I have a Nikon D5100 and want to see whats the best lens for shooting pictures of a baby and kids running around inside and outside? I would like to stay under $1,000. Thanks for the help.
So, just as Dan Wolfgang asked, I too would like to know what is your current lens and how do you use it.
I suppose you have the kit lens, the Nikon 18-55mm f/3.5-5.6G. to be honest it isn't a good lens, you may be satisfied with it but you will be amazed by the result that you can get by using a better lens on D5100.
So you got about $1000 to spend...
So you know your budget but you need a good plan to spend it.
First thing to do, review all your old photos and see what is your most used focal length.
By your current lens you're limited to 18mm and 55mm, is that ok for you?
At the wide end, the 18mm should be enough for most of your needs, that is photographing kids/people.
But what about the longer end?
Anyway, my point is you should know what is your desired focal length.
Nikon 18-105mm f/3.5-5.6G ED VR AF-S DX for $396.95 is much better and brighter than D5100's kit lens. The 18-55mm reaches f/5.6 at 55mm and it's not useful for indoor shots at all.
Tamron 17-50mm f/2.8 for $499.00, yes! a third-party lens but it's a great lens, very good for landscape and useful for portraits. the constant f/2.8 aperture is a great plus, that means sharper and brighter photos in low light situations and of course better bokeh!
Nikon 50mm f/1.8 G AF for $216.95 is a great lens for portrait photography, it is brighter than the Tamron above but the negative side is that it's a prime and you may need to change it for another shot.
Nikon 18-200mm f/3.5-5.6G AF-S DX ED VR II for $846.95 is an all around lens, you probably don't need to take it off your camera.
I have always liked prime lenses, but found zoom lenses to be far more practical -- so that's what I used. However, after having a kid I learned of two huge advantages prime lenses have: there is no zoom ring to fiddle with, and they are lighter (perhaps only slightly, depending upon your lens selections). A prime lens most likely will have a faster aperture which is invaluable to help get the shot when you can't mess around with flash because of the fast-moving kid, too.
Both reasons are related, of course. Being lighter, it's easier to hold the camera one-handed for extended periods of time. This is important when I'm holding the kid with one arm, and only have one free to shoot. Shooting at the kid's eye level is important to making an engaging photo, which means getting on the floor/ground. Having one hand to help move and balance on the floor while still being able to shoot makes it possible to get shots much easier. And of course, without a zoom ring, I'm always at the expected focal length so it's easy to recognize how to move to frame a shot.
I'm guessing you already have a lens (the kit zoom?) and have used it some. Before buying a prime, I would try to determine how you have used the lens you have to decide what focal length(s) of primes to use. The way I do this is with Lightroom, and review metadata to see what focal lengths are most-used. (Other programs likely offer such functionality.) Alternatively, try setting the lens at a specific focal length and shooting the kids for a while to see how you like it. I have found a two-lens prime kit to be hugely versatile: something on the wide end and something mildly-telephoto.
Personally, on a crop-sensor camera like the 5100 I would want a 24mm lens (which equates to 36 mm focal length in the traditional 35 mm format). I think the only option is Nikon's 24mm f1.4; a fantastic lens, but well outside your budget. I haven't used Nikon's new 28mm f1.8 ($700), however that is likely a good lens/focal length for getting shots of the kids. Sigma offers a 30mm f1.4 ($400) that is also a fine choice. Nikon's 35mm f1.8 is cheap ($200) and well-reviewed, however I find this focal length very uninteresting and simply don't use it (roughly 50 mm in the 35 mm format).
On the more telephoto end, I think the easy choice is a Nikon 50mm f1.8 ($200) or f1.4 ($500) (75mm in the 35 mm format). This focal length is too long to shoot kids who are next to you. That is, if you're holding the baby or entertaining the kid, you won't be able to use this lens -- in the time it'll take you to back up far enough to get things into the frame the moment will have passed. However, for "sneaking" a candid from the other room or outside in a wide open space, it may be just the right choice. To be honest, I waffle between using an old 50mm 1.8 and an 85mm 1.4; the 85 is often too long, but I just like the lens so much I try to make it work.
So, finally after all that justification, with a $1000 budget, I would choose the 28mm f1.8 and the 50mm f1.8.
For daylight outdoors, I mostly agree with John Cavan: If you already have the DX Nikkor 18-55mm f/3.5-5.6 kit zoom, use that. Otherwise the 18-105mm or 18-200mm DX zooms give more flexibility in focal length, and the 24-120mm f/4 FX zoom is a step up in image quality.
For indoors, the low light is a problem. I consider typical indoors to be around LV 4, which means that you would need 1/30s exposure at f/4 and ISO 3200. That's too slow for moving subjects, and ISO 3200 is about as high as the D5100 can go while still giving passable photos.
So for indoors and other low-light situations I'd suggest the AF-S DX NIKKOR 35mm f/1.8G. You sacrifice zoom, but gain two stops of low-light performance. Those stops also help with autofocus speed and accuracy in low light.
With a $1,000 budget, I would go for both: One of the zooms for general purpose use, and the 35mm DX prime for low-light situations.
I'm not sure where you're located, but if you're willing to go a little above the $1000 mark, the AF-S Nikkor 24-120mm f/4 lens is a great option as a good general purpose, do most things, kind of lens. I have one for my D800 and I've been very happy with it and it's good enough to be on the recommended list for the D800 and that's saying something.
If you're firm on the sub-$1000 mark, then something like the AF-S DX Nikkor 18-200mm f/3.5-5.6 might be a decent choice, though I think I would be more inclined to the AF-S DX Nikkor 18-105mm f/3.5-5.6 since, in my experience, the shorter the zoom range, the better the lens and the Photozone tests bear that out.
Still, I'd shell out the extra money for the the 24-120mm option. Not only is it a better lens, it's an FX lens and it future-proofs you should you ever decide to go to an FX sensor. That possibility is becoming increasingly more realistic as the price drops, so don't quickly dismiss the idea.