Equivalent Focal Length Myth

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Author: Khai Nguyen
Date: May 2019
From: PSA Journal(Vol. 85, Issue 5)
Publisher: Photographic Society of America, Inc.
Document Type: Article
Length: 1,311 words
Lexile Measure: 1260L

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Many photographers misunderstand that the focal length of a lens mounted on a crop camera would effectively be extended by a crop factor. For this reason, they could reach subjects at farther distance and make images larger. This article will disprove this myth once for all.

Background Review

Before getting into details, we need to go over some basic facts about camera and lens. There are two kinds of cameras: full frame and crop cameras. Full frame cameras are those with image sensors equivalent to 35mm film. Their sensor size is 36mm * 24mm and its diagonal is 43.27mm. Crop cameras are those with sensors smaller than 36mm * 24mm. The most popular crop camera has a sensor size of 24mm * 16mm and its diagonal is 28.44mm. The crop factor is the ratio of full frame sensor diagonal to crop sensor diagonal. The crop factor in this case is 1.52.

Nikon often refers to its full frame cameras as "FX" and its crop cameras with 24mm * 16mm sensor as "DX." Canon[R] and other manufacturers refer their full frame and crop cameras as "35mm" and "APS-C" (i.e. Advanced Photo System Type C), respectively.

Equivalent Focal Length

Each lens has a true focal length and angle of view regardless of the camera and its sensor. When a lens is used with a camera, the view it can see may change. For example, a Nikon[R] 24mm F/1.4G lens has an angle of view of 84[degrees] on a full frame FX and 61[degrees] on a DX. In order to have the same angle of view of 84[degrees], a wider lens of 16mm is needed on a DX. Therefore, some manufacturers have often claimed that a 16mm lens has an angle of view equivalent to a 24mm lens on FX. Similarly, a 400mm lens on a DX/ APS-C camera has an angle of view equivalent to 600mm lens on a FX/35mm camera.

The manufacturers' definitional claim simply means that two...

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Gale Document Number: GALE|A633523483