Friday, February 29, 2008

Lenses: Designed For Digital

Far from simple marketing hype, the term "designed for digital" encompasses the entirety of lens technologies unique to the demands of image sensors and their various sizes.
By Harlon Mitchell Published in Lenses
Lenses: Designed For Digital

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

Then 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

  • Low-Dispersion (LD) glass elements, used in Tamron lenses, are made from special proprietary glass materials with extremely low dispersion indices (an inherent tendency for glass to disperse or separate light rays into the colors of the rainbow). LD glass, in all its various incarnations, makes sure the path of light through the lens is concentrated and stable for the best possible color accuracy, image brilliance, sharpness and contrast.
  • Extra-Low-Dispersion (ED) glass elements can be found in lenses by Nikon, Olympus, Pentax and Sony.
  • Ultra-Low-Dispersion (UD) glass is a special type of optical glass developed by Canon whose properties nearly match those of fluorite. Fluorite, which is crystalline, has abnormally low refraction and dispersion characteristics, which optical glass can't achieve. The effect of two UD-glass elements gives almost the same effect as one fluorite element.
  • Extraordinary Low-Dispersion (ELD) glass was developed by Hoya (the glassmaker) and is currently used in various Sigma lenses. It has even lower dispersion characteristics than Special Low-Dispersion (SLD) glass, which Sigma has been using in its APO lenses (and some non-APO lenses as well) for some years now. Like the other types of low-dispersion glass, both types improve color accuracy,
    image sharpness and contrast by minimizing chromatic aberration.
  • Super-Low-Dispersion (SD) glass is the special proprietary glass used in select Tokina lenses. These elements, with extremely low dispersion characteristics, minimize the secondary spectrum or optical noise caused by chromatic aberration, which can result in color halos or "purple fringing" around details in your photograph.

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