Monday, April 28, 2008
Fast Lenses On Sub-Full-Frame Sensors
Quick, inexpensive and light, sub-full-frame lenses aren't just for the amateur market anymore
The speed of an interchangeable-lens system is only as good as the quickness of autofocusing. A motor incorporated into tele-zoom lenses enables them to focus at various focal lengths by enacting supersonic or oscillating movements to find the right area of focus for your subject.
Ultrafast focusing motors generally fit one of two designs—internal focusing (found in most telephoto zooms) or rear focusing (found in most wide-angle lenses). Inner focusing provides focus that's found by moving the inner lens group of elements, which is positioned ahead of the aperture diaphragm, making it easier for longer focal lengths to achieve a fast focus when the front lens element isn't moving. Rear focusing uses lens elements behind the aperture diaphragm, which has the same benefits as internal focus, but in a shorter focal length. Ordinary lenses without a drive motor are slower because they have to move all the lens elements to achieve focusing.
“One reason these lenses focus quickly is their inner-focus or rear-focus optical designs,” says Westfall. “Another reason is the use of an Ultrasonic Motor. For example, only two out of 23 elements in the EF 70-200mm ƒ/2.8L IS USM lens are used for focusing. So, even though the total lens weighs over three pounds, the focusing components weigh only a few ounces and can be moved quickly and precisely by the Ultrasonic Motor.”
The Need For Speed
One of the greatest benefits of having a fast lens for an APS-sized camera is the ability to focus much faster and achieve low-light shots. When trying to quantify how fast a lens is going to be, the ƒ-stop and other optical elements are weighed against greater control and better performance. To maintain a constant aperture in any lens takes more components than a lens with a variable aperture rating, because a constant aperture in a zoom lens has to constantly refocus to maintain such a wide-open aperture.
“It's all a matter of optical design and actual lens-element size,” says John Carlson, Product Manager for Pentax. “The larger that first lens element, the more light the lens gathers, giving the larger maximum aperture. Additionally, both of our DA* zoom lenses have aspherical lens elements that compensate for spherical aberrations that can occur when using these larger lens elements.”
The optics of a lens are comprised of groupings of glass elements of various sizes and shapes, necessary to refract light at the optimum angle to the sensor. Constant apertures will require multiple groupings of these lens elements. Other factors in between, such as image stabilization and coated low-dispersion glass elements (which can vary per lens), all play a part in the speed and weight of any digital lens. The mechanical speed of a lens, since day one, is determined by the size of the rear opening of the lens and the aperture, as well as the focal length of the lens, which determines the angle of view and the size of an object's image.
“Until it hits the sensor, the light behaves the same with digital as it would have been with film,” explains Michael Burnham from Tokina. “The light and the colors of the spectrum all have to be focused at the same point on the CCD sensor. The one thing that's different with digital, and with digitally made lenses, especially on the wide-angle side, is that you're trying to get the light rays to strike the sensor to as close to a 90-degree angle as possible. Basically a CCD or CMOS sensor is made up of little pixel sensors, and each of those sensors sits in a little site and has a little cupped lens over it. Any light striking it at too strong an angle is either going to bleed over into another pixel, or it could be possibly out of focus, depending on how the lens is made.”
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