How good, how useful, and how expensive are pickings from the the latest crop of 24+ zooms? See if you can identify the 24-90mm f/3.5-4.5 Pentax; 24-105mm f/3.5- 4.5 Minolta; 24-120mm f/3.5-5.6 Nikon; 24-135mm f/3.5-5.6 Tamron; 24-200mm f/3.5-5.6 Tokina.
原载于美国大众摄影
http://www.popphoto.com/Camera/Arti...sp?ArticleID=17
By Herbert Keppler
Wide-to-tele lenses now zoom down to 24mm focal length. Marketing ploy or worthwhile added coverage?
How do the new zooms compare?
So here you are with your faithful all-purpose 28-105mm zoom—or maybe you're still using a past-generation 35-105mm or getting by with a budget-priced 28-80mm or thereabouts.
Now the wide-angle end of tele-zooms has been dropped to 24mm by seven major optical players.
Credit Minolta for starting the 24mm ball rolling way back in 1994 when they replaced their 28-85mm f/3.5-4.5 with a 24-85mm f/3.5-4.5. We tested it and thought it a sensational lens. But Minolta did little to promote it, explaining that consumers thought the $684 list price ($340 street price today) was too dear. I became a one-person, 24-85mm Minolta cheering section, with little success. Nikon, usually credited with letting other companies innovate optically and then perfecting the designs, astounded the SLR world in 1997 with a 24-120mm f/3.5-5.6 zoom, ostensibly for the amateur market. But pros, particularly wedding photographers, glommed onto the lens, and use it extensively to this day.
Canon, the customary optical pack leader, took three years to catch up with Minolta, getting their own USM (ultrasonic motor) version in 1997. The latest to arrive on the scene is the autofocus 24-85mm f/3.5-4.5 Zeiss Vario-Sonnar for the new Contax N1.
We should have realized from the Minolta and Canon 24-85mm lenses that 28mm (like 35mm before it) was on its way out as the minimum wide-to-tele focal length and would be replaced by 24mm. If Nikon had the optical know-how to stretch a zoom from 24- to 120mm, why couldn't other lens designers break out of the 85mm-maximum-focal-length rut, too?
We were worried that other lensmakers would copycat, and all produce 24-120mm lenses since Nikon was so successful with theirs. Thankfully, Minolta, Pentax, and Tokina have gone their own ways in focal lengths offered, as you can see by perusing the photographs and charts herein. And—optical miracle of miracles, the lenses actually were often smaller and lighter than the 28mm-minimum-focal-length zooms they replaced.
Replacing? You don't think lensmakers will long continue making and trying to sell 28-105mm lenses if rivals are successfully selling 24-105mm lenses, do you?
So here we go. Smaller, lighter, 24mm minimum focal length—but higher priced.
Worth it? This will depend on optical quality, useful additional coverage, and convenience.
Optical quality: Every 24mm wide-to-tele zoom we've tested so far, either by SQF or by lines per millimeter, has gotten rave reviews: 24-85mm f/3.5-4.5 Minolta: "An incredibly good performer, a highly recommendable and desirable lens."
24-85mm f/3.5-4.5 Canon: "Well above-average optical performance, excellent contrast and sharpness at all apertures and focal lengths."
24-120mm f/3.5-5.6 Nikon: "The most outstanding large-ratio zoom lens ever tested by us."
24-105mm f/3.5- 4.5 Minolta: "Excellent central or very good resolution throughout the zoom range." Our tests showed lower results only at f/27, a
seldom-used aperture.
24-135mm f/3.5-5.6 Tamron: "Remarkably high and consistent imaging performance."
Yet to be tested: 24-90mm f/3.5-4.5 Pentax and 24-200mm f/3.5-5.6 Tokina, but we expect them to perform equally well, with the Tokina holding its own against the competing army of rival 28-200mm zooms. However, note from our chart that the Tokina close-focuses to a constant 31 1/2 inches at all focal lengths with a maximum magnification of 1:6 only at 200mm; magnification is lower at shorter focal lengths. Most 28-200mm zooms have variable close-focusing at all focal lengths.
Additional coverage: What's so hot about a silly little 4mm maximum wide-angle gain? Well, it's not silly, and 4mm at the wide-angle end of your zoom is a lot more coverage than you probably expect.
When we talk lens coverage, the most used term is "angle of coverage." While this can be measured across a picture's horizontal or vertical edge, the most universally accepted angle measurement is made by measuring the diagonal of the film coverage.
The diagonal covering angle of 35mm focal-length lenses (as in the old 35-105s) is 63 degrees. The covering angle of 28mm is 75 degrees, while our new minimum of 24mm is 84 degrees. So we gain an additional 9 degrees of coverage going from 28- to 24mm, or 12 percent more coverage.
How much do we narrow the angle going from 120 to 135mm? That's a 15mm focal-length difference; so how does the covering angle change? Would you believe it changes by only 2 1/2 degrees (from 20 degrees 30 minutes to 18 degrees)?
So, in terms of coverage difference, you get significantly more for your money from each mm of focal-length differential at the wide-angle, 24mm end of the zoom than you do at the 135mm end. Look at our comparison picture chart. See the difference in picture coverage?
I think you'll find the extra coverage at 24mm useful, both indoors and out.
Of course, if you're already covered at 24mm with another lens, you technically don't need a 24mm-minimum zoom. But in practice, having 24mm at your command without having to change lenses is very handy indeed.
In terms of physical convenience, the new 24mm+ lenses are every bit as convenient as the 28+ they have replaced—and, as we've said, often smaller and lighter too.
If you don't have a 28-85mm, 28-105mm, 28-135mm or 28-200mm zoom, but were thinking of buying one, by all means screw up your courage and your bank account, and go for 24mm. You won't regret it, as I hope I can convince you of its charms.
If you already have a 28mm+ variety, buying a 24mm+ seems frivolous, particularly if your wide-angle needs are already covered with a single-focal-length or zoom reaching 24mm (an 18-35mm zoom, for instance).
If your heart still yearns for one of the 24mm variety, maybe you can convince a rich uncle or aunt to spring for one on your next birthday.
How are the lensmakers achieving their new 24+ miracles? Designers of lenses for 35mm and APSpoint-and- shoot cameras have been forced to adapt ridiculously small apertures at tele position in order to stretch zoom-focal-length ranges. But 35mm SLR lens designers have made the stretch to 24mm while maintaining maximum apertures. Technical optical explanations are often over my head and maybe yours, but we've asked a number of makers to explain how the new lenses have been accomplished.
Can we expect other manufacturers to go 24+ with their lenses? I expect so, but can't give you any time frame. If I were a lensmaker, I'd stick a rush label on it. Will the wide-angle floor for lenses sink even lower in the future? Are there 20-105mm, 20-120mm, 20-135mm, or 20-200mm lenses in our future? Maybe, but not for some time.
How Minolta stretched 24-85mm to 105mm
"To increase our 24-85mm f/3.5-4.5 lens to 105mm we normally would need to extend the moving distance of each zoom lens group. To shorten this distance, we had to increase the refractive power of each lens group as well as reduce the number of lens elements (the 24-85mm has 14). Such a change would cause an increase in aberrations.
"We usually use spherical elements in our zooms and try to minimize the aberrations as much as possible. But then we replace some of the spherical lenses with asphericals to reduce the remaining aberrations. However the 24-105mm was designed to use aspherics at the start. The formulation would be similar to our 3X zoom lens/shutter camera design where we had used four elements only. Three aspherics would be required. The front aspheric surface of the fourth lens element, a composite, would reduce distortion mainly at wide-angle settings. The last element would be a double-sided, glass molded aspheric, the first we have ever used. These two aspheric surfaces would correct spherical aberration and also reduce image degradation caused by this group's decentration aberration—which had been the weakness of this type of zoom. By these means, we were able to reduce the number of required lens elements from 14 to 12, making the lens not only shorter but lighter."
-Minolta Corporation.
"In order to minimize spherical aberration and maximize resolution in our 11 group, 13 element 24-90mm f/3.5-4.5 lens, a unique hybrid aspheric, developed by Pentax, was used for the fourth lens element and a high-quality, molded glass aspheric for the 13th. By using aspherics fewer lens elements were needed, making the lens small and light.
"In conventional front-focusing lenses, marginal illumination is sometimes reduced. Also, with conventional lenses the front lens element must be made larger to provide macro-focusing capability. The use of internal focus technology eliminates both of these problems. With internal focusing, marginal illumination is consistent and the front element can be made smaller while still allowing macro focusing."
- Pentax Corporation
"In the consumer market, 28-200mm zooms have been very popular. But besides offering 35mm SLR owners an even greater zooming range, extending focal length down to 24mm is also beneficial for the ever-increasing use of digital cameras. The CCD sensors used in digital cameras are smaller than the 35mm format so the corresponding angle of view at a given focal length is not as wide as with 35mm cameras.
"In our 13-group, 15-element 24-200mm f/3.5-5.6 lens, Tokina has used two high-grade glass aspherical lens elements to reduce spherical aberrations and two SD (super low dispersion) glass elements which not only reduce chromatic aberrations but also maintain a high level of optical quality while reducing cost.
"To create a lens with more than an 8X zoom and overcome the problems at both the 24mm and 200mm zoom ends, internal focusing, aspherical elements and SD glass are essential.
"Our lens elements are arranged in four main groups. The second lens group is used for both zooming and focusing. Traditionally in wide range zooms, when a photographer focuses and zooms at the same time, he may find that the lens "hunts"—is unable to reach the precise focal length and focus required—because the main lens group is moving forward or backwards while individual elements within the group are also moving. But in our lens formulation, the second lens group moves very little so the chances that hunting will occur are greatly reduced.
"Using aluminum barrels in the mechanical design provided excellent durability and smooth mechanical operation.
-Tokina Company, Ltd.