Camera zoom lenses for digital single lens reflex (DSLR) and mirrorless interchangeable-lens cameras (MILCs) are not created equal. The capability of zoom lenses are usually described using f value ranges. These f values translate into different ranges of depth of field in pictures.

Before understanding how zoom lenses operate and which works best in certain situations, there is a need to understand the concept of depth of field. 

What is depth of field?

The depth of field is the range of distance that appears clear in your photographs. Perhaps you have noticed when you use single lens reflex cameras, it is possible to make an object clear relative to those around it. The distinctively sharp area is the depth of field of the camera (Fig. 1). 

Fig. 1. Mangrove skink photographed using a zoom lens (f = 5.6, exposure time = 1/400).

Notice in the above picture that the head of the lizard is relatively clear than the rest of its body and the background. The degree of sharpness is gradual, not abrupt, from the center of focus. This makes the picture more interesting and three dimensional.

The depth is not discernible by the naked eye but is dependent on the aperture adjustment of your camera's lens. The larger the opening of the lens, represented by a lower f number, the shallower the depth of field of the resulting picture. 

Shallower depth of field is evident especially in zoom lenses. When you zoom in to an object from afar, the resulting picture would pick off the subject and the rest will be blurry, fuzzy or indistinct. This is referred to by photographers as "bokeh". The word "bokeh" comes from the Japanese word "boke" (pronounced bo-keh) which literally means fuzziness or dizziness. This adds depth to the picture.

Which zoom lenses work best?

Advanced technology turn out excellent camera lenses nowadays. It is therefore difficult to spot differences between two lenses produced by well-known manufacturers of lenses. Olympus, Canon, and Nikon are well known brands in the digital camera business. But Panasonic with Olympus with its compact micro four thirds digital camera systems opens new grounds with its excellent quality Leica lenses used in the increasingly getting popular mirrorless intechangeable-lens cameras which are smaller than conventional DSLRs.

You can select from a wide variety of lenses for your own purpose. If you intend to take pictures of wild animals which are not easily approached or are dangerous, you will need to buy a zoom lens.

Zoom lenses vary in terms of speed as well as price. Zoom lenses with lower f values marked on it tend to take better pictures than those with relatively higher f values. For example, a zoom lens with 3.5 as its widest aperture is a fast lens compared to one with and f value of 4 or 5.6.

The price of zoom lenses with lower f values tend to be more expensive than those with higher values. Lenses with lower f values are also referred to as fast lenses.

Zoom lenses have usually ranges of f values marked in them, say 3.5 or 5.6. Zooming in increases the f value thus makes light less available for proper exposure. The higher the f value, the slower the shutter speed required to compensate for the smaller aperture. You therefore need an adequate amount of light when using zoom lenses, unless your camera has a versatile ISO function that can work even in low light conditions. The resulting picture, however, will be grainy (Figure 2).

Fig. 2. Asian glossy starling (f7.1, shutter speed = 1/15, ISO 400).

Even in low light conditions and very low shutter speed of 1/15, this picture looks just fine because of the high ISO number used by the camera. When this is blown up, however, the picture is grainy.

Zoom lenses also work better if the difference between the lower and higher focal length is lesser. Thus, a 45 to 100 zoom lens can work better than a 45 to 200 mm lens at the same f value of 5.6. It is not quite possible to manufacture lenses that can take crisp pictures in all the focal length ranges of the zoom lens. 

These pictures were taken using Panasonic Lumix G10, an MILC or mirrorless camera with electronic viewfinder. A 45 to 200 mm zoom lens (f5.6) produced by Leica was used.

©2011 November 24 Patrick A. Regoniel