as the magnification ratio of the subject’s image when shooting at the telephoto end versus the wide-angle end of the zoom range, specifically the ratio of image heights.

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