A portrait taken in front of a window with a great view in the backgorund sounds like a good scenario for a great photograph. However, the contrast range between your subject and the background is likely to be too great for the sensor to handle. If the camera’s meter in influenced by the background, the subject will be too dark, maybe even a silhoutte. To get a correct exposure on the face you’ll need to take a selective light reading, though of course all the background detail will then be overexposed. The only way to balance the two is to increase the light level on the face (using either reflectors, or flash) to more closely match the background level.

Bracketing

Bracketing allows you to shoot additional exposures either side of your first reading, giving you a greater chance of obtaining an optimum exposure. The camera calculates the extra exposures based on your initial reading and by what increment you wish to bracket your shot by. It then takes a series of exposures at the different settings in quick succession.

Interchangeable lenses are not unique to single lens reflex cameras, of course. Film cameras that use optical viewfinder windows (called rangefinder cameras), such as the Leica, also have interchangeable lenses. Early in my career I used a Mamiya twin lens reflex camera, which allowed unhooking both lenses (one for viewing, one for taking the picture) and substituting another set.

Swapping lenses lets you change the “reach” of a lens, from wide angle to medium telephoto to long telephoto. Interchangeable lenses let you choose a lens optimized for a particular purpose. Using an SLR lets you choose a lens, whether it’s a zoom or a fixed focal length lens (called a prime lens) that does a particular thing very well indeed. With the availability of interchangeable lenses, you can select a very fast, f1.4 lens when you need one, or choose a lens that’s particularly good in a given zoom range (say, 12-24mm). As you know, however, lenses aren’t infinitely interchangeable. Lenses designed to fit on one particular vendor’s brand of camera probably won’t fit on another vendor’s camera (although there are exceptions), and it’s highly likely that you’ll discover that many lenses produced by the manufacturer of your digital SLR can’t be used with current camera models.

The first thing to realize is that lens compatibility isn’t even an issue unless you have older lenses that you want to use with your current digital camera. If you have no lenses to migrate to your new camera body, it makes no difference, from a lens standpoint, whether you choose a Nikon, Canon, Minolta, Olympus, Pentax, or another dSLR. You’ll want to purchase current lenses made for your camera by the vendor, or by third parties such as Sigma or Tamron, to fit your camera. In that case, you’ll be interested in whether that older lens will fit your new camera.

It’s easier to design a whole new line of lenses for a new camera system than to figure out how to use older lenses on the latest equipment. Some vendors go for bleeding edge technology at the expense of compatibility with earlier lenses.

The next most important component of a digital SLR is the lens-or, more properly, lenses- because, unlike other types of digital cameras, the lens of a dSLR is interchangeable. Galileo and Leeuwenhoek came up with improved telescope and microscope gadgets, and most of the optical breakthroughs hence have involved different kinds of shapes of glass and other materials (including non-spherical “aspheric” elements) special coatings, and clever combinations of lenses to create zooms, fish-eyes, and other innovations.

Lenses consist of precision-crafted pieces of optical glass (or plastic or ceramic material) called elements, arranged into groups that are moved together to change the magnification or focus. Lenses contain an iris-like opening called a diaphragm that can be changed in size to admit more or less light to the sensor. Lenses are mounted in a housing that keeps the elements from rattling around and provides a way to move them to adjust focus and magnification. The lens housing can include a microprocessor, a tiny motor for adjusting focus (and, in non-dSLR cameras, for zooming), and perhaps a mechanism for neutralizing camera shake (called vibration reduction). You might find a switch or two for changing from autofocus to manual focus, locking a zoom lens so it doesn’t extend accidentally while the camera is being carried, and a macro lock/lockout button to limit the seeking range of your autofocus mechanism so your lens won’t seek focus from infinity to a few inches away every time you partially depress the shutter release.