Macro lens advice for nikon camera?

I have recently bought a nikon D5100 and i would like some advice on macro/micro lens for it.
it must be an af s lens, and i was hoping not to spend more than 400 usd, but i can increase the budget if it's really worth it. I would mainly use it to snap shots at insects and small critters in general.
All advice is welcome, and if you could link personal pictures or stuff from the web as examples all the better!

The only macro lens Nikon sells that is less than $400 is the 40 2.8 DX, but that isn't a long enough focal length to shoot insects. The 18-55 is actually decent for close-up work; just use it @ 55mm.

Don't get a macro lens, get a set of extension tubes. These go between your lens and body and allow you to focus n tiny objects. They will cost much less than a lens. CHeck with Samy's CAmera or B&H Photo for the correct set. They normally come in groups of 3 which you can stack to get really close.

You can have Nikon 40mm f/2.8G AF-S DX Micro NIKKOR Lens The AF-S micro nikkor 40mm f2.8G is moderately fast or bright, contrasty and extremely sharp lens, just like all its siblings in the Nikkor Micro (Macro for other brands) interchangeable lenses. It appears that Nikon has consistently improve and exceeded expectations in this "special purpose lenses" category. Historically, every Nikkor (brand name for Nikon lenses) which are stamped with "Micro-Nikkor" are stellar performers and tops its class versus the competition, this new AF-S DX Micro Nikkor 40mm f2.8G is no exception.

If you want to do "true" Macro Photography, which is usually defined as 1:1 or greater magnification, you have a number of options but make sure you understand that this is highly technical and demanding area of photography.

At magnifications of 1:1 (Life size on the film or sensor) there's very limited depth of field so focus must be VERY precise and, because the range of sharp focus is very short it is tricky to get background detail in focus when you wish. For this reason such things as extension tube sets, extension bellows and lens reversing rings often require manual focusing and "focusing rails" that allow the camera and close up apparatus to be moved on a geared track mounted on a tripod. Such set ups are also likely to require a semi-automatic exposure method because it is technically difficult to automate the exposure function in the lens with these devices.

The great advantage to extension tubes comes from the fact that they have no optical components. This means they are less complex and simpler to manufacture, therefore costing less and not compromising optical quality. They simply shift the focusing range of the lens to nearer objects. Many manufacturers make special extensions tubes (usually a single tube with electronic couplings) that work with a number of lenses and preserve auto-focus and all or most exposure metering modes. The drawback to tubes is that focusing is shifted closer to the camera and the lens will no longer be able to focus on more distant objects. Here is a detailed explanation of extension tubes and sample photos -

http://www.shutterfreaks.com/Tips/ExtensionTube.htm

On the other hand, if your are primarily interested in getting good close ups of insects and want a flexible tool for images at about 1/2 life size or less some of the so called "macro zoom lenses" are a good alternative. These usually do not achieve 1:1 magnification but you can expect about 1/2 life size and it will be easier to get sharp focus, background detail and automatic exposure operation. The advantage to using this type of lens is that they incorporate a design to allow the lens to focus much closer than normal and, especially if they are an appropriate focal length such as a moderate telephoto, they give a convenient working distance to avoid disturbing, or worse provoking, subjects such as wasps, spiders or other undesirable creatures.

Unfortunately these require complex and therefore often expensive optical systems. For the purposes you describe and your price range I think you will get the best results with a used or refurbished Nikkor lens in the 40~60 mm range. Here is a link to one of the best deals I could find.

http://www.adorama.com/...028UR.html

Hope that helps. Good luck. Email me if I can be of any further help.