While digital technology has produced the most exciting images ever, it also has made lens design more difficult to fully understand. With that in mind, we take a look at what exactly it means for a lens to be designed for digital.
The light that produces an image with digital cameras behaves exactly the same as it did with film. It’s refracted by the optics to a focal point where a miniature version of the image forms. D-SLR sensors react differently than film, however. Made of tiny photodiodes that require light to be collected evenly, sensors are perfectly flat, making it even more important for light to hit the sensor at as close to a 90-degree angle as possible. For this reason, especially on the wide-angle end, lenses need to transmit light precisely, and thanks to modern technology, designed-for-digital lenses do.
The optics of a modern digital lens are made of glass element groupings with various sizes and shapes for refracting light at optimum angles to the sensor. One of the most important lens-design developments has been the advent of aspherical lenses. Aspherical optics use glass and other materials with specialized surfaces to converge light rays to exact focal points from many different directions, which reduces aberration significantly. Aspherics are particularly useful for correcting distortion in wide-angle lenses so that lenses can achieve sharpness from edge to edge. And the best thing about them is that they’re also lighter designs than traditional glass, keeping the overall weight of the lens minimal.
Bigger is often better, but when it comes to D-SLRs, that’s not always true. Smaller sensors can have big advantages when it comes to cutting costs. Because of this, most popular D-SLRs have a sub-full-frame sensor, so named because it’s smaller than the 36x24mm dimensions of traditional 35mm film (35mm refers to the diagonal length of the frame). To a manufacturer, the advantage of creating a sub-full-frame sensor is that they‘re easier to design and less expensive to produce. Thanks to the meticulous designs, sub-full-frame sensors still offer excellent quality with images big enough for most users, and the lower production costs pass savings on to the buyer so that more photographers are likely to invest in new D-SLRs.
The only problem with sub-full-frame sensors is that most camera manufacturers have their own unique design. With film, there was a unifying size to plan cameras around, the 35mm frame. With digital, instead of one standard, there are now competing sensor sizes from many manufacturers, and lenses need to be designed accordingly.
Canon, for instance, has two sub-full-frame sensors within its own line of cameras, the APS-C standard at 22.2×14.8mm and the APS-H at 28.7x19mm. Canon also offers full-frame, as does Nikon, which also has an APS-C sensor at 23.6×15.7mm. Other companies were able to build entire digital systems from the ground up. Olympus offers the Four Thirds System at 17.3x13mm. Sigma offers its Foveon design at 20.7×13.8mm, and Sony, who also has entered the full-frame market with the Sony Alpha DSLR-A900, offers a sub-full-frame APS-C at 28.0×22.3mm.
For Canon, Nikon, Pentax and other companies who already had a large base of photographers invested in their lenses, it was important to develop sensors that were compatible with preexisting lenses in order to persuade people to buy new digital cameras. Naturally, since the sensor is smaller than the size of a 35mm piece of film, and the image is being projected from a lens designed for the larger dimensions of a 35mm image circle, sub-full-frame sensors have a magnification factor. Expressed as 35mm equivalence, this standard is still being used, even for digital-specific lenses. The math by each individual sensor differs, but a 50-200mm zoom on a Four Thirds sensor, for instance, which provides a doubled focal length, actually gives you a 100-400mm zoom.
So if extra magnification is beneficial in most cases, why not use full-frame lenses on sub-full-frame bodies? For one thing, there’s the extra cost of producing full-frame lenses, which are more expensive not only because of specialized production, but also because of a lower market demand. More importantly, magnification is actually a crop factor, which reduces image quality when compared to a lens that has been designed to match a particular sensor.
With smaller sensors, the image circle (the image projected to the sensor plane) also is smaller so optics can be designed to be more compact. Less material is used to manufacture the lenses, so they’re less expensive to make. Lenses designed specifically to match the precise dimensions of a smaller sensor also provide higher resolution by matching the dimensions of a sub-full-frame image field.
Conversely, lenses designed for sub-full-frame sensors can’t be used with full-frame sensors because the image circle is too small, which would cause extreme vignetting—so extreme, in fact, that the image won’t fit the whole sensor, which produces a perspective similar to the circle of a telescope.
MORE THAN JUST OPTICS
With analog lenses, flaws in design would have required an entirely new lens to be produced. Cameras have become computers themselves, though, and lenses now feature digital technology that has been incorporated into the body. Digital communication between the lens and the camera provides autofocusing that’s precise and fast, and often lenses can be updated through firmware fixes that optimize performance even after the lens is in the hands of photographers.
Digitally designed lenses also utilize special coatings for reducing optical aberration, ghosting and flare. This is especially important with the highly reflective surfaces of D-SLR sensors. These coatings are able to shape the light as much as the glass does, and thanks to their introduction, lenses are now much lighter and more precise. All of this should give you an idea of just how complex lens design is. However, while lens construction has certainly become more advanced, the same goal remains: a sharp image without sacrificing speedy shooting.
Canon’s EF-S line of lenses are designed to meet the exact specifications of its APS-C sensors, present in the majority of its D-SLR cameras on the market. The efficient image circle is smaller and, therefore, the lenses are lighter, more compact and more affordable. The EF-S 18-200mm ƒ/3.5-5.6 IS, for instance, has a wide magnification range from wide-angle to telephoto with a 35mm equivalent of 29-300mm. The lens features an Optical Image Stabilizer for up to four stops of compensation, even at full zoom. Estimated Street Price: $599.
Nikon’s sub-full-frame (DX) format is roughly two-thirds the size of its full-frame FX sensor, providing an approximate 1.5x magnification factor, making the 35mm equivalent of the Nikon AF-S DX Nikkor 18-105mm ƒ/3.5-5.6G ED VR lens a healthy 27-157.5mm range. The lens also includes a built-in Silent Wave Motor for quiet focus and Vibration Reduction for up to three stops of camera-shake compensation, which can be seen through the viewfinder. The lens uses an Extra-low Dispersion (ED) glass element with an aspherical lens element for exact image reproduction. Estimated Street Price: $324.
Olympus offers only sub-full-frame sensors, a move that coined the term “designed for digital,” thanks to its Four Thirds System, and more recently the Micro Four Thirds System, codeveloped with Panasonic. The E-System of Zuiko Four Thirds lenses features compatibility with Four Thirds cameras from Olympus, Panasonic and Leica. Thanks to the smaller sensor sizes, Four Thirds sensors also feature a doubled focal range in a 35mm equivalent lens. The Zuiko Digital ED 9-18mm ƒ/4-5.6 (18-36mm equivalent) uses a Dual Super Aspherical lens to keep this extreme wide-angle compact at 280g and 73mm long. Estimated Street Price: $541.
Pentax’s DA lenses are designed specifically to match Pentax digital sensors in an efficient design at a good combination of price and performance. Using Extra-low Dispersion glass to compensate for chromatic aberration and other optical flaws, the 82.5-450mm (35mm equivalent) smc Pentax DA 55-300mm ƒ/4-5.8 ED zoom covers a versatile range for indoor shooting, portraiture, sports, landscape, wildlife and more. Pentax’s Super Protect (SP) coating repels dust, water and grease. Estimated Street Price: $325.
With the Sony APS-C-sized sub-full-frame sensor, the Sony DT SAL-18250 18-250mm ƒ/3.5-6.3 lens becomes a 27-375mm equivalent, and if you have Sony’s full-frame model, the A900, the lens is still compatible thanks to an APS-C capture mode in-camera. With a 14x range, the SAL-18250 is Sony’s widest focal length champ, and the long telephoto uses an internal focusing system for faster focusing and weighted balance. The optics include Extra-low Dispersion glass for minimizing flare and reducing chromatic aberration and a 16-element, 13-group lens design for correcting visual aberration across the entire zoom range. Estimated Street Price: $549.
INDEPENDENT LENS MAKERS
Independent lens manufacturers like Sigma, Tamron and Tokina are in a unique spot, as they have to design high-quality lenses in multiple iterations for the various sensor types. Thankfully, sub-full-frame digital camera sensors are close enough in size that the optics can be designed to produce an image circle adequate for sensors across brands. On the other hand, lenses need to be able to function with the individual characteristics of each system’s autofocus, TTL flash and other considerations, which is why a version of each lens must be released for each system separately.
Available in models for Canon, Nikon, Pentax, Sigma and Sony, Sigma’s 50mm ƒ/1.4 EX DG HSM lens provides an 80mm equivalent on APS-C-sized image sensors and a 100mm on Four Thirds cameras. The DG lenses in the Sigma series have bright peripheral illumination for eliminating fall-off around the corners of the image, making them ideal for use with digital sensors, and thanks to the prime design, the aperture on the 50mm is an incredibly fast ƒ/1.4 for a tight depth of field and quick shooting. Estimated Street Price: $499.
Tamron’s Di II lenses feature a design exclusively tailored for sub-full-frame sensors, providing optimum angles of view all the way to the widest zoom setting. With an extreme 15x zoom, the AF18-270mm ƒ/3.5-6.3 VC Di II (28-419mm equivalent) is compatible with Canon and Nikon APS-C mounts. The lens also incorporates Tamron’s Vibration Compensation (VC) for stabilizing camera-shake and the viewfinder image. Estimated Street Price: $599.
The DX series from Tokina are carefully crafted for the image circle of sub-full-frame sensors. Available for Canon and Nikon APS-C mounts, the Tokina AT-X 116 PRO DX AF 11-16mm ƒ/2.8 provides approximately a 16-24mm equivalence. The ultrawide zoom features a fast and constant aperture of ƒ/2.8 throughout the zoom range. The front element is Water Repellent (WR), and there’s a One-Touch Focus Clutch Mechanism for switching back and forth from manual to autofocus with your finger. Estimated Street Price: $569.