We usually don’t give it much thought other than that it enables us to photograph objects at a distance or really up close, but how many of us know how a digital camera’s interchangeable lens works? Most of us take it for granted when we simply twist on the new lens to give us a different perspective, but there are a lot of intricate processes that go into bring the image to the sensor.
We generally assume that the lens is the one single piece that we clip into the lens socket on the camera body, and while that is correct, it is actually a number of lenses combined into one unit. While a single lens inside would be able to produce an image, it will be warped and distorted to the point where it won’t be usable. These multiples smaller lenses are arranged along a common axis, and if you look closely at a lens’ curved surface, you might just notice that it is coated to reduce abrasion, flare, and surface reflectance, and to adjust the colour balance. Behind all the fancy glass lenses sit the aperture and shutter control. Most lenses also have a manual focus control, which is located near the tip of the lens.
The layman’s term for image magnification in a lens is zoom, but the correct terminology would be the camera’s focal length – it is this that determines how much zoom a camera has or what type of images it can take, such as wide-angle, macro or telephoto. Focal length is measured in millimetres and is often written on the side of the lens to indicate which focal length is in use. The measurement has nothing to do with the actual length of the lens, but is all about the optical distance from the point where light rays converge, calculated to the digital sensor inside the body.
The longer the focal length, the narrower the angle of view and the higher the magnification. A good example of thinking about it, is a projector. When projecting something on a canvas, to make the slide bigger you would move the projector further away. By turning the lens to increase the focal length, the distance that the image’s light beams has to travel to get to the sensor is increased, which in turn zooms the end result.
While all lenses work the same in theory, lenses do behave according to their design. For wide-angle lenses, the focal length would be rather small (minimal zoom) but the angle of view would typically be wider than 60° – getting as much in the picture as possible. A long-focus lens on the other hand, has huge magnification capabilities as the focal length typically over 300mm, but a small viewing angle reduces the over image.
[Image – CC by 2.0/Vincent Diamante]