There are a lot of ways to get close to small subjects with a DSLR. If you have the money, a dedicated macro lens is great, but don't expect a macro lens to solve all of your problems in terms of getting closer. I own a 105mm Micro Nikkor, for example, and without additional accessories, it can only go to a close-up ratio of 1:2, which means half life sized--that's simply not close enough for insect photography.
There are other ways you can get closer (even if you already own a macro lens) and some of them are very inexpensive. One cheap way is with what they call close-up filters. These are just glass discs that screw to the front of your existing lenses and they reduce the lens-to-subject minimum focusing distance, which in turn makes your subjects look bigger. Their downfall is quality: any time you add an extra glass element to a lens you degrate sharpness and contrast and typically these filters are not great in quality.
A better alternative to consider are relatively inexpensive extension tubes. These tubes come in a variety of lengths (measured in millimeters) and they also decrease the minimum focusing distance. However, because they are only spacers and don't contain any glass, they don't degrade your lens' optical quality in any way. I shot this photo of a wasp (at least I think it's a wasp) on a black-eyed Susan with my 105mm Nikkor and a Kenko 36 mm extension tube and without the tube you wouldn't even see the wasp. Another great thing about extension tubes is that you can combine them to get even greater magnification.
Be aware that there is a light loss with extension tubes, but the exact amount depends on the amount of extension, the focal length of the lens and the maximum aperture. But since most cameras have TTL (through-the-lens metering) the camera will compensate automatically. Also, depending on the exact marriage of electronics between the tubes and your lens and camera body, your flash exposures may require some experimentation.
Be sure that the extension tubes you buy are compatible with your camera body so that you know your auto-exposure and autofocus systems will work properly. I recently tested a set of three Kenko tubes in sizes 12, 22 and 36mm (about $179 street for all three) and they worked beautifully with both my autofocus and auto-exposure systems on my Nikon D90. Of course, the more you extend, the more shallow depth of field becomes, but that's always an issue with close-up subjects. Even at f/11 I had a tough time keeping this fly in sharp focus with a 36mm extension. As long as your camera doens't have a bad noise issue, consider bumping up the ISO by one or two stops to get more depth if you need it--the small amount of additional noise is probably a price worth paying to get very close to tiny subjects.
As I said there are other close-up photography options, some cheap, some not so cheap. But give extension tubes some consideration if you're trying to save some money and still get good results. They're lightweight, simple to use and as I said, they don't degrade optical quality in any way.
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