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Compare:
  • Nikon 35-70mm f/2.8 ($550)
  • Nikon 28-80mm f/3.5-5.6 ($100)
What is the difference, and why does the price reflect this extreme?

The is some difference in the construction, the 35-70 has a bit more metal, but not enough to warrant $400. Likewise half a stop at maximum aperture isn't that much. The zoom range would seem to indicate that the 28-80 was the better lens. The 35-70 also weighs in at 24 ounces while the 28-80 is 7 ounces.

There is some difference in the constant aperture. Notice that the 35-70mm has a single quoted number while the 28-80mm has two. The f/2.8 means that the maximum aperture stays the same all through the zoom range. To do this, it means that the lens has to be a f/2.8 at 70mm and that as the camera zooms in and out, the size of the aperture changes.

To do this, it means that at the 35 end there is a fair amount of glass that isn't being used at that end. In the diagram below you can see something along this lines that the aperture changes. Please realize that it is not to scale within the comparison - and certainly not between the two. The f/2.8 @ 70mm is much longer than the f/5.6 @ 80mm.

Constant Aperture  | Variable Aperture
-------------------+-------------------
---                | ---
 |   ---   ...     |  |   ---   ---
 |    |    ---     |  |    |     |
 |    |     |      |  |    |     |
 |    |     |      |  |    |     |
 |    |    ---     |  |    |     |
 |   ---   ...     |  |   ---   ---
---                | ---  
foc  f/2.8 f/2.8   | foc  f/5.6 f/3.5
len  @70   @35     | len  @80   @28
The '...' around the f/2.8 @ 35mm shows the glass that is not used. Even if it was 'capable' of using this area to get a larger aperture (which would mean even a larger and heavier lens) thus having a 35-70mm f/1.8-2.8 (or something) lens, those who look for the constant aperture would be most displeased. The reason for this is rather simple: the market for the constant aperture is the higher end, the professionals.

Variable aperture lenses reflect the f-stop to within 1/3 of a stop, which is fairly good. However, when it comes to a professional they want to know that when the f-stop is dialed to f/3.5 it really is f/3.5 and not something slightly off.

On the other hand, people who want a lens and are not too concerned with the speed of the lens or if it may be a fraction of a stop off are quite happy with the smaller, lighter and cheaper lenses that are out there. It takes less glass and gadgetry to have a variable aperture - hence, a cheaper lens.

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