The focal length of the lens you use determines how the camera records the scene. A short focal length includes a wide view of the scene, which is why a lens with a short focal length is a wide-angle lens. Wide-angle lenses cover focal lengths from 12mm (very wide field of view) to 35mm. A long focal length magnifies the scene, essentially capturing a small part of the scene (or the field of view) and magnifying it to fill the frame. Lenses with long focal lengths are telephoto lenses. Telephoto lenses begin with a focal length of 80mm and exceed 500mm. A lens with a focal length that is 50mm encompasses the same field of view as the human eye. A lens that encompasses a range of focal lengths is a zoom lens. You can zoom in on your subject to focus on a small area, or zoom out for the big picture. You may see zoom lenses referred to as wide-angle to telephoto zoom, or normal to telephoto zoom.
One very important thing to remember: digital focal lengths don’t act the way they do on 35mm film cameras if you have a sensor that is smaller than a 35mm frame of film. If you do have a camera with a smaller sensor, your camera doesn’t capture as much of the scene in front of you as a 35mm film camera. In essence, the focal length crops to a smaller area of the scene, which is the same as zooming in. This is great when you’re a wildlife photographer. You can get closer to your subject without having to purchase an expensive telephoto lens with a long focal length. However, when you shoot landscapes, you’re at a disadvantage if you own a camera with a sensor that is smaller than a frame of 35mm film. A full-frame sensor has dimensions of 36mm x 24mm. If your sensor is smaller than that, you need to calculate your focal length multiplier and apply it to the focal length of the lens you’re using to get the 35mm-equivalent focal length. The focal length multiplier is also referred to as the crop factor.
The following figure shows two images of the same scene taken with two cameras. The image on the left was taken with a camera that has a full-frame sensor. The image on the right was taken with the same focal length on a camera with a sensor that is smaller than a 35mm frame of film. Notice that you see more of the scene with the picture taken by the camera with the full-frame sensor.
Figuring out your focal length multiplier
If you own a camera with a sensor smaller than a frame of 35mm film (36 x 24 millimeters), the sensor records only part of what the lens captures. The net result is that the lens acts the way a longer focal length would on a full-frame sensor. The focal length multiplier depends on the size of your camera’s sensor in relation to a full-frame sensor. Generally, the focal length multiplier falls in a range from 1.3 to 2.0. If you place a lens with a 50mm focal length on a camera with a focal length multiplier of 1.6, the resulting 35mm equivalent is 80mm (50 x 1.6). If you put the same lens on a camera with a focal length multiplier of 1.5, you end up with a 35mm equivalent of 75mm (50 x 1.5).
Know your camera’s focal length multiplier when choosing accessory lenses for your camera. Most Canon cameras that don’t have full-frame sensors have a focal length multiplier of 1.6, with the exception of the Canon EOS 1D MK IV, which has a focal length multiplier of 1.3. Nikon cameras without full-frame sensors have a focal length multiplier of 1.5.
If you can’t find the focal length multiplier for your camera, you can easily calculate it. For example, the sensor on a Canon EOS 7D is 22.3mm x 14.9mm. To find the focal length multiplier for the camera, divide the width or height of a 35mm frame of film by the width or height of your camera sensor. In the case of the EOS 7D, 36 divided by 22.3 is 1.614, which rounded off is 1.6. Therefore, the focal length multiplier for that camera is 1.6.
The right focal length for the right photo
The following table shows several common focal lengths and the types of photographs you would use them for.
| Focal Length | Type of photography | Comments |
|---|---|---|
| 24-35mm (wide angle) | Landscape photography, large buildings, a large group of people | Use this focal length with a small aperture (large f/stop number for a large depth of field. |
| 50mm | Buildings, people | Use with a medium aperture (f/7.1 to f/11) for a sharp image |
| 85-100mm (medium telephoto) | Portrait photography | Use with a large aperture (small f/stop number) for a shallow depth of field |
| 150mm plus (long telephoto) | Wildlife photography, photographing details of objects | Use with a large aperture (small f/stop number) for a shallow depth of field |
| Macro (focal length varies) | Close-up photography | Mount your camera on a tripod to ensure a blur free image |
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Source:http://www.dummies.com/how-to/content/focal-lengths-and-your-digital-slr-camera.html
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