Image sensors themselves have depth, like infinitesimally small buckets that collect photons instead of water, but on top of these buckets is a perfectly flat and annoyingly reflective surface.
“Because the surface of film was never completely flat, compromises could be made,” says John Carlson, Product Manager for Pentax. “With a completely flat sensor, lenses need special anti-reflective coatings to minimize ghosting and flare, and they also need designs that correct for spherical aberration. This is done by using aspherical lens elements and by designing lenses that minimize the field curvature, basically, flatter final lens elements.”
With the exception of Sigma’s Foveon sensor, sensors don’t have separate layers for recording Red, Green and Blue, as does film. Light from all three colors must be aligned properly, on the same focal plane, or you’ll have problems with chromatic aberrations like color fringing.
And since the original design standard for many SLR-turned-D-SLR lenses is 35mm film, the image circle they create is intended to cover the area of a 35mm frame: 24x36mm. With the Nikon D3 or the Canon EOS-1Ds camera line, there’s no need to apply a magnification factor to focal length because their sensors are the same size as 35mm film.
However, if the sensor is smaller than a full-frame, and a majority of them are, a portion of the frame becomes cropped because the image circle is much larger than the sensor actually needs. If the sensor is two-thirds the size of a full-frame, a 28mm focal length will look like a 42mm focal length, as if you had zoomed into the scene you’re photographing.
To maintain the same angle of view, which is especially important with wide-angles, manufacturers have to shorten the focal length. That’s why you often see a 35mm-equivalent focal range listed with interchangeable lenses. Because so many of us still think of focal range in terms of the 35mm standard, manufacturers include that as a reference.
That way we know a 12-24mm DX Nikkor, designed for Nikon’s DX sensors (15.8×23.6mm) will have a 27-52.5mm perspective, or a Canon 17-55mm EF-S, designed for its smaller APS-C-sized sensors (15×22.5mm), will perform like a 27-88mm EF lens, and so on.
Even though a majority of lens manufacturers make high-quality lenses that can be used with full-frame sensors, sub-full-frame or 35mm film, they saw definite advantages in making a new line of lenses for smaller sensors. Olympus decided to do this exclusively with the Four Thirds System, so the smaller sensor and the lenses were matched perfectly from the start. Panasonic and Leica have since adopted the Four Thirds System in their cameras as well.
Chuck Westfall, Director of Technical Marketing for Canon USA, says, “The benefit of a lens designed for sub-full-frame sensors is a combination of things. By making a lens that has a shorter back focus, the glass is sitting in closer to the focal plane than a regular lens. That helps us actually reduce the size, weight and cost of the lens substantially compared to a conventional lens. The other thing that becomes an issue is, because of a smaller sensor, you need a shorter focal length than you would for a regular SLR.”
If you have a telephoto zoom, you may like the fact that it has seemingly become more powerful because of the magnification factor. For your standard lens, though, you’ll probably want to invest in one of the alternative short focal-length lenses that will give you back the standard focal length to which you’re accustomed.
Steve Heiner, Senior Technical Manager for Nikon, says, “In creating lenses exclusively for Nikon’s DX format digital SLR cameras, decreasing the size of a lens to two-thirds of what’s necessary for the FX (full-frame) format wasn’t all that simple. There are many obstacles that have to be overcome, otherwise it would only make the size of conventional lenses smaller, but not improve aspects of lens performance such as sharpness or eliminating vignetting.”
Many DX Nikkor lenses have focal lengths similar to many non-DX Nikkor lenses says Heiner, but they’re designed to present a smaller image circle to the DX format sensor, thus solving the restrictions of more conventional lens designs. The goal was to achieve a good balance between miniaturization in size and overall improvement in performance.
But even before these smaller-format lenses, as of December 2000, manufacturers began modifying their lenses to improve performance with D-SLRs, without affecting their performance with 35mm SLRs. Vuslat Tatar, Sigma’s International Division Section Manager, says, “When D-SLR cameras were first introduced to the market, several problems were experienced because the image sensor surface causes ‘specular reflection,’ which causes more flare and ghosting compared to the film surface in film SLR cameras.
“The other serious problem was light falloff in the corners, due to the micro-lenses located on the image sensors, which can’t sufficiently capture the light coming from an oblique angle, thus causing vignetting. Therefore, lenses used with D-SLR cameras require a special lens coating that increases overall light transmission and a special design that aims to channel light at a more vertical incident angle that minimizes such problems.”
Pat Simonetti, National Service Manager, Tamron USA, says this is critical if a CCD or CMOS sensor is going to do its job accurately and capture the original image as it was intended. Explains Simonetti, “If the primary glass or, in this case, the lens itself gives the sensor poor-quality light or color, then the sensor will only magnify this and, therefore, give you an image with some sort of unwanted flaw. At Tamron, we were able to control these factors while not sacrificing our original design specifications. The goal was to produce these lenses without increasing the size of the lens and also maintain an attractive price point.”
Collimation or Straightening of Light Rays
hen there’s the issue of needing a lens that focuses light rays in as much of a telecentric manner as possible, especially with wide-angle lenses, says Richard Pelkowski, Product Manager of D-SLRs for Olympus. “When the light exits the rear element of the lens, the light rays are traveling straighter than they would have on a wide-angle analog counterpart from years past. Olympus needed to incorporate this telecentric design in its lenses made for digital sensors, particularly for the periphery of the image-pickup elements on the sensor. That way each pixel is capturing as much light as possible across the entire surface of the sensor, so you don’t get vignetting or a darkening around the edges of the frame.”
Low-Dispersion & Aspherical Glass
Correcting other problems like chromatic aberration and spherical aberration is done with low-dispersion and aspherical glass elements. Low-dispersion elements, with such designations as ED, LD, UD, SD, ELD and SLD, make sure the path of light through the lens is concentrated and stable, guaranteeing color accuracy and image brilliance, as well as improved sharpness and contrast.
To maintain optical quality throughout the zoom range, lenses also use one or more aspherical elements to correct spherical aberrations like pincushioning and barrel distortion and to increase sharpness. Zoom lenses, in particular, are susceptible to bowing toward the edges of the frame at very short focal lengths. Sometimes this is a desirable aesthetic as with fish-eye lenses, but often these optical distortions aren’t welcome, and aspherical glass helps keep your straight lines straight.
According to Tokina, standard lens designs are made up of a combination of spherical lens elements that have an even curve to the surface of the glass. This means that light entering the center of the lens and light entering at the edge may not be perfectly focused at the same point, which leads to unwanted spherical aberration. Wide-angle zoom lenses and wide-angle lenses with large apertures are especially at risk for spherical aberration.
The aspherically shaped surface of the lens element focuses light rays entering both the center and edge of the element correctly at the focal plane for an accurately focused image. In addition, these elements help to evenly distribute light clear to the edges of the sensor, thereby minimizing problems with vignetting. That’s why most, if not all, “designed for digital” lenses are going to have aspherical elements, in addition to the myriad of other advanced technologies we’ve highlighted. So rest assured, the next time you purchase a “designed for digital” lens, you’re definitely getting more than your money’s worth.
The Lowdown On Low-Dispersion