What are the advantages of a lens having a Fixed Maximum Aperture? Previous answers have indicated that "pros tend to want constant apertures in zoom lenses".
While in general lenses with a fixed maximum aperture tend to be more expensive and thus have a larger aperture, this is not always the case. To take a random example, the Canon 10-22 has a variable maximum aperture from f/3.5 - 4.5, while the Tokina 12-24 has a constant maximum aperture of f/4. In cases such as these, is there a reason to prefer a fixed maximum aperture?
Answer
One big reason, apart from the fact that these are usually better built, higher quality, lower aperture cameras - your exposure stays the same regardless of your zoom. You don't have to worry if you need to take the shot a little wider, just to keep the shutter speed down or if you'll drop too low in shutter speed by zooming in. It's one less thing to worry about.
Think of it like this, you should be able to treat the composition and exposure as two separate creative forces to be controlled. With a non-constant zoom, now your composition drives your exposure creativity within a certain limit - you may no longer have the ability to do the shot you thought you could. You wanted to zoom in and freeze the action - oh wait, your aperture stopped down, you can't raise ISO more, so you're forced into a slower shutter speed and its a whole different photograph than you thought you had.
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