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Tuesday, May 26, 2009

Lens Tech

What’s under the hood of modern photography optics?


This Article Features Photo Zoom

lens techWhat is a photographic lens? Essentially, just a tube that holds a number of glass or plastic elements that make light do something we want it to do: produce an image of the subject/scene at a certain magnification and angle of view, sharply focused on the image plane in the camera. A focusing ring adjusts the elements so the lens can focus on subjects at various distances, an aperture ring controls the diameter of the opening through which the light travels so we can control exposure and depth of field, and on a zoom lens, a zoom ring adjusts the elements to provide various focal lengths. Pretty simple, right?

Hardly! The elements inside the lens do their thing by refracting (bending) the light as it travels through the tube. Bending light rays produces the desired effects (a specific focal length and angle of view, focus at a specific distance, etc.), but also a number of undesired effects: aberrations and distortions. Dealing with these unpleasant characteristics requires clever design. State-of-the-art technology goes into today’s optics and lens construction.

Aspherical Elements

Conventional spherical lens elements can’t focus light rays coming through the edges of the lens at the same plane as light rays coming through the center of the element. The result: When the area of the subject in the center of the photo is sharp, areas at the edges will be unsharp (and vice versa). This is called “spherical aberration.” Spherical aberration is especially a problem with fast (large-aperture) lenses.

One solution to spherical aberration is to use an aspherical shape for the curved surface of the lens element. This “uneven” curve focuses rays at the same plane regardless of where they go through the element. Aspherical lens elements are difficult to produce, which drives up the cost of a lens. But the improvements in image quality make them well worth it. Aspherical elements help reduce distortion in wide-angle lenses, as well as minimize spherical aberration.

Precision ground-glass aspherical elements are very expensive. Manufacturers now also use molded aspherical elements (produced by shaping the glass in molds instead of by machining it) and compound or replica aspherical elements (produced by forming a resin aspherical layer on a spherical glass element). All are helpful in producing large-aperture lenses with minimal spherical aberration, wide-angle lenses with minimal distortion and compact high-quality zoom lenses.

Ultralow-Dispersion Elements

A simple lens element can’t focus wavelengths of all colors at the same plane. Green (middle) wavelengths are focused at one plane, blue (shorter) wavelengths are focused closer to the lens than the green ones, and red (longer) wavelengths are focused behind the green ones. The result is called “longitudinal (or axial) chromatic aberration.”

Chromatic aberrations cause a loss of image quality and those annoying color fringes you sometimes see around subject edges in high-contrast situations like backlighting. Stopping the lens down can reduce axial chromatic aberration, but not tangential chromatic aberration. Chromatic aberrations are especially problematic with telephoto and tele-zoom lenses, and wide-angle lenses.

Lens designers correct for both types of chromatic aberration by using combinations of exotic glass elements that have different dispersion characteristics. Proper combinations will cancel out one another’s aberrations, resulting in all wavelengths being focused at the same plane and all wavelengths from a given point on a subject focused at the same point in the image.

These exotic glass elements have designations such as LD (low dispersion), UD (ultralow dispersion), ED and ELD (extra-low dispersion), SD and SLD (super-low dispersion), AD (anomalous dispersion) and fluorite.


 

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