I use a Sony NEX-5R, and I bought the Sony 18-105 F4 constant-aperture lens.
I'm aware that the constant-aperture F4 gathers more light than, say a F3.5 - 5.6, and it gives you more control over the depth of field. I'm also aware that constant-aperture lenses are heavier, bigger and more expensive.
What are the other tradeoffs? Would I get a lens of better quality for the same price if I don't insist on a constant aperture?
Do constant-aperture lenses make a practical difference for low-light shooting, in aperture or shutter speed priority mode? If the aperture changes as you zoom in with a variable-aperture lens, the camera can always compensate by varying the shutter speed and / or ISO, so does it make a noticeable difference in practice, for low-light shooting?
And, taking a step up from F4 constant-aperture zooms, how do F2.8 constant-aperture zoom lenses compare? Are they generally as sharp as prime lenses? In other words, if I don't need a wider aperture than F2.8, would an F2.8 constant-aperture zoom lens substitute for multiple prime lenses in its focal length?