Focal Length and Angle of View
The focal length of a lens, together with the size of the image sensor in the camera (or size of the 35 mm film), determines the angle of view. A lens is considered to be a "normal lens", in terms of its angle of view on a camera, when its focal length is approximately equal to the diagonal dimension of the film format or image sensor format. The resulting diagonal angle of view of about 53 degrees is often said to approximate the angle of human vision; since the angle of view of a human eye is at least 140 degrees, more careful authors will qualify that, for example as "similar to the angle of crisp human vision." A wide-angle lens has a shorter focal length, and includes more of the viewed scene than a normal lens; a telephoto lens has a longer focal length, and images a small portion of the scene, making it seem closer.
Lenses are not labeled or sold according to their angle of view, but rather by their focal length, usually expressed in millimeters. But this specification is insufficient to compare lenses for different cameras because field of view also depends on the sensor size. For example, a 50 mm lens mounted on a Nikon D3 (a full-frame camera) provides approximately the same field of view as a 32 mm lens mounted on a Sony α 100 (an APS-C camera). Conversely, the same lens can produce different fields of view when mounted on different cameras. For example, a 35 mm lens mounted on a Canon EOS 5D (full-frame) provides a slightly wide-angle view, while the same lens mounted on a Canon EOS 400D (APS-C) provides a "normal" or slightly telephoto view.
In order to make it easier to compare lens–camera pairs, it is common to talk about their 35 mm equivalent focal length. For example, when talking about a 14 mm lens for a Four Thirds System camera, one would not only indicate that it had a focal length of 14 mm, but also that its "35 mm equivalent focal length" is 28 mm. This way of talking about lenses is not just limited to SLR and DSLR lenses; it is very common to see this focal length equivalency in the specification of the lens on a digicam.
Values in the following table are approximate, and apply to rectilinear lenses only, not to fisheye lenses.
| Sensor size: | 35 mm or full-frame | APS-H | APS-C | Foveon X3 | Four Thirds | |
| Crop factor: | 1.0 | 1.3 | 1.5 or 1.6 | 1.7 | 2.0 | |
| Lens view | Angle of view (deg. diagonal) |
Focal length (mm) | ||||
| Ultra wide-angle | 118 | 13 | 10 | 8 | 8 | 7 |
| 111 | 15 | 12 | 10 | 9 | 8 | |
| 100 | 18 | 14 | 12 | 11 | 9 | |
| 92 | 21 | 16 | 14 | 12 | 11 | |
| Typical wide-angle | 84 | 24 | 18 | 15 | 14 | 12 |
| 75 | 28 | 22 | 18 | 16 | 14 | |
| Slightly wide-angle | 63 | 35 | 27 | 23 | 21 | 18 |
| "Normal" | 59–47 | 40–50 | 30–38 | 25–32 | 23–29 | 20–25 |
| Portrait lens | 29 | 85 | 65 | 55 | 50 | 43 |
| 23 | 105 | 81 | 68 | 62 | 53 | |
| Telephoto | 18 | 135 | 104 | 87 | 79 | 68 |
| 14 | 180 | 138 | 116 | 106 | 90 | |
| 12 | 210 | 162 | 135 | 134 | 105 | |
| Long telephoto | 8 | 300 | 231 | 192 | 176 | 150 |
| 6 | 400 | 308 | 258 | 235 | 200 | |
| 5 | 500 | 385 | 323 | 294 | 250 | |
| 4 | 600 | 462 | 387 | 353 | 300 | |
| 3 | 830 | 638 | 535 | 488 | 415 | |
| Very long telephoto | 2 | 1200 | 923 | 774 | 706 | 600 |
| Some cameras using this sensor size: |
35 mm film Canon EOS-1Ds Canon EOS-1Ds II Canon EOS-1Ds III Canon EOS-1D X Canon EOS 5D Canon EOS 5D Mark II Canon EOS 5D Mark III Canon EOS 6D Nikon D3 (X, S), D4 Nikon D700, D800, D600 Sony α 850, α 900, α 99 Leica M9, M9-P, M Monochrom, M-E, M |
Canon EOS-1D Canon EOS-1D II N Canon EOS-1D III Canon EOS-1D IV Kodak DCS 460 - 760 Leica M8 |
Canon EOS 7D Canon EOS D30, D60 Canon EOS 10D - 60D Canon EOS 300D - 1100D Nikon D1 (H,X) Nikon D2H (Hs,X,Xs) Nikon D40 (x) - D90 Nikon D100 - D300 Nikon D3000-D7000 Pentax *ist D-Series Pentax K10D - K200D Pentax K-5, K-5 II, K-7, K-30 Pentax K-m, K-r, K-x Sony α 33 - α 700 (except α 99) Konica Minolta 5D, 7D |
Sigma SD9 Sigma SD10 Sigma SD14 Sigma DP1 Sigma DP2 Canon EOS DCS 3 |
Leica Digilux 3 Olympus E-3 Olympus E-30 Olympus E-620 Panasonic DMC-G1 |
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Read more about this topic: Lenses For SLR And DSLR Cameras
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