Suggest a good macro photography lens for the Nikon D5100?
Thanks
Of course you need an AF-S lens to be able to autofocus. Don't let anyone sell you the premise you will be loving manual focus too much to miss autofocus.
The micro Nikkor 40mm AF-S DX is relatively inexpensive and reputed to have excellent sharpness. A disadvantage is the short working distance. It is also kind of short to double as a good portrait lens. Moving up to the more expensive Micro Nikkor 60mm AF-S, it could be a good portrait lens, and there's a bit more working distance than the 40mm. For live insects the working distance is still pretty short. For that you may need to step up to the expensive Micro Nikkor 105mm AF-S VR.
I guess it depends upon what you consider macro. For close ups of flowers, an inexpensive set of close up lenses that screw onto the front of your existing 18-55 lens will get the job done. You're looking at something like $20-$30 for decent ones. I'm sure critics will point out all the deficiencies in photo quality and such, but most people will never see it. For real macro work, a set of extension tubes lets you get really close. Using them, made an entire photo of the date on a penny and I've taken photos of the individual grains of pollen on a dandelion stamen and pistil. Non-automatic tubes won't work so hot with your 18-55 lens because there will be no communication between the lens and camera body, meaning the lens will stay at f32. This will make it too dark to see the subject. Other than spending $80-$100 for automatic tubes, the option is to buy an old manual Nikon lens or something like a Vivitar with Nikon mount (you'll have to use the M manual mode on the camera). The latter are pretty inexpensive (like $20-$30) while something like a Nikon 50mm f1.8E will cost $40-$50. However, a really fast lens like the f1.8 is overkill because when the lens is about 1/4 inch from the subject, the depth of field is so shallow that you need to stop down to f5.6 or smaller. Using the extension tubes, I've gotten photos of a dew drop with the reflection of the sky and other grass in it or a drip of water magnifying the edge of a grass leaf enough to see the little saw like edges. Just know that for every 100 or so exposures you make, there will probably only be 3-4 that are what you want after you blow it up on the computer screen.
As for extreme macro, I don't know if an auto focus lens could focus on what you want and maintain that focus since you moving the camera even 1/16" is enough to throw the subject out of focus. In short, if you want to try real macro, buy a $20 set of ebay extension tubes and another $25-30 for a Vivitar-Nikon mount 28-50mm f2.8 manual lens. If you get good enough to require something better, then you can spend $400 for a good marco lens. I say go cheap at first because true macro just doesn't interest some people.
Focal length will dictate angle of view. Wide angle lenses (less than 35mm) may bring you closer to the subject but the subject may not be that big in the picture. Long telephoto lenses (longer than 55mm) may bring you close to the subject without disturbing it from a distance. Short telephotos (longer than 35mm but less than 55mm) is most often just right for most macro jobs being close enough and big enough.
How far do you think you'll be from your subjects? Check minimum focusing distance. How big do you want your subjects be? Check magnification ratio. 1:1 or 1x is life size. 1:5 is 5X smaller than life-size. 5:1 is 5X larger than life-size.