I think the biggest advantage of a prime is the weight. They're a lot lighter than a zoom -- especially if you're dealing with L glass. Let me tell you the 70-200 f/2.8L IS gets heavier as the day progresses!
The second advantage you will get is speed. Quality is I think down the list -- especially if you're comparing lenses of similar build and you're comparing them at their own optimums. For example, compare the 85mm prime L at f/4 to a 70-200 f/4 L and the prime will be sharper hands down. But that's as much due to the fact that the 85mm was used in a sweet spot vs the zoom being used at its extreme as anything.
And remember, primes give you less control over DOF and perspective than do zooms. (Getting closer with an 85mm to be equivalent to a 200mm lens does not give the same DOF and perspective as the 200mm).
As others have said, if the aperture is changing as you zoom it's ONLY because you started with an aperture that was higher than the maximum focal length could support. It changed as you zoomed in. There is no other reason. If you have a f/3.5 - f/5.6 lens and you set up at f/5.6 or smaller that will work for all focal lengths. If you set up for anything below f/5.6, then you will only be able to zoom so much until your camera will jump to work with the lens.
|