When you just have to have 200mm ...
One of the problems people encounter with the "All-in-one" lenses, e.g., the 18-200mm(250mm) is that 200mm does not necessarily mean you are getting a 200mm shot.
That particular style of lens uses the focus barrel to makes up the telephoto length of the lens. If you set your lens to an indicated 200mm focal length, marked on the side of the lens, and then focus on something just 18-inches away, when you have the subject in focus, the actual telephoto length is more like 135mm. If you focus on something 27-feet or more away ... it becomes the indicated 200mm.
Well, not all lenses are quite as predisposed to this focal lengthening by the focus ring.
Below are four examples of different lenses that can provide a pretty accurate 200mm shot, no matter what the distance (outside the MFD).
- TAMRON AF 70-300mm f/4-5.6 LD (MFD = 60-inches),
- TAMRON SP AF 200-500mm f/5-6.3 Di LD (MFD = 98.4-inches),
- TAMRON SP AF 70-200mm f/2.8 Di LD (MFD = 37.1-inches), and a
- Minolta AF 200mm f/2.8 APO G HS (MFD = 60-inches)
Each lens has it's own qualities and you have to decide what it is you want in your photography, to make the appropriate choice. Here are a few qualities that I have found to base my own decisions on.
- The first lens produces a reasonable image, but has some serious purple fringing in high contrast (light/dark) situations. It takes the 200mm image at a base aperture of f/5. It is "cost effective" to the extreme.
- The second lens is rather long, but delivers a sharp image, no fringe ... but, also at base aperture of f/5. There is a price for this kind of overall reach, but it is reasonable compared to lenses of similar focal reach.
- The third lens had a medium focus speed and delivers a terrific image at f/2.8. Most cost effective 200mm f/2.8 shot on the market.
- The last lens delivers a sensational image, the fastest focus response of any 200mm lens, and is roughly the lightest and most pricey of the bunch. Yep, the sharpest 200mm f/2.8 you will see, too!
Now, of course, there are many other lenses that have a 200mm shot in their design, but for argument, these four should do well. Each one is just about the same in apparent magnification as the other three. As such, depending on the Minimum Focus Distance (MFD), they should deliver an identically sized subject in your shot, when focused.
Please realize one of the advantages of the "All-in-one" lens is its incredibly short MFD (18-inches) throughout its focal range of 18-200mm(250mm). As a convenience, that is hard to appreciate until you try to take shots indoors with the other lenses described above. Start backing up. :rolleyes:
I hope this might explain some differences that are being experienced out there. :)