انوش جان من درمورد این دولنز و اینکه چقدر فاصله کانونی 135 در روی این لنز واقعی است اطلاعی ندارم چون گاهی سازندگان برای روند کردن اعداد کمی در فاصله کانونی لنزها اغراق میکنند اما مطابق محاسبه باید لنز با 135 روی دی300 به 200 میلیمتر برسید و با 85 همانطور که گفتید 128 و اختلاف به نظر زیاد میرسد اما کن راکول نکته جالبی را مطرح میکند که خواندنش خالی از لطف نیست:
و اینجا هم نقل قولی از تام هوگان در مقایسه لنزهای 200-18 و 85-16Nikon 16-85mm VR vs. 18-200mm VR
Due to the optical tomfoolery used to design modern do-everything zooms, there actually is much less difference between these lenses if you're only a few feet away from the subject. This is because these zooms actually shorten their focal lengths as you focus more closely!
If you point the 18-200mm at your feet and zoom from 85 - 200mm, you'll see little change.
If you're comparing them for yourself, be sure to point them at something very far away. You'll see less difference at 20 feet (6m) and very little difference at just five feet (1.5m).
Such a range, of course, is very useful. You get a nice taste of wide angle at the one end (16mm [24mm equivalent]), plus a modest amount of reach into the telephoto end (85mm [technically, 128mm equivalent]). Some people would call this the ultimate "walking around" zoom range, as rarely would you need wider and you can always try to get a little closer to your subject if you need more telephoto. Indeed, that's about how I think of the lens: for casual photography where I don't have any particular type of shot in mind and want to be ready for whatever subject presents itself. The addition of VR to this lens augments this "walk-aroundability," as it means you can continue to photograph in situations where you otherwise might want a tripod or other type of support.
Many Nikon DX users had already picked the 18-200mm VR as their walkaround lens, partly because they were seduced by numbers. Quick question, which gives you more range: the 18-200mm or the 16-85mm? The answer might surprise you a bit. The 16-85mm has a horizontal angle of view range of 16 to 73 degrees, the 18-200mm has an angle of view range of 7 to 66 degrees. However, because the 18-200mm changes focal length so much at the long end when focused close, for many situations its angle of view is only 10 to 66 degrees, which is not looking a lot better than the 16-85mm. I personally value those extra 7 degrees at the wide end much more than the extra 6 to 9 degrees at the telephoto end--they make a more dramatic impact on my photography.