About Camera Lenses

A lens is the camera’s eye. Like the human eye, a camera lens frames and focuses the subject. And like the human eye, a camera lens contains a diaphragm that adjusts the amount of light that passes through the lens.

When you look at the specification sheet for almost any camera (or removable lens), you see two numbers-the focal length and the maximum aperture, or f-stop-that describe the lens. The focal length describes how the lens sees the world, and the maximum aperture measures the lens’ efficiency.

The focal length of a lens determines the camera’s angle of view. Because most cameras have zoom lenses, you usually see the focal length expressed as a range of numbers, like 28-105mm. Shorter focal lengths provide a wider angle of view, and longer focal lengths provide a narrower field of view. Figure 5.1 shows the same scene photographed with several different lenses.

As you can see, there’s a huge difference in the angle of view in the example pictures. In the widest shot, you can barely tell that there’s a boat at the dock; in the closest shot, you can read the lettering on the sides and back of the boat.

The diagonal of a 35mm film frame is about 43mm; a normal lens on a 35mm camera is around 50mm. Different digital cameras use different-sized image sensors, so the focal length of a normal lens on a digital camera depends on the sensor size. To avoid confusion, most digital camera manufacturers state their lens focal lengths in their 35mm equivalent. There are two good reasons for this. First, many people are familiar with 35mm cameras, so they’re also familiar with 35mm focal lengths. Second, the use of 35mm equivalent focal lengths makes it easy to compare lenses on cameras with different-sized image sensors.

The maximum aperture of the lens is usually expressed as a formula like f/2.8. The aperture is also called the f-stop, and it measures the ratio of the lens’ focal length to it’s diameter (see Figure 5.2). A lower f-stop allows more light to pass through the lens. A lens with a wider aperture is said to be "faster" than one with a smaller aperture because the larger aperture allows more light to pass through the lens. This in turn enables the photographer to use a higher shutter speed to obtain the correct exposure for a given scene.

Most lenses contain a diaphragm that enables the user to adjust the amount of light passing through the lens. The adjustment is necessary to obtain the proper exposure over a wide range of light conditions and shutter speeds. This adjustment is called the aperture adjustment or f-stop adjustment. As you adjust the diaphragm to make a smaller opening, you are also reducing the amount of light that passes through the lens. F-stops are calibrated as a series of numbers. The standard f-stop numbers are 1.4, 2, 2.8, 4, 8, 11 ,16 ,and 22, although many cameras enable you to select a number in between the standard stops. Each standard f-stop passes half the amount of light as the previous f-stop, so a lens set for f/2.0 passes twice as much light as the same lens set for f/2.8. This confuses many users, because a higher f-stop number represents a smaller lens aperture, which in turn passes less light.

As we just saw, smaller f-stops (higher f-stop numbers) pass less light than lower numbers. But something else happens as the size of the aperture decreases; the depth of field (also called depth of focus) increases. Depth of field describes the area of view that is in sharp focus. Larger openings produce less depth of field, and smaller openings increase depth of field.

Most (but not all) digital cameras allow you to manually select an aperture setting. This gives you creative control by enabling you to decide what is and isn’t in sharp focus. Many SLR cameras (including most digital SLRs) have a feature called Depth of Field Preview that lets you see the effect of different aperture settings in the viewfinder.

The vast majority of cameras-both film and digital-on the market today include zoom lenses. A zoom lens is one that can adjust to cover a range of focal lengths. Zoom lenses are very convenient, because they allow the photographer to adjust the framing and field of view of a photograph by simply adjusting the lens zoom. You could accomplish much the same effect with a single focal length lens by moving the camera closer or further away from the subject, but a zoom is faster and easier. As you’ll see later, different focal lengths give different perspectives on the subject.

Zoom lenses are often measured by their zoom ratio. A lens with a range of 50 to 150mm has a 3X zoom ratio; one with a 35-350mm range has a 10X ratio. Don’t confuse zoom range with the magnification ratio, which is something else entirely. The magnification ratio tells you how large (or small) a lens is in relation to a normal lens. A 200mm lens, for example, has a 4X magnification ratio compared to a normal 50 mm lens.

Zoom lenses are convenient, but many people don’t understand how to use them most effectively. When you zoom in on a subject, two things happen. First, longer focal lengths provide less depth of field (at the same aperture setting) than short focal lengths. As you zoom in on a subject, focusing becomes more critical because a smaller area is in sharp focus. This can work to your advantage if you want to isolate an object that is placed against a busy or distracting background. It’s a favorite technique of fashion and portrait photographers, because it places the emphasis on the main subject and leaves everything else blurry.

The second thing that happens as you increase a lens’ focal length is a phenomenon called foreshortening. Wide-angle lenses used very close to a subject tend to accentuate items (noses, for example!) in the center of the picture, making them appear larger than they are. Longer lenses tend to "flatten" the view, rendering near and far objects more realistically. Unless you want to accentuate someone’s nose, you should never use a wide angle lens for portraits.

Longer focal lengths make for attractive portraits, and they let you get in closer to distant subjects like zoo animals, players at sporting events, and distant scenery. But many users aren’t aware that longer lenses magnify not only the subject, but vibration, too. In most cases, you can use most P&S cameras handheld in bright light with no ill effects. But if your subject is in the shade or it’s getting late in the day, you need to use a tripod to hold the camera steady and prevent blurring caused by camera shake. If a tripod isn’t available (ie: you left it in the car because you knew you wouldn’t need it), you can always steady your camera by leaning against a building or tree.

Many cameras have a close focusing feature-often called a macro focusing feature-that enables you to get very close to a subject. Technically, a macro lens is one that can produce a life-sized image on the film (or image sensor), but camera makers have stretched the term to include any camera that focuses closer than 1 foot or so.

Most digital cameras have a macro feature, but some are much more effective than others. Some cameras can only focus close when they are zoomed to their widest setting. This reduces the effectiveness of the macro feature because the shorter focal length provides the least amount of magnification.

Some cameras can’t use their flashes in macro mode because the flash is designed to illuminate objects more than a few feet away from the camera. This is especially true for cameras with pop-up flash heads.

Macro focusing lenses are great for getting very close to small subjects like bugs and flowers. They’re also great for taking pictures of small items like jewelry, which is especially helpful if you plan to use your camera to create a home inventory for insurance purposes.

Lenses are complex, precision-made optical instruments. Precision costs money, so some lens designers take shortcuts to reduce the cost and size of lenses, especially on P&S cameras. Here are some common lens faults to watch for when testing out a new camera:

  • Vignetting – A vignette is a small portrait, usually placed in a frame with rounded corners. Vignetting in a lens happens when the image is darker in the corners than in the center of the picture.
  • Chromatic Aberration – This is a fancy name for a prism-like effect that is especially noticeable at the corners of the picture. It’s most visible when there’s a sharply-defined object like a tree limb or roofline at the edge of the picture. The aberration shows as a sharply colored (usually purple) line or blur in an area where there is no color.
  • Barrel and Pincushion Distortion – Barrel distortion happens when a vertical object appears thicker in the middle than at the ends-just like a barrel. Pincushion distortion is the opposite effect; objects seem to be pinched towards the center.