Glass for concerts/dark shooting, should be at f2.8 or lower if you want to get the best quality from the most available light.
Can someone describe what happens if you'd use something like a f4.0 in these situations? I.E. no shadow detail, too much grain, whatever?
Thinking about beginning the DSLR trek when these stimulus checks come through and wondering just how cheaply I can jump in.
An aperture of f/4 allows
exactly half as much light to reach the film/sensor as does f/2.8. That means for a given exposure at f/2.8, the equivalent exposure at f/4 will require you to either double the shutter speed or double the ISO setting.
For reference, the standard stops you'll see on 35mm lenses are f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22. Each one lets in either half or twice as much light as its neighbor. The number is actually a mathematical ratio between the focal length and the "entrance pupil."
I agree with all of Sunjan's points... except that I have found that 21st century consumer-grade lenses can be surprisingly good! The manufacturers skimp on the body and mechanics, but the optics are designed by companies with many decades of experience and very good lens modeling software. The optics of my Nikkor 17-55 f/2.8 produce amazing results (my favorite lens right now, it pretty much lives on the camera), but to say that their 18-70 f/3.5-4.5 is technically inferior is not exactly an insult!
In my experience, the biggest drawback to consumer lenses with small maximum apertures is that they don't allow you to generate much background blur (bokeh), and the blur they do create isn't nearly as "pretty" as with fancy pro lenses. The other drawback is that consumer lenses are outperformed in terms of contrast and saturation by pro lenses that use special high-quality glass elements in their construction. But that's general photography. You want to shoot concerts, too.
Bottom line is that you
can shoot a concert with a f/4 lens, but the faster your lens, the better your chances. If you're just starting out, I'd recommend getting one of the kit zoom lens with the camera body and a dirt-cheap 50mm f/1.8 prime for low light concert shooting.
[Edit, in response to stirinthesauce...
It's my philosophy that a kit lens allows an inexperienced photographer to start taking a variety of photographs on day one. Concert photography is a tiny niche in the photographic world, and do you REALLY want to spend most of your time behind the lens taking photographs of men?? If you don't know what you're doing, it doesn't matter how fancy your lens is. Only after you get some skills is better glass is a must-have. Also, I would not recommend buying a used DSLR body. I'd buy a used film camera in a heartbeat, but digital--no way. Those sensors really DO get dusty and there's no telling how the previous owner dealt with the issue (the guy could have cleaned it with his shirt for all I know). ]