LENSES

Other than the image sensor, the lens is the part of the camera that has the greatest effect on the quality of your images. In this section we look at some aspects that have an impact on the pictures you get.

Focal length

One of the most important characteristics of any lens is its focal length. It's the focal length that determines a lens' angle of view-wide angle, normal, or telephoto.

Lens focal lengths are indicated in millimeters (mm). On a more familiar 35mm camera, a lens with a focal length of less than 35 mm is considered a "short" or wide angle lens and one over 65mm is considered a "long" or telephoto lens. Lenses between 35mm and 65mm are considered normal and the 50mm lens is the most common normal lens.
As the focal length of a lens increases, its angle of view decreases. Here the short focal length lenses are on the left and the long focal length lenses are on the right. As you move from left to right the focal length increases and the field of view decreases.


When you change focal lengths, two important effects are immediately obvious in the lens' angle of view and in its magnifying power, both of which you can see in your viewfinder.

- Angle of view describes how much of a scene the lens "sees". A short lens has a wide angle of view; as the focal length gets longer, the angle of view becomes narrower. A short lens will capture a wide expanse of a scene; a long lens with its narrower angle of view will isolate small portions of the scene without your having to move the camera closer to the subject.

- Magnification is related to the lens' angle of view. Since a short lens includes a wide sweep of the scene, all of the objects in the scene are reduced to fit onto the image sensor. Long lenses have a much narrower angle of view, so objects in a scene appear larger.

Your choice of lens depends in part on what you plan to do with the camera. Wide-angle lenses are best for photographing buildings, landscapes, interiors, and street photography. Telephoto lenses are best for portraits and many nature scenes. Normal lenses are a compromise.

With a wide-angle lens you get a lot of coverage and a lot of depth of field. Also focusing isn't as critical because the lens has great depth of field. Long lenses let you isolate out details from a scene but have shallow depth of field which makes focusing more critical. Also, because long lenses magnify more, they are harder to handhold and still get sharp images. The slightest movement causes blur.

So what makes a lens focal length wide, normal, or long? When the focal length of a lens is close to the diagonal measurement of the film format, the lens is said to be "normal" or close to the magnification of the human eye. When the focal length of a lens is longer than the film diagonal, it's a "long" or telephoto lens. When the focal length is shorter than the film diagonal, it's a "short" or wide-angle lens. Since the placement of lenses into these categories is based on the film size being used, a given focal length might be considered normal on one type of camera, wide angle on another, and telephoto on a third.

Since 35-millimeter film has an image area measuring 24-by-36mm, its diagonal is 43 mm-not quite 50 mm. The 50mm lens has become the normal lens because there is some latitude in characterizing a lens as normal and it's more a range than an exact number.

Digital cameras use the same relationships as other cameras to determine wide-angle, normal, and telephoto lenses. However, the focal lengths are much shorter because solid-state image sensors are much smaller than the smallest film. For example, while 35mm has an area of 36 x 24 mm, a 2/3-inch image sensor is only 8.8 by 6.6 mm and many sensors are even smaller.

So What's Normal, and Why?

A lens is called normal because it sees a scene just as the human eye does. This seems to violate common sense, because the eye's angle of view is much wider than any normal lens. However, you can demonstrate for yourself why a specific focal length is normal for your camera. If you are a passenger in a car and have a camera with a zoom lens, try zooming as you watch the traffic ahead through it. The longer focal lengths make distant cars appear right on top of you; in reaction you might even try to put on your brakes and then discover the cars are nowhere near as close as you thought. With shorter focal lengths, cars look far ahead, even when relatively close. A normal focal-length makes the cars appear in the same distance relationship as you perceive them ordinarily.

Another demonstration is to take two photographs of greatly different size and tape them to a wall. Look at them one at a time through the camera viewfinder with a normal focal-length. Move close enough so each fills the viewfinder frame. Take the camera from your eye without moving, and you'll discover you are at the correct distance for viewing the prints. With longer focal-lengths you would feel too far away from the distance and with shorter ones too close.

Zoom lenses

Many digital cameras come with zoom lenses that let you change the focal length of the lens on the fly. The range of focal lengths a zoom lens covers is usually specified by its magnification. A 3X zoom lens will enlarge or reduce the subject in an image by three times depending on which way it's zoomed over its full range. The equivalent range when used on a 35 mm camera is also usually given, for example, "38mm ~ 114mm."

Zoom lenses come in two varieties; optical and digital zooms. An optical zoom lens actually changes the amount of the scene falling on the image sensor. Every pixel in the image contains unique data so the final photo is sharp and clear. A digital zoom lens uses sleight of hand by taking a part of the normal image falling on the sensor and then saving that part as is, or enlarging it to fill the sensor. It does this by adding new pixels to the image using interpolation. The interpolated image doesn't have as many unique pixels as one taken with an optical zoom so is inferior. In fact, you don't even need this zoom feature because you can get exactly the same effect just by cropping a normal image in a photo-editing program and then enlarging it.

Although many zoom lenses work by pressing buttons or a zoom lever, some cameras have zoom rings on the lens just like those on zoom lenses for 35mm cameras.

By turning the zoom ring during an exposure, you can get special effects, especially when combined with flash. I've now take a hundred of these photos and find that it really is a matter of trial and error. Shoot a lot of them to find out what works and what doesn't.

Macro mode and macro lenses

Many digital camera lenses have a macro mode. These lenses act like any other lens until you switch them to macro mode. In this mode you can get very close to objects so they are greatly enlarged in the image. The trade-off is that depth of field, the area that's sharp from foreground to background is very shallow.

Cameras with interchangeable lenses have some lenses with macro modes but also have special macro lenses dedicated to macro photography.

Interchangeable lenses and lens accessories

For years photographers using 35mm SLR cameras have had the advantage of being able to change lenses on their camera to suit the situation they are shooting. Digital cameras don't yet give you this advantage except at the high and more expensive end of the spectrum.

Lens converters

Although you can't change lenses on most cameras, the better cameras make it easy to screw on or otherwise attach lens converters and other accessories. For example, you can attach wide-angle or telephoto lens converters or filters. Although it seems odd, for some unknown reason many cameras make it difficult to attach these kinds of accessories. Another odd thing is that most camera companies make these kinds of accessories, but then they don't promote them or even make it easy to find out what they have.

Interchangeable lenses

Many camera companies have designed their digital cameras so they accept the lenses originally designed for film cameras. This causes some confusion about focal lengths. Lens focal lengths are based on the physical characteristics of a lens so they are absolute values. However, a given focal length lens may have an "effective" focal length on one camera that's different from its effective focal length on another camera. This is because the effective focal length depends on the size of the film or image sensor being used. As these get smaller, a given focal length lens magnifies the scene more.

Since the smaller sensor essentially captures only the central section of the focused image, the effective focal length increases by a factor of 1.6 times compared to the indicated focal length of the lens. This multiple works across the entire range of focal lengths, making wide-angle lens less so on the digital than on the film camera, and making telephoto lenses dramatically longer.

There are two ways to resolve this "problem." The first is to design an image sensor the same size and shape of a frame of 35 mm film. These "full-frame" image sensors are available, but not yet widely used because of their expense. The other solution is to design lenses to match the sensor. Since there is no standard sensor size and shape, this hasn't been done until recently. Now such interchangeable lenses being designed specifically for digital cameras by members of the Four Thirds System group started by Olympus, Kodak, and Fuji. These cameras and lenses have a standard mount so lenses are usable on any camera produced by group members. Because they have been designed specifically for digital cameras, they don't have the multiplier effect that lenses from film cameras have.

Image stabilization

If you've ever photographed in dim light, or tried to hand-hold a long telephoto lens, you know how easy it is to get blur in your images from camera shake. In most cases, we resort to tripods or other camera supports. However, some lenses now have image stabilization built-in. Lenses with this feature contain gyro sensors that sense movement of the lens and micro-motors that instantly shift a special image stabilization lens group vertically to compensate for the motion and keep the image steady on the sensor. These lenses break the old rule that you should never handhold a lens using a shutter speed slower than the reciprocal of the lens focal length. For example, when using a standard 125mm lens, you shouldn't use a shutter speed slower that 1/125. Image stabilized lenses let you add two stops to that calculation. You can handhold an image stabilized 125mm lens at 1/30 of a second shutter speeds.

Note that when using an image stabilized lens on a tripod, you should turn off image stabilization. If you don't do so, you can actually add blur to the image. (A few lenses have a mechanism that prevents this problem.) This is because when image stabilization is on, the special image stabilization lens group is free to move. If it moves while everything else is perfectly stable, blur results. When image stabilization is off, the image stabilization lens group is locked in place so it can't move at all.

Maximum aperture

When you press the shutter release to take a picture, the camera's shutter opens and light from the scene is focused by the lens onto the image sensor. To get a perfect picture you want just the right amount of light to strike the image sensor. If there is too much light, you need to decrease it. If there is too little, you need to increase it. One way to increase or decrease the amount of light passing through the lens is to open or close the len's aperture, an adjustable hole through which the light passes. "Stopping" down the aperture makes it smaller and lets in less light. "Opening" it up lets in more. One way to think of this is as if it were a water faucet valve that you could close or open to decrease or increase the amount of water flowing out the faucet.

Aperture settings are called f-stops and each f-stop lets in half as much light as the next larger opening and twice as much light as the next smaller opening. From the largest possible opening on down to the smallest, the f-stops are f/1, f/1.4, f/2, f/2.8, f/4, f5.6, f/8, f/11, f/16, f/22, f/32, f/45. No lens has the full range of settings; for example, the standard lens that is supplied when you purchase a digital camera will range from about f/2 to about f/16. Notice that as the f-stop number gets larger (f/8 to f/11, for example), the aperture size gets smaller. This may be easier to remember if you think of the f-number as a fraction: 1/11 is less than 1/8, just as the size of the f/11 lens opening is smaller that the size of the f/8 opening.

How much you can open up a lens depends on it's maximum aperture. Larger maximum apertures (such as f/1.8) let in more light then smaller maximum apertures (such as f/3.2) so you can take images in dimmer light or freeze faster actions.

Rotatable lenses

Lenses on digital cameras don't have to be rigidly mounted in a fixed position as they do on a film camera. Some camera designs take advantage of this to let you rotate the lens separately from the body. Instead of tilting or swiveling the monitor, you twist the camera so you can view the image on the monitor while still pointing the lens where you want it. For example, you can hold the camera over your head while looking up into the monitor as you rotate the lens down to get the picture you want.

Glass or plastic?

The lens' job is to sharply focus the scene onto the surface of the image sensor. The better it does this, the better the picture will be. Generally, lenses made from glass are sharper and more damage resistant than lenses made from plastic. However, even glass lenses are not all created equally. The best way to find out about lens quality for a specific camera is to read reviews. Generally lenses made by a name company with a long tradition of making lenses such as Nikon, Canon, or Olympus are considered superior.