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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

fast lenses
Generally speaking, the priority for travel photography is wide focal-length range in order to minimize the number of lenses to be carried. Assuming this thinking is the priority, one of the best choices would be the SAL-18250 Zoom Super Wide-Angle Telephoto DT 18-250mm ƒ/3.5-6.3, due to its more than 13x range. However, other photographers might have other priorities, particularly lens speed, which would lead to other choices.
—Mark Weir

“In the case of an APS-C or full-frame 35mm, you have to design the optics to fit the same area of coverage, and it doesn't matter whose system it is,” Andre Costantini, Technical and Creative Supervisor at Tamron says. “The optics for all the different mounts are the same, but the linkage to the autofocus, flash, TTL and all that is different.”

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.”

fast lenses
There are a few lenses, including the SP AF17-50mm ƒ/2.8 XR Di II LD—it's very compact and it's a great wide-angle lens to shoot normal focal lengths. The SP AF70-200mm ƒ/2.8 Di LD will cover up to a full-frame, but will work fine with the smaller sensors as well. I guess it's the closest-focusing 70-200mm ƒ/2.8 that gives you a 1.3:1 magnification at 200mm. It also may have the lightest weight. We also make the SP AF28-75mm ƒ/2.8 XR Di LD, which will cover a full-frame chip and is a great option for people who are doing portraits, landscapes and that sort of thing.
—Andre Constantini

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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