Friday, May 25, 2007
Lens Design And Technology In The Age Of D-SLRs
Lens quality has grown exponentially in recent years. The optics being produced for professionals today incorporate some high-tech miracles to get the job done.
The performance of a lens deteriorates the closer you get to the close-focus distances. The primary problem is pronounced curvature of field, which reduces sharpness toward the image borders. A similar problem exists in large-aperture lenses that suffer from increased spherical aberration. Most “normal” lenses shift a single group of elements as the focus is changed. By floating additional internal elements, these issues can be reduced or eliminated. Professional-quality lenses often float more than one group of elements, but at a high cost. The design becomes much more difficult to manufacture with multiple floating groups, and cost goes up but so does image quality.
Image stabilization, vibration reduction—depending upon whose lenses you use, the name is different. But the upshot is the same. Lenses built with this technology make it possible to shoot sharp photographs as much as four shutter speeds slower than your normal handholding shutter speed. The technology is fascinating.
The Canon Image Stabilizer uses sensors to detect motion and generate a corrective signal to reduce blur caused by camera movement. An image-stabilizing lens group along the optical axis is shifted instantaneously in response to the detected motion, providing effective cancellation of unwanted lens movement and vibration. If you're panning along with action for a background blur, there's a mode that disables a degree of the corrective effect so as not to fight the panning motion.
The Nikon Vibration-Reduction system works similarly. To compensate for image blur caused by camera shake, the angular velocity (amount of camera shake) is detected by angular velocity sensors. One sensor detects pitching, while the other detects yawing. Diagonal movements are detected by combining the results of both sensors. These sensors detect movement every 1⁄1000 sec. The angular velocity data is relayed to a microcomputer built right into the lens, which calculates the amount of compensation needed and adjustments are made instantaneously. Commands are sent to two motors, which move the VR lens in a particular direction. This is controlled by the electric current inside the motors' magnetic field. One motor controls the vertical, while the other controls the horizontal, with both working together to compensate for diagonal movement.
Ultimately, the lens is what makes the picture, not the camera. That's not to discount the technology built into the modern digital SLR, which is extraordinary, but without the lens, the best image sensor in the world would be recording just a whole lot of nothing. The lens is what brings the image to focus on the image sensor, and today's lenses are being designed and built with technology that continues to surge ahead at a furious pace.
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