A Short Course Book
Curtin's Guide to Digital Cameras
And Other Photographic Equipment


Point and shoot cameras often have 3x zoom lenses but they can range much higher. This S3 IS from Canon is equipped with a 12x zoom.
SLR cameras from major camera companies let you choose from a wide array of lenses. Here are those offered by Canon.
Click for a PDF of Canon lenses.
Lenses with larger maximum apertures let you use faster shutter speeds and are often called "faster" lenses.
Click to explore apertures and their role in exposure.
The focal length of a lens determines its angle of view.
Click to explore how the focal length of a lens determines its angle of view.
Click to explore the sizes of image sensors.
Sounds like science fiction but liquid lenses that focus by changing shape are now being used in some camera phones. Courtesy of Varioptics.
Click to explore how the size of an image sensor determines the focal length of a lens.

These two photos were taken with the same camera. One was taken using optical zoom (top) and the other with digital zoom from farther away (bottom). The one taken with optical zoom is much sharper.
Click to explore optical and digital zoom.
Click to explore how a wide-angle lens can distort a subject.
Click to see how extreme wide-angle lenses can be used to create 360 degree interactive panoramas.
It's hard to design a lens that works great at both wide-angle and normal/ telephoto focal lengths. To remedy this, Kodak designed a camera with two lenses.
Wide-angle lenses can distort objects near the edge of the frame. This is called "barrel distortion."
Long lenses make the sun and moon look much larger.
Digital SLR cameras use electrical connections to set the lens aperture and adjust focus.
Most digital cameras have a fixed zoom lens that can't be removed or replaced. One big advantage is that the camera is sealed so no dust can get on the image sensor. Digital SLR cameras have removable lenses so you can change them when circumstances dictate.

Lens Information

Many lenses display information that is useful in your photography. Be sure to consider this information when choosing a lens and take the time to read any printed information that comes with a lens.
Information around the lens may include:
  • The focal length of the lens or the zoom range in mm. Here the range of a zoom lens is 6.0-72.0mm. On fixed lens cameras the zoom range is often given as a multiplier. For example, 6.0-72.0mm is 12x (72 divided by 6).
  • The maximum aperture determines how wide the lens will open. It's listed on the lens as a ratio such as 1:2.4 or 1:2.8-3.7. On most zoom lenses, two maximum apertures are given because the aperture changes as you zoom the lens in and out. However, some lenses don't change the aperture as you zoom them. This lets you set exposure and zoom all the way through the lens's zoom range without the aperture or shutter speed varying. A larger maximum aperture is better because it lets you use shallower depth of field, a faster shutter speed to freeze action, and increases the range of your flash. A lens with a maximum aperture of f/1.4 is three stops faster than a lens with a maximum aperture of f/5.6. This means that instead of using a shutter speed of 1/15 you can use one of 1/125. The problem with lenses having large maximum apertures is that they are expensive, large, and heavy. A lens' maximum aperture is determined by dividing the actual diameter of the aperture opening into the focal length of the lens. That's why the aperture on most (but not all) zoom lenses changes as the lens is zoomed in and out to change the focal length.
  • The size of filters or other accessories that can be screwed into the threads. The diameter is often preceded with the symbol f as in f85mm.

Choosing Focal Lengths

The focal length of a lens has a huge impact on your images and is one of the most important tools in your creative tool box. On fixed lens cameras you change the focal length by zooming the lens. On SLRs you can do the same or change lenses. The various focal lengths you can use are referred to by synonyms that can be confusing at first.
  • Wide-angle, short focal length, short lens and zoom out refer to the same thing lenses that capture a wide expanse of a scene.
  • Telephoto, long focal length, long lens and zoom in refer to the same thing lenses that bring distant subjects closer.
The focal length you choose is a creative choice because it has two effects on your images:
  • Angle of view refers to how much of a scene the lens covers. Fisheye lenses, the widest available, can capture up to 180-degrees. As you zoom in or change lenses to increase the focal length, the field of view narrows and you can isolate small portions of the scene without moving closer to the subject.
  • Magnification is related to the lens' angle of view. Since using a short focal length lens or zooming out includes a wide sweep of the scene, all of the objects in the scene are reduced to fit into the image. Zooming in or using a longer focal length lens gives a much narrower angle of view, so objects in a scene appear larger.
Lens focal lengths are based on the physical characteristics of the lens so they are absolute values. However, a given focal length may be a wide angle lens on one camera and a telephoto lens on another. This is because descriptions such as "wide-angle" or "telephoto" depend on the size of the film or image sensorbeing used. As these get smaller, a given focal length lens magnifies more. There are currently a number of differently sized image sensors used in digital cameras. For that reason, different focal lengths are needed to give the same image coverage on different cameras. Because of the confusion this causes,most digital camera companies give the actual focal length of their lenses and then an equivalent focal length were the lenses to be adapted to a 35mm camera. For example, a camera may list its lens as 7.5mm (equivalent to 50mm on a 35mm camera). Because digital equivalents vary widely, we often use the more familiar 35mm focal lengths in this book.
The impact of sensor size on focal length isn't limited to fixed lens cameras. Digital SLRs are often adapted from 35mm film cameras and use lenses designed to project an image circle large enough to cover a frame of 35mm film. When these lenses are used on a digital camera, the angle of view captured in the image depends on the size of the sensor placed within this image circle.
  • When the image sensor is the same size as a frame of 35mm film, called a full-frame sensor, the lens' angle of view, and hence its focal length, is the same as it is on a film version of the camera.
  • When the image sensor is smaller than a frame of film, as many are, it captures a smaller area of the image circle, effectively increasing the lens' focal length by a factor of 1.5 x or so compared to the focal length indicated on the lens. Therefore, a lens that is 100mm on a film camera will be 150mm or 160mm on the digital version of the camera. This multiple works across the entire family of lenses that work with the camera, making wide-angle lens less so on a digital SLR, and making telephoto lenses more so.
A lens projects the image as a circle and the size of the film or image sensor determines what area of the circle is captured. Here the frames (from largest to smallest) show the areas captured by 35 mm film or a full-size sensor, an APS-H sensor, and a APS-C sensor.
A smaller sensor penalizes you when used with shorter focal length lenses (top left). Its smaller sensor captures a smaller part of the image circle (the white outline) than a camera using a full frame sensor or film so it has a longer effective focal length

A smaller sensor gives you a bonus when used with long focal length lenses or macro lenses (bottom right). Its smaller image sensor captures a smaller part of the image circle (the white outline), increasing magnification.

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.

If your camera has a zoom ring on the lens, you can turn it during a slow exposure to streak lights.

Zoom Lenses

Most fixed lens cameras have a built-in zoom lens and zooms are also very popular with SLR users. These lenses are popular because they let you choose any focal length within the range the lens is designed for.

Zoom comes in three varieties; optical, digital and cropping:

  • An optical zoom 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, found on many fixed lens cameras, uses sleight of hand by taking a part of the image falling on the sensor and 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. Ignore advertising claims for digital zoom and total zoom and focus on optical zoom. If they don't use that term in their ads or specifications, beware.
  • Cropping zoom, called Smart Zoom by Sony, is just like digital zoom but it doesn't inflate the cropped image by adding pixels. It just uses some of the image sensors pixels to record an image and has exactly the same effect as cropping a picture in a photo-editing program.

Normal Lenses

A "normal lens" for a 35mm camera usually refers to a fixed focal length lens of 50mm or a zoom lens zoomed in a little from its widest angle. When using a lens of this focal length, the scene looks about the same as it does to the unaided eye. With a longer focal length, everything appears closer than it actually is. With a shorter focal length, everything looks farther away.
A normal-focal-length (50mm) lens isn't necessarily the one photographers normally use. Many photographers prefer the wider angle of view and greater depth of field provided by a slightly shorter focal length.

Short Focal Lengths

Using a short focal length lens or zooming out gives you a wide-angle of view that lets you capture a wide expanse of a scene. This view is ideal for use in tight spaces, such as when photographing landscapes and in small rooms where you can't position the camera a great distance from the subject.

A short lens also has great depth of field so it's good for street or action photographs. When responding to quickly unfolding scenes this depth of field lets you respond quickly without worrying about focus.

Short lenses also let you focus very close to your subject, and the effect this can have on the perspective in your images can be dramatic. Objects very close to the camera loom much larger than those farther in the background. This distortion in the apparent size of objects can deliberately give emphasis and when carried to an extreme, give an unrealistic appearance to a scene.

Long Focal Lengths

A long focal length lens acts somewhat like a telescope in that it magnifies the image of your subject. This is especially useful when you can't get close to your subject— or don't want to. Long lenses are ideal for wildlife, portrait, and candid photography, whenever getting close to a subject might disturb it. The long focal length lets you keep your distance and still fill the viewfinder frame with the subject. Keeping at a distance eliminates the exaggerated perspective caused by working very close to a subject with a shorter focal length lens. It also helps relax your subjects if they get uneasy, as many people do, when a camera comes close.

As the focal length of a lens increases, the depth of field gets shallower so you must focus more carefully. Also, a long lens visually compresses space, making objects in the scene appear closer together than they actually are. The primary drawback of a long lens is that most (but not all) such lenses have a smaller maximum aperture. This may force you to use a slower shutter speed. Also, since a long lens magnifies movement, just as it magnifies the subject, you may also have to use a tripod instead of hand-holding the camera.

One thing birders are doing is called digiscoping. They use an adapter to mount a digital camera, often a point and shoot model, to their spotting scope. Many of them already have spotting scopes from which they can get great magnifications. For example, by combining a 4x optical zoom camera with a 60x birding scope, you get a combined 240x. A 35mm lens with that kind of magnification would be expensive and extra weight to carry. To get an idea of the effect an extreme lens can have on your images, turn on your digital camera and handhold it up to a binocular or spotting scope eyepiece. You should be able to see the image on the monitor although you may also experience some vignetting. Take a few pictures to see what you get.

Depth of Field Preview

To check depth-of-field some cameras have a depth-of-field preview button. Pressing this button closes the lens aperture down to the f/stop you've selected so the viewfinder gives you an idea of what's sharp and what isn't. However, when using small apertures, the viewfinder image is very dark. When the maximum aperture is selected, as it often is in dim light, you'll see no change at all.

Depth of Field Scales

Some wide-angle lenses have a depth-of-field-scale that lets you use an old technique of focusing on the hyperfocal distance. When you do so, the depth of field extends from halfway to the hyperfocal distance all the way out to infinity. For landscapes, this provides you with the deepest possible depth of field that you can obtain with the current aperture and lens focal length you are using. For action photography, you can use a variation of this technique, called zone focusing, to prefocus and set depth of field so a specific range is always in focus. If anything happens within that range you can quickly capture it without focusing.

Minimum Focus Distance

The minimum focus distance of a lens determines how close you can get to a subject. If too close, the image will be blurry. Generally shorter focal length lenses let you get closer. For example, Canon's 14mm lens will get closer than a foot and their 600mm can't get closer than 18 feet. This information is usually found in the manual or lens specifications.

OEM and 3rd Party Lenses

Canon and Nikon, with their vast arrays of lenses, have a real advantage over other camera companies that make digital SLRs. Since developing a lens lineup is hugely expensive, there are only two ways companies can overcome this disadvantage:
  • The easiest way is for the company to license the lens mount from one of the leaders so their lenses work with your camera. Fuji has done this with Nikon so you can buy a Fuji camera and use it with Nikon lenses. Sony has also acquired Minolta and is using their lens mount in a range of new cameras and lenses.
  • A more expensive way is to join with other companies to share the costs of development. Olympus (along with Kodak, Fuji Photo Film, Panasonic, Sanyo and Sigma) are doing this with the Four Thirds System.
There are also independent third-party companies that make lenses for cameras. To reduce costs, their lenses can be used with more than one camera make just by changing the mount, sometimes using an adapter. Many of these lenses are quite good and usually less expensive than those from camera companies.

Older Lenses

Digital SLR cameras use electrical connections to set the lens aperture and adjust focus. When you upgrade to digital you can't assume that your older lenses will work with your new camera. It depends on how old they are. Lenses with mechanical mounts definitely won't work, but even some older lenses that have electrical connections may not work or loose some of their features.

Newer Lenses— Looking Down the Road

Many camera companies that use smaller image sensors in their digital SLRs are introducing lenses designed specifically for these sensors. Because they create a smaller image circle, these lenses can be lighter and less expensive. However, if you ever upgrade to a model that has a full-frame sensor you won't be able to use these lenses because the image circle will be too small to cover the sensor. Camera companies make this obvious by designing the lens mount so it won't attach to a full-frame camera.

Lens Accessories

If your camera has a fixed lens, you may be able to use lens converters to decrease or increase its focal length. There are two popular accessories for cameras with interchangeable lenses. Both fit between the lens and the camera body.
  • Extenders extend the range of the lens. For example, a 2x lens converter will make a 100mm lens into a 200mm lens.
  • Extension tubes are used to increase magnification in macro close-up photography.

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