Photography

Zoom and Focal Length

Zoom is the ability of a camera to magnify or demagnify an image to a certain range. A 3x zoom factor indicates that the camera is capable of magnifying from its lowest zoom value up to a point where the image is three times its size. Most camera manufacturers provide Zoom values that may sound confusing or misleading so it is best to understand how they come up with these values.

There are two types of zooms used in most digital cameras, the digital zoom and optical zoom. Digital zoom is merely a method of image manipulation where the image is enlarged using interpolation by which artificial pixels are generated from the actual pixels and provided with a color calculated from nearby pixels. This is very much the same method used by most graphic software like Photoshop to artificially enlarge images. The color of the artificial pixels are often inaccurate resulting in lack of detail and sharpness. Optical zoom enlarges the image by changing the overall properties of the lens. The images produced are sharper, more detailed and the colors are more accurate.

When looking for a camera with good zoom, make sure you are looking at the optical zoom and not digital zoom. Some manufacturers will also advertise the zoom value as total zoom or combined zoom -- this is merely obtained by multiplying the optical zoom by the digital zoom, so a combined zoom of 6x for example may be just 2x optical and 3x digital or 3x optical and 2x digital.

How does the camera perform optical zooming? You might notice that in most camera specifications, certain values accompany the zoom factor such as 28mm - 70mm. Let us first examine the basics of lenses in order to understand how these numbers are obtained and why they are significant in choosing a good camera or lens.

When parallel rays of light passes through a lens, they converge in a single point called the focal point. The distance from the middle of the lens to the focal point is called the Focal Length, measured in millimeters, and is used as a basis in determining the magnification properties of a lens. The focal lengths of camera lenses are often based on the full-sized 35mm equivalent sensor (36mm x 24mm) so cameras with smaller sensors use their "35mm equivalent" to provide users with a standard idea of what they are getting.

Lenses are classified as Wide Angle, Telephoto, or Normal based on their focal lengths. A focal length of 50mm is considered normal. This value is used because it is with a 50mm lens on a 35mm film camera that we can see about the same view as a normal eye can see. Focal lengths below 50mm are considered Wide Angle and above it are called Telephoto. Zoom refers to the capability of a lens assembly to vary its equivalent focal length and thus changing its magnification to a certain range.

With this in mind, we now understand that it is not enough to know the zoom factor of a camera, it is also important to know the focal range of the camera. In the example above, 28mm-70mm means that this particular camera can vary the focal length of its lens from 28mm (wide angle) up to 70mm (telephoto). The zoom factor is derived by dividing the maximum focal length by its minimum, giving this particular camera a 2.5x zoom factor (70/28). Other cameras may also have a zoom factor of 2.5x but may have a different focal range. For example: a focal range of 36mm-90mm still gives you a zoom factor of 2.5x but most of its zoom is in the telephoto range. The 28mm-70mm lens therefore can give you more wide angle view of the image than the 36mm-90mm lens but the latter can bring you closer to the subject although they both have a 2.5x zoom.

For cameras with fixed lenses, it is important to know the focal range of the lens to match your usual type of subjects. If you like to photograph landscapes, or are more inclined to take pictures of groups rather than individuals, a wide angle lens will be more useful. For portraits, close-ups or for taking a closer picture of a subject from a distance, a camera with more telephoto would be ideal. Many sub-compact cameras with super zooms are now available with zooms of 12x to 18x. SLR cameras have the advantage of versatility on this area because the lens can be detached and replaced with a more appropriate lens for any occassion. That is why most SLR camera owners own not just one but several lenses depending on their needs and budget.