Sony DSC-W80/W Sony® Guide to Digital Photography (Spring 2007) - Page 10

Shooting the digital way CAMERA SYSTEMS The lens To the casual user, it's obvious that the camera takes the picture. But to accomplished photo professionals, it's really the lens that takes the picture. The lens is responsible for so much of what defines a great image, including field of view, focus (and the selection of what objects are in focus), color, contrast and detail. Focal length The angle of view that a lens takes in is most often described by the focal length (the distance from the image sensor to the lens's "rear nodal point"). Longer focal lengths correspond to narrower angles of view (telephoto). Shorter focal lengths correspond to wide angles of view. In the world of 35mm film lenses, a 50mm lens approximates the angle of view of natural human vision and is considered a "normal" lens. A 28mm lens is "wide angle" and a 200mm lens is "telephoto." "35mm equivalence" Unfortunately, lens focal lengths are related to image sensor sizes. And while a 35mm film frame is always the same size, digital camera image sensors vary greatly from model to model, even within a single manufacturer's line! The same 16mm focal length that would be considered an extremely wide "fisheye" lens on a 35mm camera can actually be a telephoto lens on a compact digital camera. To compare "apples to apples," the industry uses a "35mm equivalent" specification that converts every lens to its equivalent 35mm angle of view - regardless of image sensor size. Optical zoom Optical zoom lenses are specified by a range of focal lengths, such as 24-120mm. Because the 120mm image is magnified five times compared to the 24mm image, this is also called a "5x optical zoom" lens. Maximum aperture The lens is like a window, admitting light. The wider the window, the more light will be let in. The width of the window is called the "maximum aperture" and it's expressed as an "f" number, the ratio of the focal length, divided by the aperture diameter. For example, a 100mm lens with a maximum aperture of 25mm is an f4. This generates the same brightness as a 40mm lens with a maximum aperture of 10mm (also f4). The lower the "f" number, the brighter the lens. Zoom lenses are often brighter at the wide end than at the telephoto end. So it's not unusual to see a zoom lens specification such as 28-200mm, f2-2.8. Lens and sensor size The lens and the image sensor work together as a team. In a fixed-lens camera, the lens is carefully tailored to the specific size and resolution of the image sensor. Small image sensors work with smaller lenses - ideal for ultra compact cameras. But if you want the higher performance and creative control of a large image sensor, you'll need a larger lens to go with it. The difference becomes even more dramatic in telephoto and high magnification zoom lenses. For example, the Sony DSC-H7 15x optical zoom lens extends from 31 to 465mm (35mm equivalent). On a 35mm camera, such a lens would be gigantic. Yet the DSC-H7 is quite compact. 35mm film / 24mm lens APS-size DSLR / 38.4mm equivalent Take a lens designed for 35mm. Put it in front of an APS-size digital image sensor. Result? A focal length conversion of about 1.5x. (Sample photo for illustration purposes.) 34mm 50mm 200mm Wide, normal and telephoto views of the same subject taken with 34, 50 and 200mm focal lengths (35mm equivalent). (Sample photos for illustration purposes.) 16 CAMERA SYSTEMS Integral and interchangeable lenses Most digital cameras have "integral" lenses, fixed to the body. Some offer interchangeable lenses that can be swapped and upgraded. Both types have their advantages and Sony makes both types. (See chart for more information.) Resolution When we think of digital camera resolution, we immediately think of the image sensor. But an image sensor can only resolve the detail that the lens presents. A good lens maintains high contrast at high resolution. In fact, a special family of graphic curves called the Modulation Transfer Function (MTF) describes how well a lens maintains both resolution and contrast. Geometric accuracy The lens is also responsible for rendering straight lines as straight. While the task seems simple enough, it is difficult to achieve at the wide end of zoom lenses, which tend toward either pincushion or barrel distortion. Color convergence When Isaac Newton demonstrated that a prism could break white light into its component colors, he also demonstrated what would become a drawback in lenses. Without careful correction, lenses cause colors to break apart. You can see color fringing or "chromatic aberration" as unwanted color along the edges of objects in the picture. Chromatic aberration is especially noticeable on the edges between very bright and very dark areas of the scene. INTEGRAL LENS INTERCHANGEABLE LENSES Size Compact. Tailored to the specific size Larger. Typically optimized for 35mm image size. of the sensor. Zoom range Good. Varies by camera. Sony 15x optical zoom cameras have phenomenal range from full wide to full telephoto. Best. Varies by lens selection. However, it's often difficult to get wide angle because 35mm lenses undergo a 1.5x telephoto conversion when used with the smaller, APS-size image sensor. Live Preview off the main image sensor Yes. You see what the camera sees on the LCD monitor, with indication of white balance, exposure and depth of focus. Versatility Good. Zoom lens and conversion adaptors cover many needs. No. The main image sensor is blocked by the mirror until the moment of exposure. You frame via the optical viewfinder. Best. A selection of optional lenses gives you maximum choice. Migration No. The body and lens are permanently joined. Yes. As a system evolves, you may be able to keep your lenses and migrate to compatible new camera bodies. The a100 works with Minolta Maxxum film camera lenses from as far back as 1985! 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 0 LENS MTF CURVES KEY 10 Line Pairs/mm Sagittal 10 Line Pairs/mm Meridional 30 Line Pairs/mm Sagittal 30 Line Pairs/mm Meridional 3 6 9 12 DISTANCE FROM CENTER This set of four MTF curves describes the ability of a lens to maintain contrast (vertical scale) across the image area (horizontal scale). As you can see, a lens can't be described by a single resolution number. mm Barrel distortion (left) and pincushion distortion (right) are inaccuracies that lenses can introduce. (Sample photos for illustration purposes.) Chromatic aberration takes the form of unwanted color fringing, especially on the edges between very bright and very dark areas of the scene. (Sample photos for illustration purposes.) 17