Choosing Lenses

Sometimes the choice of lens is dictated by the situation- a telephoto lens when the photographer is unable to get closer the scene and a wide angle when he or she cannot move any further back to get the entire scene in.

You may think that brand is the first thing to decide. If you are still searching for a camera, this would be true. Looking at lenses when deciding which camera brand to buy is very wise. Each camera body is best when paired with lenses made by the same manufacturer, specifically designed for that particular camera.

It is true that you can get a third-party lens by companies such as Sigma, Tamron, and Tokina. Many people do get such lenses and are perfectly happy. It mostly depends on how much shooting you are going to do; if you are really serious about photography, we recommend getting Canon lenses for Canon, Nikkor lenses for Nikon, and Minolta lenses for Minolta... The may cost more but they are worth every extra penny.

While the brand you choose is determined for the most part by which camera you own, other decisions still need to be made.
The first decision to make is whether to go with a zoom or a fixed focal length lens. What's a fixed focal length lens, you ask? It is basically a lens that does not zoom.

Lenses are measured in millimeters. Forget about what the millimeters refer to and just learn this:
• 20mm or less = Extreme Wide Angle
• 24mm to 35mm = Wide Angle
• 50mm to 80mm = Normal
• 100mm to 300mm = Telephoto
• 400mm and above = Super Telephoto

Whether it is a zoom or fixed focal length, each lens will most likely fall into one of these areas. Some stretch the envelope; others can cover everything from 35mm wide angle shots to 350mm telephoto shots. You pay a price for this kind of convenience, though. The lens is heavier and does not allow as much light to reach the film. This latter fact means that you will have a more difficult time shooting in relatively low light conditions. You will need to use a tripod more often, and the lens may not catch fast action as well as you would like.

If you are just starting, get at least a normal/wide-angle lens. A zoom in the range of 28mm to 80mm works best. If you can afford it, also get a medium to long telephoto, say 100mm to 300mm. One camera body and two such lenses makes an awesome combination when out in the field.

From this foundation, you can add an extreme wide angle if you like to do panoramic, expansive scenes or wacky, wild angles.

Also consider adding a super telephoto, especially if you like shooting wildlife, sports, etc. These long-range lenses are a lot of fun and open up many new opportunities. Children, for one, are much easier to photograph from a slight distance.

Specialties and Options

If you find that you love to shoot abstract close-ups of things like flowers, bugs, and dripping water droplets, you might like a good macro. Those who know this is what they are going to be doing can save themselves a lot of money but just getting one 50mm macro instead of a 50mm normal lens and an additional macro lens. A 50mm macro works just as well for normal photography; its design simply allows you to get much closer to your subject than you could with a normal lens. This gives you the added ability to do fun close-up work.

You can also get tele-extenders that will magnify your lens by 1.4 of 2. For example, putting a 2x teleconverter on a 200mm lens will give you a 400mm. However, such teleconverters cut out a lot of light, making it more difficult to maintain fast shutter speeds and shoot without a tripod or in low-light conditions.
Lens elements

A lens or lens assembly on a network camera performs several functions. They include:
• Defining the field of view; that is, defining how much of a scene and level of detail are to be captured.
• Controlling the amount of light passing through to the image sensor so that an image is correctly exposed.
• Focusing by adjusting either elements within the lens assembly or the distance between the lens assembly and the image sensor.

Field of view

A consideration to take into account when selecting a camera is the field of view required; that is, the area of coverage and the degree of detail to be viewed. The field of view is determined by the focal length of the lens and the size of the image sensor; both are specified in a network camera’s datasheet.

A lens' focal length is defined as the distance between the entrance lens (or a specific point in a complicated lens assembly) and the point where all the light rays converge to a point (normally the camera’s image sensor). The longer the focal length, the narrower the field of view.

The fastest way to find out what focal length lens is required for a desired field of view is to use a rotating lens calculator or an online lens calculator, both of which are available from Axis. The size of a network camera’s image sensor, typically 1/4”, 1/3”, 1/2” and 2/3”, must also be used in the calculation. (The drawback of using a lens calculator is that it does not take into account any possible geometrical distortion of a lens.)

The field of view can be classified into three types:
• Normal view: offering the same field of view as the human eye.



• Telephoto: a narrower field of view, providing, in general, finer details than a human eye can deliver. A telephoto lens is used when the surveillance object is either small or located far away from the camera. A telephoto lens generally has less light gathering capability than a normal lens.



• Wide angle: a larger field of view with less detail than in normal view. A wide-angle lens generally provides good depth of field and fair, low-light performance. Wide-angle lenses sometimes produce geometrical distortions such as the “fish-eye” effect.



Different fields of view: wide-angle view (at left); normal view (middle); telephoto (at right).



There are three main types of lenses:

• Fixed lens: Such a lens offers a focal length that is fixed; that is, only one field of view (either normal, telephoto or wide angle). A common focal length of a fixed network camera lens is 4 mm.
• Varifocal lens: This type of lens offers a range of focal lengths, and hence, different fields of view. The field of view can be manually adjusted. Whenever the field of view is changed, the user has to manually refocus the lens. Varifocal lenses for network cameras often provide focal lengths that range from 3 mm to 8 mm.
• Zoom lens: Zoom lenses are like varifocal lenses in that they enable the user to select different fields of view. However, with zoom lenses, there is no need to refocus the lens if the field of view is changed. Focus can be maintained within a range of focal lengths, for example, 6 mm to 48 mm. Lens adjustments can be either manual or motorized for remote control. When a lens states, for example, 3x-zoom capability, it is referring to the ratio between the lens’ longest and shortest focal length.