Nikkor AS-F DX 18-200mm 镜头的国外评测译文（还没译完）

 

基本信息

Nikon发布D200的一个令人惊讶的事是与此配套的DX系列广角和远摄镜头。对于Nikon来说，为消费级相机发布新套头并不是件新鲜事，但引入注目的是这是一款18-200mm焦段加上防抖和AF-S自动对焦的产品，除了超广角、超远摄、超微距和超低照度以外，看起来是一个无所不能镜头。换句话说，你可以“一镜走天下”。这个完美的怪物在市场上被叫作“超级镜头”，尽管大多数指标其实并不是超级。

每个人马上就要问：拥有这么大焦段的镜头好不好？我会劝你不要去看它的性能列表：是的，超出你的想象。但是我们在讨论细节之前，我们先从好的细节看起。

首先我们讨论DX意味着什么。所有Nikon单反（包括富士的）的感光器都是小于35mm画幅。这通常被称为APS尺寸，这是因为它很接近我们快遗忘的胶片的尺寸。准确说来，35mm画幅的长轴是36mm，而Nikon单反都是24mm的长轴，所以任何35mm的Nikkor镜头的成像半径都远大于机身所需要的。DX系列镜头其实设计用来和较小感光器的数码产品配套的，因此，这些镜头完全满足Nikon单反（以及富士的）的需要，DX的优点就是它比同样覆盖35mm画幅的镜头更小和更轻。

我现在有七支DX镜头：17-55mm f/2.8G，10.5mm 全画幅鱼眼镜，12-24mm f/4G AF-S，18-70mm f/3.5-4.5G AF-S，18-55mm f/3.5-4.5G，55-200mm f/4-5.6G和本文提到的这个超级镜头。我希望我们能看到更多的DX镜头出现。

你首先注意到18-200mm DX 镜头的事可能是它比说明书上说的要小得多，那么这个200mm和VR功能是如何折腾进去的？不管怎么说，它是Nikon制造的中距镜头中较小和较轻的一款。

18-200mm的焦段提供了8-76度的视角，这等效于在35mm机器上使用28-300mm镜头，对于有些用户来说，这是stay-on-camera 范围。

The 18-200mm focal range gives you angle of views from ~8 to 76 degrees on a DSLR; it's effectively the same as using an 28-300mm lens on a 35mm body. For some users, that's a stay-on-camera range. There's no denying that this is a much-asked-for focal length range. Indeed, even I find it interesting to walk around with this lens, the 10.5mm, plus a Canon 500D close up lens--that makes for a very compact, light kit that covers a pretty incredible focal length and focusing range. Still, I would have loved for it to be 16-180mm instead of 18-200--I miss that little bit extra at the wide end and rarely use the extra bit at the telephoto end.

This is a two ring design; like most recent consumer Nikkors, the zoom ring is the front most ring and the focusing ring is closer to the camera. Yuck. The lens does have a distance scale, but with no depth of field or infrared markings. For that matter, we only get three markings other than infinity for feet, and three in meters. You know, even if the lens manufacturers don't want to give us complete depth of field markings, the least they could do is mark the hyperfocal point at one useful focal length and aperture combination and give us a few more markings. In the case of the DX lenses, this is even more important, as standard 35mm depth of field charts won't help you; you need one designed specifically for the APS-sized sensors (copies of which are in my DSLR books, by the way). On the left side of the lens (from the back of the camera) three buttons:

• Manual Focus button: In the M/A position the lens works as usual (autofocus with manual override). In the M position, the lens focuses only manually.
• VR switch: VR can be turned On or Off.
• VR type: VR can be set to Normal or Active.

VR技术现被Nikon特称为VR II，按Nikon的说法，他们声称VR II可以达到你通常能持稳的程度以外的4级快门的效果。

VR is specified by Nikon as being VR II--this new variant supposedly gives a bit more range to the VR usefulness. In Nikon's literature, they claim that VR II manages to get you four stops beyond what you'd normally be able to handhold. As far as I can tell, that's a valid claim at 200mm. But, be forewarned that VR is often much less useful than you might think. When you start hand holding a lens at 1/30 or lower, you're definitely going to be fighting subject motion. If your subject is absolutely static, fine, no problema as my South American friends like to say (sorry Brazil--I haven't been there to make friends yet ;~). But most of the folk that'll be using this lens aren't shooting completely static subjects with it, I think. The primary use for VR on this lens in my mind is to keep you from having to significantly boost ISO in common situations at 200mm (which would be f/5.6 maximum aperture). If the choice at that focal length were between shooting at f/5.6, 1/300, and ISO 1600 versus perhaps f/5.6, 1/60, and ISO 400 and I wasn't shooting a moving subject, I'd pick the latter just to keep noise under as much control as possible.

What VR (or VR II or even VR III when it appears) won't do is to allow you to shoot your daughter's night soccer game at 200mm and 1/15. That's a recipe for fuzzy players on sharp grass.

The HB-35 hood supplied with the lens is the bayonet type butterfly style. It can be reversed onto the lens for carrying, but it adds significant diameter to lens when you do so. It's a bit shallow, so doesn't present a lot of shading for the front element from the sides, but it's far better than nothing, and it is more substantial than some of the butterfly hoods that have appeared for other Nikkor lenses. The lens itself uses 72mm filters, an odd size for Nikon and one that'll force almost everyone to buy a step-up ring of some sort (I use a 72-77mm step up, as my primary filter set is 77mm). For the consumer target of this lens, though, the filter size isn't a big deal, as this is likely to be their only lens, and thus they'll just buy 72mm filters.

You get AF-S focusing with this lens, and that'll take you down to a bit over 16 inches (.5m) at most focal lengths and a bit over 14 inches at 18mm. For those of you who keep track by magnification, that's about 1:15 at 18mm and about 1:4 at 200mm, both pretty good figures for a superzoom.

The supplied lens cap is the pinch-front type.

Finally, one word about focal length. As with most zooms, focus point shifts the focal length a bit. At infinity, the lens is 18mm at its wide end, and I think a few millimeters short of 200mm at the tele end (I've seen one measurement that says 193.5; all I know is that it's a bit shy of my 70-200mm at infinity). At very close focusing distances, which is where I'm at most of the time, the lens is almost down to 17mm at the wide end, with very little perceptible change at the tele end. Perfect! Just the way I want it to be.

Other tests of this lens you might want to look up:

• Amateur Photography, 4 March 2006, p.39.
• Popular Photography, April 2006, p. 67.
• Bjorn Rosslett's mini-review.

Handling

Handling is not the strongest point of this lens, though good enough for the target audience.

The zoom ring is stiff and has a very different feel at different focal lengths (I should note that some users claim it is loose and subject to zoom creep; that isn't the case on my sample). As you zoom in to 200mm, the zoom ring has a sudden "loose point" just before you get to the maximum focal length. Zooming the other direction is smoother, though still what I'd call rough. The barrel extends almost two-and-a-half inches (6cm) at 200mm, so you're moving a fair amount of plastic and glass during the zoom. The barrel doesn't rotate during zoom, which means that you don't need to readjust filters. The focus ring is better than the zoom ring, though it's thin and has nothing to distinguish it in feel from the zoom ring other than position and width. Remember, the focus ring is not in a "normal" spot, and this takes some getting used to if you're coming from the standard 35mm Nikkor lenses.

Focal length changes slightly when you're shooting at close distances. As with most modern "zoom" lenses, this is really a vari-focal lens, so you should be focusing after framing; I don't see this as a big deal.

Maximum aperture doesn't change the way you might expect with focal length. The relevant values are:

18mm f/3.5
24mm f/3.8
35mm f/4.2
50mm f/4.8
70mm f/5
135mm and higher f/5.6

The 18-200mm is pretty small and light for its range. It balances well on the front of a D200. I can see why the D200 and 18-200mm intros were simultaneous--they're a very good match for many users. On bigger cameras, such as the D2x, the 18-200mm is a little less balanced, but still okay. Either camera, I felt comfortable.

The switches are well located--as long as you can remember which is which--and can be easily moved from one position to another without looking away from the viewfinder. Still, I would have liked a bit more differentiation in feel so that I could tell by touch which is the focus on/off and which is the VR on/off switch (the VR type switch is smaller than the other two). Even something as simple as a round raised point versus an X raised point would have been welcomed.

性能

我前面曾提到过这一点，现在我们直奔主题：几乎每个性能参数都表明这是一款极其出色的镜头，不算完美，但各方面也是相当好了。

Sharpness is excellent throughout most of the range, with f/11 being the point of maximum sharpness on my sample once you get to 100mm. At the wide angle end, though, there's actually little to distinguish the central area in one aperture from another--go ahead and use maximum aperture if you'd like. At all apertures at 200mm you'll see a bit of softness, with the best apertures being f/11 through f/16. (I should note that I've seen tests from others that say the opposite--that their sample was a tiny bit soft, especially in the corners at 18mm but pretty darned sharp across the board at 200mm. Sample variations do abound at the lower price points. But the conclusions are usually the same: this is a sharp lens across virtually all of its range.) From 18-100mm my lens was everything you'd ever want in terms of sharp, so the way I look at it is that I've gotten a superb mid-range zoom that can give me a usable long telephoto in a pinch.

Vignetting (light falloff) was better controlled than I expected, considering the 18-200mm. At full aperture and 18mm, there's a good half stop of falloff in the corners, but by 24mm that's dropped to a third of a stop, and by 50mm a little less (it returns to a bit more than a third of a stop by 200mm, but at such telephoto extremes that's actually less light falloff than most shooters would desire). Two stops down from maximum aperture you'll never get more than a third of a stop of light falloff in the corners, an excellent performance.

Chromatic aberration is decently controlled, though not quite as good as the 17-55mm DX, especially at the wide end.

Images taken with the 18-200mm were sharp and contrasty, and in some situations, the VR also helped keep acuity high. I was a little worried that a superzoom design would show some inability to keep up with 10 and 12mp cameras, but that's simply not the case. Nikon is really showing that they can design excellent consumer zoom lenses lately. Indeed, of all the so-called superzooms I've used and tested, this is the first one that I'll keep.

Put the 18-200mm on an F6 and it becomes obvious why the lens has little light falloff at 18mm: the image circle isn't a circle! At this focal length the lens almost manages to cover the full 35mm frame; but it has a wavy pattern at the top and bottom of the frame that barely cuts off the corners. As you zoom past about 30mm the extension in the image circle disappears and you now have the traditional DX pattern of clipping the full frame corners significantly. This increases beyond 50mm so that the 200mm image circle is pretty tight to the APS sensor size. There's no point at which this lens covers the full 35mm frame.

Distortion performance is good. At both ends there's measurable distortion (about 1 percent complicated barrel at the wide end, slightly less than 0.5% simple pincushion at the telephoto extreme). Curiously, the best distortion performance is somewhere around 20mm (above that and we start getting pincushion, below that and you have barrel). By complicated barrel I mean that there's just a bit of mustache, or wave, distortion in the barreled lines. Simple tools won't correct it fully. This isn't an architectural lens, but it's far from a fun-house lens.

Autofocus is fast and hunt-free. As I noted in my 17-55mm review, the primary focus speed difference between that lens and the 18-200mm comes in terms of the maximum aperture and in low light. In bright light, both are very sure and fast. In low light, especially at the telephoto end, the 18-200mm starts to lag, as it's just not letting as much light hit the autofocus sensors.

Flare performance is quite good except for direct into the sun. Unless light is hitting directly on the front element, I've not seen any visible contrast degradation, and even then it's better than some Nikkors. Fortunately, the supplied butterfly type hood does a reasonable job of keeping light from hitting the front element in all but oblique side-lighting conditions. If the light source is directly in the picture, things get a little wonkier, and you're likely to see contrast reduction if not ghosting. That's typical of virtually all modern zooms, though.

I'm still evaluating bokeh, but my initial impressions are that the bokeh is acceptable, if not great (the lens features a 7-blade aperture diaphragm).

 

Drawbacks

• Variable aperture. The big issue is that at 200mm this is an f/5.6 lens, which means that autofocus in low light can be compromised slightly. Still, not a big issue, though a negative one.
• Complicated distortion. At the wide end, a bit of wave distortion is added to barrel; at most of the focal range, there's enough pincushion distortion to just be visible.
• Build quality. Build quality doesn't exceed the price point. I always worry about that slightly wobbly extension at 200mm--I'm not sure the lens would survive even a mild drop fully extended. And the zoom ring is rough on my sample (but apparently loose on some others).

Positives

• Super optics. Really, no significant flaws worth mentioning. Considering the price and the long focal range, superb performance.
• The 28-300mm for the digital world. Yes, the superzoom can be super. Considering the close focus ability, this lens really is going to suffice for 80%+ of the shots most folk take, at least if you don't need an f/2.8 or faster aperture.
• Price/Performance winner. This is a far better lens than you'll ever expect, even for US$750. Far better. Good enough that it has replaced both my 18-70mm and 24-120mm as the walk around lens of choice for when I want to go light and with one lens.