May 2000
President's
Message
Portrait of Steve Scott
Article: Seeing the Big Picture
Article: Q&A Forum
Question & Answer Forum
- Erik Kissa
The following questions were submitted to
Erik Kissa as a result of his recent offer to answer general photographic
puzzlements. Erik kindly offers to answer any questions, excluding digital
photography and manipulations at (302) 478-6737 or ekissa@
aol.com.
What is an aspherical lens?
A conventional lens consists of several elements (single lenses), which have surfaces shaped like a section of a sphere. The shape of the element is defined by the surface area of the lens and the radius of the curvature. Usually the front and the back surfaces of a element are not similar, the element does not have to be symmetrical. To correct various aberrations of the lens, several elements are needed, especially for wide angle lenses of a large aperture. This adds to the weight and the cost of the lens and may not cure all optical problems entirely. The aspherical lenses contain one or two aspherical elements which are not shaped like a section of a sphere but have an irregular curvature. Hence the name aspherical. Aspherical elements can effectively correct aberrations and minimize distortion. This allows one to design a lens with a smaller number of elements and, consequently, to reduce the size and the weight of the lens. Aspherical elements are found mainly in wide angle lenses, but in some telezooms as well. For example, the Sigma APO 135-400mm and 170-500mm zoom lenses are aspherical. Some aspherical elements are made by precision grinding of a glass element. This is a very costly process, for example the Nikkor f/1.4/24mm costs about $1,500 and the Nikkor f/2.8/20-35mm costs about $2,200! In contrast, molding of glass or a resin is a low cost operation for making aspherical elements. Most aspherical elements used in quality lenses are hybrids made by molding a resin onto a glass element to give it an aspherical shape.
Are aspherical lenses sharper than the conventional lenses?
Not necessarily, aspherical is not synonymous with high quality. For example, the super sharp Nikkor f/2.8/28mm is not aspherical. The aspherical elements are used mainly to reduce aberrations and distortion. For example, almost all Sigma wide angle lenses and zooms with a wide angle are aspherical. The aspherical elements allow the lens to be made smaller and lighter and they improve the overall optical performance, of which sharpness is only one facet.
What are the low dispersion elements in some lenses?
One of the aberrations common in telelenses is chromatic, the rays of a different wavelength (color) do not focus at the same point. Chromatic aberrations are corrected with one to three low dispersion elements. Calcium fluoride (fluorite) used in the past for making low dispersion elements is brittle and temperature sensitive. Nowadays special low dispersion glasses are used instead. Low dispersion elements improve sharpness but increase the cost of the lens. For example, the Sigma APO f/4-5.6/70-300mm zoom has three low dispersion elements, while the DL version of this lens has only one. The price difference: $120.
Which camera bags and cases are most useful?
Here are my favorites:
1. When I travel by car, I use the Firmani Supercase. This magnificent
case is a 18" long Arctic Zone cooler equipped with cardboard portioning
obtained from a wine case. The cooler costs about $30, but discounts may
be available. The advantages of the Firmani Supercase: Rigid sidewalls
offer solid protection. Insulation keeps camera and film cool. Plenty
of space for cameras, lenses and accessories. Stealthy, not an obvious
camera bag. Affordable price. On location I transfer the immediately needed
equipment and film into a photographers vest.
2. When I travel by air, I use the Lowepro Trekker AW backpack (the letters
AW indicate an air-line regulation carry-on). The Mini-Trekker AW (about
$ 110) fits under the seat and holds two 35mm cameras with telelenses
on, two other lenses, two flash units, film, diffuser, filters, small
flashlight, a sweater, a plastic poncho etc. On location I load some of
the equipment and film into my vest.
3. A very versatile bag is the Tenba convertible slim pack P215 (about
$120, if still available). A rugged insert has four partitions for camera
equipment and a zippered outside partition. The insert can be removed
and on my lecture tours I used the bag without the insert as a briefcase
as well. By opening a zipper the bag can be expanded to a spacious (over
8" wide) camera case.
My other camera bags, including a pro Tamrac, are used for storage and
seldom leave the house.