Focal Length and Zoom
Capture the whole image or focus in on the tiny details
The focal length of the lens (measured in millimeters) and the zoom ratio determine how much or how little of the scene you are photographing will appear in the final photograph. When you want to show the whole scene, you use a short focal length and when you want to focus on a detail you use a long focal length.
Fixed Focal Length vs. Zoom
A fixed focal length lens cannot zoom in and out on a scene. If you want the subject of your photo to be larger, you have to walk closer. If you want it smaller, you have to walk away. Common fixed focal length lenses are 35mm and 50mm.
Zoom lenses have variable focal lengths. For example, a typical zoom may be able to change from 28mm (wide-angle) to 135mm (zoom or telephoto). The benefit here is that if you cannot walk toward or away from your subject (maybe you're on the edge of a cliff) then the only way you can get closer is by zooming in with your lens.
Fixed Focal Length and Zoom Lens
On the left is a 50mm fixed focal length lens.
On the right is a 70mm — 300mm zoom lens. The zoom lens is currently set to 70mm, so at 300mm it is almost twice as long as in this photo.
Most compact digital cameras on the market today have zoom lenses. A zoom lens is more versatile for the everyday photographer.
In order to simplify things for the consumer market, most compact digital cameras are not going to describe their zoom lenses in terms of focal length. Instead, they use a zoom ratio. A zoom ratio is a multiplier number that lets you know just how much of a zoom you are going to get.
Some examples of common zoom ratios are: 2x, 3x and 10x. A 10x zoom is going to allow you to get a much closer photograph of something you can't get next to than a 2x zoom. An example of this would be a bird in a tree. With the 2x zoom, the photo will include a lot of the tree, and it might be hard to tell there is a bird in it at all. With the 10x zoom, you can make the bird fill the entire photograph, and most of the tree will not be in the photo.
All zoom ratios have their focal length equivalents. Many times when you are reading about a compact camera, they will give both types of measurements. For example at 2x zoom has a focal length equivalent of 35mm to 70mm and a 4x zoom has a focal length equivalent of 35mm to 140mm.
Optical Zoom and Digital Zoom
When looking at digital cameras you will hear about optical and digital zoom. Cameras ads will say something like "4x optical and 3x digital for a total 7x zoom!"
Optical zoom is what the lens is actually capable of. This is an unmanipulated zoom, and the result of changing the focal length of the lens. Digital zoom is an artifical zoom, where the size of the image is digitally enhanced to create the impression of a longer focal length.
While that might sound like a great idea it really isn't. Every digital zoom I have ever seen makes your photograph look like it has gone through a laundry cycle. Images are blurry, fuzzy and clearly display the pixels that make up the digital photo. Only the optical zoom will produce clear, crisp photographs.
Keep this in mind when you are evaluating cameras. A camera that has a 5x optical and 5x digital zoom is not the same as a camera with a 10x optical zoom. The camera with the 5x digital will produce blurry photos at 10x zoom whereas the 10x optical photos will be clean and clear.
Zoom and the Real World
Zoom lenses can be habit-forming. It's fun just to stand in one spot and zoom in and out on the scene until you get the composition just right. While many professional photographers will tell you to actually move your body, zoom lenses can be quite convenient.
Zoom lenses are really used for those situations when you can't get close to your subject. The zoo is a good example. Say you want to take a nice photo of a gorilla's head. The gorilla is sitting far away behind a barrier and across a moat. The only way to get the photo that you want is if you have a powerful zoom lens on your camera.
Here are some shots you might want to take, and the zoom that you might need:
|Situation||Zoom Setting||Multiplier Required|
|You want to take a landscape photograph that shows the whole entire scene||Wide-angle (28mm)||Any — at wide-angle you are not even using the zoom|
|You enjoy taking portraits of people||Moderate (100 - 125mm)|
If you use a wide-angle when taking portraits the facial features of your subject will be distorted. It is much more flattering to use a longer focal length
|You want to photograph birds||Full zoom (300 - 600mm)|
Because of their small size, birds tend to disappear in photos if you are not close enough to them