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Digital SLR Camera Basics

Focal Length and Angle of view

AF-S DX NIKKOR 18-55mmF3.5-5.6G VR
An AF-S DX NIKKOR 18–55 mm
f/3.5–5.6G VR lens

The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters (e.g., 28 mm, 50 mm, or 100 mm). In the case of zoom lenses, both the minimum and maximum focal lengths are stated, for example 18–55 mm.

The angle of view is the visible extent of the scene captured by the image sensor, stated as an angle. Wide angle of views capture greater areas, small angles smaller areas. Changing the focal length changes the angle of view. The shorter the focal length (e.g. 18 mm), the wider the angle of view and the greater the area captured. The longer the focal length (e.g. 55 mm), the smaller the angle and the larger the subject appears to be.

Lens with a short focal length (wide angle of view)

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  • The illustration is an artist's conception.

Lens with a long focal length (small picture angle)

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Lenses with a wide picture angle are referred to as wide-angle lenses, lenses with a small picture angle as telephoto lenses.

Lens
Picture angle
Focal length
Area captured
Apparent size
Wide angle
Wide
Short
Large
Small
Telephoto
Small
Long
Small
Large

A zoom lens with a focal length of 18–55 mm offers the widest picture angle at 18 mm and the smallest picture angle at 55 mm. If the only lens available is an 18–55 mm zoom lens but you want wider angles—say, for landscape shots—you should buy a lens that offers focal lengths shorter than 18 mm, for example, a 10–24 mm lens. If, on the other hand, you want to photograph at greater distances, you should choose a lens that offers focal lengths longer than 55 mm, for example 55–200 mm.

Picture angle

Focal length: 12 mm
Focal length: 12 mm
Focal length: 24 mm
Focal length: 24 mm
Focal length: 50 mm
Focal length: 50 mm
Focal length: 70 mm
Focal length: 70 mm
Focal length: 135 mm
Focal length: 135 mm
Focal length: 200 mm
Focal length: 200 mm

The shorter the focal length, the greater the extent of the scene captured by the lens. On the other hand, the longer the focal length, the smaller the extent captured by the lens. If the same subject is photographed from the same distance, its apparent size will decrease as the focal length gets shorter and increase as the focal length gets longer.

DX and FX Formats Compared

The DX and FX formats use different crops, resulting in different picture angles (and magnifications) with lenses of identical focal lengths.

Crop
Subject Magnification
DX format: high (about 1.5× FX format)
The DX format picture angle for a lens of a given focal length is equivalent to the FX picture angle of a lens with a focal length about 1.5× longer.
FX format: low
FX format: low

DX and FX Format Cameras

FX format cameras can be identified by the “FX” label on the camera body.

DX Format Cameras
No format label on the camera body.
No format label on the camera body.
FX Format Cameras
FX format cameras bear an “FX” label.
FX format cameras bear an “FX” label.

DX and FX Format Lenses

Lenses are available in DX and FX formats. The former are adapted to the DX format crop, while the latter support both DX and FX format crops. DX lenses can be identified by the “DX” label on the barrel.

DX Format Lenses
DX format lenses bear a “DX” label.
DX format lenses bear a “DX” label.
FX Format Lenses
No format label on the lens.
No format label on the lens.

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