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14
Nov
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. I’m not going to waste a lot of time discussing the science of optics, primarily because it’s an ancient technology. Indeed, in the Western world, Romans invented glass in the first century, and found that different shaped pieces of glass could be used for magnifying objects such as insects. In fact, early magnifiers were actually called “flea glasses” for that reason. They could also focus the sun’s rays to burn things (probably fleas); they were also called “burning glasses.”
After a mere 1,500 years of viewing fleas and setting stuff on fire, two genius Dutch spectacle makers named Janssen (father and son) figured out how to combine several lenses in a tube to produce much greater magnifications. 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.
As I noted, I’m not going to tell you much about how lenses work. Most of the rest of this section will deal with practical matters relating to interchangeable lenses on a dSLR. The only things you really need to know about lenses are these:
■ 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. The elements may be based on slices of spheres, or not (in which case they are aspherical), and given special coatings to reduce or eliminate unwanted reflections.
■ Lenses contain an iris-like opening called a diaphragm that can be changed in size to admit more or less light to the sensor. In addition to adjusting the amount of light that passes through the lens, the diaphragm and its shape affect things like relative overall sharpness of an image, the amount of an image that is in focus, the brightness of your view through the viewfinder, and even the shape and qualities of out-of-focus highlights in your image. I’ll describe these aspects in more detail as they come up.
■ 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). Also included are threads or a bayonet mount for attaching filters, a fitting that attaches to your camera, and various levers and electronic contacts for communicating with the camera body. 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.
