edstubbs Loc: East Coast; 1st state, Delaware

Sorry, I am lost to that comment. Can you explain how a F4 lens can be one stop slower than a F2.8? Can F4 lens be used in some low light situations?
:?:

MT Shooter Loc: Montana

To compensate for a 1 stop slower lens, you simply shoot at one stop slower shutter speed, or increase your ISO by one stop. Shooting at F2.8 yields a very shallow DOF for most shots so even with an F2.8 lens you don't SHOOT at F2.8 that often. The larger diameter of the F2.8 lens will allow more light through so the AF sensor can AF better in low light than with an F4 lens, but for image quality there is almost no difference at all if you compare shots taken at identical aperture settings (F5.6, F8, F11, etc.)
If you do not fully understand the relationship of these designations then I highly recommend a book called "Understanding Exposure", it will help you to understand the "exposure triangle" and how each of those three components work together for your photography.

Weddingguy Loc: British Columbia - Canada

The Tamron 24-70 F/2.8 is not as good as your Nikon 24-70 F/2.8, but far superior to the kit lens . . . and much lighter than top Nikon , Canon or Sigma glass. Go to your local camera shop and check one out. Take along your camera for some test shots that you can later take back to your computer.

pixelwars

I was trained in needs base selling where it was of paramount importance to discover what the customer needed before selling them anything. It seems that you may need a f/2.8 lens to satisfy your wife's irrational desire. Good luck.

So the real question to this, and so many other similar posts, is: "What does she expect to gain with the new lens?" Some of the possible answers are; ego gratification, keeping up with your equipment, sharper images, longer zoom range, and so on.

In spite of knowing better I must confess I have almost one of everything but my "go" bag has the; 24-120 f/4, 70-200 f/2.8, and the 200-400 f/4.

My 24-70 f/2.8 in spite of being one of the best lenses ever made sits neglected on the shelf. I must feel the need to support my local camera shop.

lamiaceae Loc: San Luis Obispo County, CA

Because it is.

Stops are multiples of the square root of 2 apart and are factors of 2 in exposure, e.g.

f/ or F sequence

1.4 2 2.8 4 5.6 8 11 16 22 32 45 64

You do realize that the larger numbers mean smaller openings (apertures)?

Yes, at higher ISO settings. Not the best situation and a 2.8 is easier to focus in low light, both with AF and manually.

lamiaceae Loc: San Luis Obispo County, CA

I do that all the time. I have primes at f/1.4 to f/2.8 with focal lengths from 24mm, 28mm, 35mm, 50mm, and 135mm.
I also have slower primes at f/3.5 to f/4, f.l. of 19mm, 28mm, 35mm, 50mm, 100mm, 135mm.

And I have zooms that are both the fast constant stop pricey ones and variable stop cheaper ones. I have an old film f/4 28-135mm that weighs a ton (seemingly).

romanticf16 Loc: Commerce Twp, MI

That would be too heavy, IMHO.

tradergeorge Loc: Newport, Kentucky

I have a Sigma 18-50mm f2.8 lens that I love. It is older and I am not sure they make it anymore, but they probably make something similar. Mine is a bit bigger than the kit lens, but not horribly so.