A common question among new photographers concerns high-speed lenses. Why get the f/2.8?
Advantages of f/2.8 zoom lenses (14-24mm, 24-70mm, 70-200mm, 300mm, 400mm for Nikon users):
More depth of field control. A big advantage f/2.8 has over f/4 and f/5.6 is depth of field control. You can throw a background out of focus more easily with larger apertures. Consider the 50mm and 85mm f/1.4: Incredibly thin depth of field.
However, you must be careful in how you exercise that depth of field. In portraiture, it's generally preferable to have the subject's eyes in focus. With a couple centimeters that are in focus, it's easy to misfocus and have the ears in focus. Oops.
Focusing advantage. A larger aperture gives the camera's autofocus sensors more light to work with, generally enabling faster and more accurate autofocus. It's also easier to manually focus a fast lens because you can see what you're focusing on a little bit better.
The thinner depth of field comes back to haunt you, because it's easier to misfocus. If you're just a little off you're back to having ears instead of eyes in focus.
Optical and build quality. Generally, f/2.8 lenses are in the professional ranks of a manufacturer's product lineup. So they are among the top performers optically, especially compared to slower lenses. They are well-built, to take abuse from professionals who are really good at crunching them into rock faces or dropping them off waterfalls.
But, there are many slow lenses that are great performers. There are f/2.8 lenses with all sorts of weaknesses. Consider Nikon's 70-200mm f/2.8. Great on small sensors, but rather lacking on 135 format cameras. Consider Nikon's 18-70mm f/3.5-4.5. Its only real problem is vignetting at large apertures.
And many of Nikon's lower-end lenses can take abuse. I have kicked my 18-70mm around for five years and it's still going.
They're big. If you've got assistants and budget, you can carry five or six f/2.8 zooms. The rest of us have limited bag space and limited transportation space. Slower lenses are lighter and cover fewer cubic centimeters. I'd rather carry five pounds of gear than 10.
So basically, consider your needs and the limitations you'll encounter when planning a lens purchase. Everything in photography is a compromise.
