The aperture is the opening in the lens through which light gets to the sensor (or film). Adjusting its size—"stopping down" or "opening up"—controls the amount of light that reaches the sensor or film. The aperture is described by an ƒ-number, which is the ratio between the diameter of the aperture opening and the focal length of the lens; ƒ/4 means the diameter of the opening is one-quarter the focal length—25mm on a 100mm lens. From a practical standpoint, the effective aperture is the ratio between the size of the entrance pupil (the aperture as viewed through the front of the lens) and the focal length. Movie lenses are often calibrated in T (true) stops, which are based on the measurement of light actually transmitted by that individual lens rather than calculated.
Obviously, the larger the opening, the more light transmitted. So fast lenses—those with large maximum apertures—are desirable when shooting in dim light: You can shoot at a faster shutter speed or use a lower ISO setting in a given light level. Wider apertures also produce shallower depth of field, handy when you want to concentrate the viewer’s attention on a specific subject (or portion of the subject). On the downside, fast lenses are bulkier and more expensive than slower ones.
Number Of Aperture Blades
Lenses with a greater number of aperture blades have "rounder" apertures, even when stopped down, which produces a more pleasant blur effect in backgrounds ("bokeh") and foregrounds than the polygonal-shaped apertures produced by fewer blades. Rounder apertures also produce rounder, more pleasing specular highlights.
Simple lens elements focus different wavelengths at different distances behind the lens (longitudinal chromatic aberration) and light rays traveling at an angle through the lens at different positions, depending on their wavelengths (lateral chromatic aberration). These aberrations appear as colored artifacts in color images, but also reduce the sharpness of monochrome images. Chromatic aberrations are most evident in supertele lenses, but can also occur in shorter designs. Low-dispersion and extra-low-dispersion glass elements can minimize chromatic aberrations. These elements have designations such as LD, ED, ELD, SD, SLD, HLD, FLD and UD. Fluorite elements are especially good at reducing chromatic aberrations, but are very costly to produce, and thus found mostly in high-end pro lenses. Canon’s Diffractive Optics (DO) also effectively reduce chromatic aberrations, while also permitting more compact lens designs.
If you shoot handheld, you want a lens with built-in image stabilization (unless you use a camera that has sensor-shift stabilization, which works with all lenses; Pentax and Sony DSLRs offer sensor-shift stabilization).
Canon’s IS, Nikon’s VR, Sigma’s OS, Tamron’s VC and Tokina’s VCM lenses have built-in optical image stabilizers, which move a special group of lens elements to counter camera shake. In the mirrorless world, Pentax and Samsung OIS lenses provide optical stabilization (Olympus mirrorless cameras have sensor-shift stabilization). These systems really work, allowing you to get sharp handheld images at two to four shutter speeds slower than would be possible otherwise.
In Canon’s original IS system, a vibration gyro detects the angular velocity component of lens motion caused by camera shake and transmits this data to a microcomputer, which converts it to a stabilizer optical system drive signal, which is transmitted to the magnet-and-coil stabilizer optical system drive circuit, and the compensation is applied by shifting the compensating element group appropriately. Stabilization Mode 2 lets you pan the camera to track action subjects while still having compensation for vertical camera shake. Tamron’s VC system uses a tri-axial configuration with three pairs of driving coils and low-friction ball bearings around the lens’ shake-compensating group to, in effect, provide a free-floating shake compensation for diagonal as well as up-down and side-to-side shake. Olympus’ higher-end mirrorless cameras provide 5-axis image stabilizing, compensating for horizontal shift, vertical shift, yawing, pitching and rolling motions. Generally, new stabilization systems are more effective than earlier ones, but even the early ones are much better than no stabilization.
Conventional spherical lens elements focus light rays traveling through the center farther behind the lens than rays traveling through the edges. This is called spherical aberration, and it’s most evident in shorter focal lengths and at wider apertures (especially with fast lenses). Aspherical elements can minimize spherical aberration, as well as distortion, so are often found in wide-angle and fast lenses, as well as zooms.
Internal focusing means that elements move inside the lens barrel to focus, so the physical length of the lens doesn’t change during focusing, and the front of the lens doesn’t rotate (handy when using orientation-sensitive filters like polarizers and graduated ND filters). Because only lighter internal elements move during focusing, manual focusing has a lighter feel and autofocusing is faster. Internal focusing also produces closer minimum focusing distances, helps reduce vignetting and allows for more compact lens designs. The downside is that the focal length can be reduced at closer focusing distances, especially with zooms. It’s not uncommon for a 70-200mm zoom’s maximum focal length to reduce to 150mm or less at minimum focusing distance.
The first AF SLRs had focusing motors in the camera body, which physically turned a key in the lens to adjust focus. These were relatively noisy and inefficient, and you could damage the motor by rotating the lens’ focusing ring while in AF mode.
Canon came out with the EOS system, which featured a new all-electronic lens mount and the focusing motor in the lens rather than in the camera body. This had a drawback, where owners of previous Canon SLRs couldn’t use their lenses on the new EOS bodies, but the advantage of quicker, smoother AF operation, with each lens containing an AF motor optimized for it.
Today, most new AF lenses have focusing motors, even those from companies like Nikon, Pentax and Sony, whose DSLR bodies contain AF motors so they can also use older motorless AF lenses (and manual-focus lenses). (If a lens with an AF motor is attached to a body with an AF motor, most systems deactivate the in-body motor. And
note that entry-level Nikon DSLR bodies don’t have AF motors; they must be used with lenses that have AF motors if AF is desired.)
Canon introduced the Ultrasonic Motor (USM), which makes for quicker, quieter autofocusing, and in higher-end versions (those with manual focusing rings and focusing scales), allowed you to fine-tune focus manually without leaving AF mode. While conventional motors convert electromagnetic force to rotational force, USM generates force from ultrasonic vibrational energy. This provides high torque at low speeds, quick starting and stopping, great precision and quiet operation. Today, most lens makers offer lenses using similar AF motors (Nikon AF-S, Pentax SDM, Sigma HSM, Sony SSM and Tamron USD and PZD lenses, for example).
When video capability was added to DSLRs, noise emitted by the AF motor took on new importance. So new, quieter AF motors have been produced especially for video, such as Canon’s STM (Stepping Motor). But Canon USM, Nikon SWM, Pentax SDM, Sigma USM and Sony SSM motors are very quiet. A tip: Pro video shooters generally record sound via an external microphone connected to an external recorder, not using the camera’s built-in mic, which can pick up all the camera noises.
A lens’ focal length is the distance from the "optical center" of the lens (technically, the rear principal point) to the image plane when the lens is focused at infinity. The focal length determines the magnification (longer focal length = bigger image of the subject at the focal plane) and the field of view (longer focal length = narrower angle of view). Note that sensor size affects apparent magnification (smaller sensors "crop in" on the image formed by the lens, making the subject fill more of the frame even though the size of the image produced by a given focal length at the focal plane is the same) and field of view (the smaller sensor crops out some of the image that a larger sensor would include; see Figure A).
Each glass-air interface in a lens can produce reflections and flare that reduce contrast and sharpness, along with the amount of light transmitted to the image sensor (or film). So most lenses have their elements coated with antireflection coatings. Multilayer coatings are even more effective, and today we even have nano-coatings, whose thickness is less than the wavelengths of visible light.
Number Of Elements And Groups
It’s impossible to correct all the lens aberrations and distortions with a single element, so lens designers use multiple elements, singly and in groups, to more effectively deal with all the things that can reduce lens performance.
A single-focal-length supertelephoto lens can work with just a few elements (Pentax’s DA 560mm ƒ/5.6 ED AW supertele makes do with just six elements, including two ED ones, in five groups), while a wide-range zoom might require two dozen elements to deal with all the aberrations at a wide range of focal lengths (Canon’s EF 28-300mm ƒ/3.5-5.6L IS USM uses 23 elements, including three UD and two aspherical ones, in 16 groups). More elements and groups generally mean better correction of aberrations and other lens faults (and a heavier lens), especially when those elements include low-dispersion and aspherical ones.
Designed For Digital
Early DSLRs were adapted from their manufacturers’ 35mm film SLRs and used the same lenses. But digital sensors aren’t film. Digital sensors are flatter, and each pixel is a little "light well" that has depth, so the light has to strike its surface more directly. Additionally, image sensors are highly reflective, and can cause reflections inside a lens designed for film use. So manufacturers have been updating their lenses for digital, using designs and coatings that reduce these problems.
"Designed for digital" sometimes means designed specifically for smaller (than full-frame) digital sensors: APS-C and Micro Four Thirds. This allows the manufacturer to optimize performance for the smaller sensor and produce smaller lenses. But keep in mind that lenses designed for smaller sensors can’t be used on full-frame DSLRs (or 35mm SLRs) because their images would vignette. Note that if you attach a DX (APS-C) lens to a full-frame Nikon DSLR, the camera automatically will crop the image to APS-C format. The same is true for Sony full-frame DSLRs when a Sony DT (APS-C) lens is attached. Canon’s EF-S APS-C lenses can’t be mounted on full-frame (or APS-H) EOS cameras, and all Pentax DSLRs have been APS-C. You can use full-frame lenses on smaller-format DSLRs.
Flange-back distance is the distance between the camera’s lens mount and the image plane (the image sensor, in a digital camera). A DSLR must provide room for the SLR mirror assembly, so it has a fairly long flange-back distance. Mirrorless interchangeable-lens cameras don’t have SLR mirrors, so they don’t have to be as thick; mirrorless cameras can have shorter flange-back distances.
The mirrorless cameras’ short flange-back distances provide a couple of benefits, besides smaller cameras. First, you can use just about any lens for which an adapter is available on a mirrorless camera because, even including the thickness of the adapter, the lens can still focus out to infinity. If you add an adapter’s thickness to the DSLR’s already longer flange-back distance, the lens might not be able to focus out to infinity. Second, lenses can be smaller. A wide-angle lens for a DSLR must employ a retro-focus ("inverted telephoto") design because the typical compact symmetrical wide-angle design would require the lens to protrude into the camera body, interfering with the mirror. Mirrorless cameras can use more compact wide-angles of the symmetrical type since there’s no mirror.
The flange-back distance for Sony E-mount mirrorless is 18mm, for Micro Four Thirds, 20mm, for Leica M, 27.85mm, for Canon EOS, 44mm, for Sony A mount, 44.6mm, for Pentax K mount, 45.46mm, and for Nikon F mount, 46.5mm. Lenses designed for longer flange-back distances (e.g., Nikon F) can easily be adapted to cameras with shorter ones (e.g., mirrorless), but lenses designed for shorter flange-back distances (Leica M, for example) can’t be used on bodies with longer ones (Nikon DSLRs, for example).
With modern mass-production capabilities, quality is very high, but sometimes a given lens and a given camera body may not work perfectly together, even if both meet production tolerance criteria. If the acceptable tolerances are +/-5, and the body is +5 and the lens is +5, the combined +10 offset will result in a noticeable loss in image quality. Back in the film era, when we didn’t "pixel-peep" at 100% or more, we didn’t notice such problems. With 36-megapixel digital cameras and big LCD monitors, we do.
So, most DSLRs today come with a feature that lets you test and adjust your specific camera/lens combo(s) to correct for individual lens/body quirks. Canon and Sony call this AF Microadjustment, Nikon, AF Fine Tune, and Pentax, AF Fine Adjustment. If your autofocused images don’t seem quite sharp, shoot an image of a test subject, then apply some microadjustment (generally, +/-20 are provided, in tiny steps), and shoot another image. If the second image looks sharper, apply some
more adjustment in the same direction and try another image until the result becomes less sharp, then go back to the previous correction setting. Conversely, if the second image looks less sharp than the original image, apply adjustment in the other direction. It’s a bit tedious, but not all that difficult. Products like Zig-Align make it easier and more accurate. Most cameras allow you to store corrections for a number of lenses, and will apply the correction whenever a tested lens is mounted.
Lens designers also counter internal flare and reflections via baffling, a process that mechanically reduces internal reflections. This is particularly important with digital, as reflections from the shiny sensor surface to the lens rear element can greatly reduce image quality. Some photographers create their own baffling for legacy full-frame lenses when adapting them to smaller-sensor cameras.