Update:

There is an updated version of this article available here:
http://markusjaisphoto.com/lenses/choosing-a-macro-lens/

Choosing a macro lens

This article contains my thoughts on how to choose a lens for close-up and macro work in nature. This is not a simple task, as there are many choices out there. The most obvious one would be to buy a dedicated macro lens like Canons EF 2.8/100 Macro or Nikons 2.8/105 Micro VR. But a Macro lens is not the only solution. If you are only starting to get into macro photography, you may not want to buy a macro lens immediately, but use your existing lens. If you have a zoom lens like a 28-105mm or a 17-85mm (or similar), the chances are good that those lenses allow you to do some macro work. Many of those lenses give you a magnification ratio of up to 1:4. This is enough for a lot of subjects like many flowers, mushrooms, leaves and even big insects like dragonflies. If the 1:4 ratio is not enough, you can get even closer with extension tubes. Other options are diopter lenses, bellows and more.

In this article I want to focus on close-up lenses and describe the various options you have. For serious macro photography I think a dedicated macro lens is your best choice. They normally give a a magnification ratio up to 1:1 and are designed to perfom optically superb at close distance.

Close-up or macro lenses normally come in three different ranges. The first group has a focal length of about 50mm (some have 60mm). The second group has a focal lens of 100mm (some have 90mm, Nikon and Sigma offer lenses with 105mm). The third group usually has 180mm (Canon, Sigma, Tamron) or 200mm (for example Nikon). In the following sections, I used the focal length from the lenses Canon offers as a headline. Please replace it with the ones your manufacturer offers while reading.
After describing the different macro lenses, I also describe how a wide angle, a telephoto zoom or even a super telephoto lens can make a good makro lens.

50mm macro lens

Those lenses are the smallest of the macro lenses. That makes them very easy to carry in the field due to their low weight. But even despite this advantage I do not recommend a 50mm or 60mm macro lens. For insects, you often have to get very close for a frame filling picture. And even if you have a camera with a crop factor (for example using Nikons 60mm Micro with a Nikon D300 will give you a 90mm lens), you will have a hard time to get close to many insects without disturbing them and scaring them, and leaving you behind without a picture. The other huge disadvantage is, that a 50mm lens makes it much more difficult to get a calm background, due to its wider angle of view. A calm background crucial in many macro photographs and it is much harder to achive this with a 50mm lens than with a longer lens.

100mm macro lens

A 100mm lens (Nikon and Sigma offer 105mm, Tamron 90mm) is very popular among many nature photographers. Many books recommend this as your first lens. In comparison with a 50mm lens, it allows you to work at a greater working distance (especiall important for insects) and also makes it easier to get a calmer backbround due to it's narrower angle of view.
Imagine photographing a beautiful red flower against a green background with a 50mm lens. Behind the red flower are many white flowers. Including an unsharp white flower in the image would normally distract the viewer from the red flower. Sometimes, you can get rid of the white flower with changing the position of the camera. But this is not always possible and you might just add another white flower somewhere else in the picture.
If you now change your lens to a 100mm lens you have a much narrower angle of view than with the 50mm lens. That makes it a lot easier to get rid of the white flowers in the background.
A 100mm macro lens is also quite light and small (compared to a 180mm) and easily fits into your bag or backpack.

180mm macro lens

As just explained under the last paragraph about the 100mm lens, the longer the lens, the easier it is to get a calm background. This is the reason why my favorite macro lens is my Canon EF 3.5/180L Macro. With the 180mm lens mounted on a tripod head (I recommend the Manfrotto 410 head) it is pure joy to compose an image of a flower or any other subject. The very narrow angle of view allows you to set the subject apart from the background. The longer focal length is also great for insects. If 180mm is still not enough, you can add a 1.4x extender (or even a 2x) to get even more focal length. This disadvantage of the 180mm (or 200mm) lenses are the higher price and the bigger weight. They all come with a tripod collar as they are too heavy for mounting the camera on the tripod. But if you are doing a lot of close-ups, especially if want to photograph insects, you definitely should consider buying such a lens. If the Canon and Nikon lenses are too expensive for you, check out the Sigma and Tamron 180mm lenses. They are very popular, very sharp and they got great reviews.

Wide Angle Lens

Wide angle lenses can offer interesting possibilites for close-ups. For example, you can get pretty close to a flower and also show it's habitat. This is normally not possible with a telephoto lens like a 180mm macro lens. A wide angle lens is not very well suited for high magnificationa like 1:1 as you would have to get very close to the subject. Still, it is worth putting a wide angle lens in your camera bag when going out for close-ups. After you've taken a shot from a subject like a flower with a macro lens, also try to see if also works well when shot with a wide-angle lens. Which wide-angle lens you choose depends on your camera. If you are using a full frame camera like Nikon's D3 or Canon's 5D and 1K Mark III, you will probably own a "normal" wide-angle zoom like a 16-35 or similar. If you are using a camera with a smaller sensor like Nikons D300 or Canon's 40D you may want to get a wide-angle lens especially designed for those cameras. Nikon offers a 4/12-24, Canon a 3.5-4.5/10-22 to give you real wide-angle with such cameras.

70-200 lens

Canon offers a wonderful 4/70-200 zoom lenses. It comes in two versions, one with IS, one without IS. I have the version with IS and absolutely love it. The lens is extremely sharp and IS works very well. Also, the lens is very light (much lighter than a 2.8/70-200). That lens also make an interesting close-up lens, as a zoom often is more flexible than a prime like a 180mm macro lens. The lens offers a magnification rate of about 1:4,8 according to Canon. I like using that lens for flowers and mushrooms when I don't have to get to close. I also like using it with a Canon EF 25mm II extension tube. It is not a replacement for a real macro lens, but when you already own that lens, try using it with an extension tube. Nikon does currently not offer a 4/70-200 lens, only a 2.8/70-200 (with VR). It does not offer a very small close focusing distance, but with extension tubes it should work fine. But as I already wrote, a 2.8/70-200 is much heavier than a 4/70-200. Hopefully Nikon will offer such a lens in the future and hopefuly with a better close focusing distance.
Getting a new 70-200 just for close-ups might be "overkill", but if you already have one, try using it.
Nikon also offered a 70-180 macro zoom, but as far a I know, that lens is no longer available. You may want to check if you can get a used one. This lens was designed for macro work and has a much smaller close focusing distance than Canon's 4/70-200L.

100-400mm lens / 200-400mm lens

Canon offers a 4.5-5.6/100-400 and Nikon and Sigma offer a similar 80-400mm lens (all with image stabilization). The Canon has a mignication rate of about 1:5 at 400mm. This is interesting for flowers and also for many dragonflies. I prefer the 4/300L from Canon as it is lighter and also sharper. The 100-400mm is a great lens, though. And for a zoom it is very sharp. I made some very sharp close-ups with that lens. When photographing subjects that are moving but you can't change your position, the zoom can be a great advantage.
A really amazing lens is the 4/200-400 VR from Nikon. That lens is especially popular with bird and wildlife photographers as it offers a very interesting zoom range and superb image quality. The lens is as sharp as a prime. It also has a very interesting close focusing distance of 2 meters. This is much better than most 400mm prime lenses. The magnification rate is about 1:3.7. which is normally enough for many big insects and flowers. The Nikon 4/200-400 VR is unfortunately a very expensive and also quite heavy lens (much heavier than the 4.5-5.6/80-400 VR from Nikon). But if you can afford it, it might be a very interesting addition to your Nikon lens collection. If you are shooting Canon (like I do), you are out of luck. Currently Canon does not offer such a lens. I hope that this will change soon. I am sure many photographers would be very interested in a Canon EF 4/200-400L IS.
The Nikon 4/200-400VR also delivers great results with the 1.4x extender.

300mm lens

I recently bought a Canon EF 4/300L IS for my EOS 40D. It is great for flowers, mushrooms and large insects. About the size of a my EF 3.5/180L Macro, it is not too heavy to carry over long distances and is also easy to hand hold (when necessary - I prefer to use a tripod whenever possible). It gives a magnification rate of about 1:4 (the Nikon 4/300 gives you even 1:3.7) and when used with a 1.4x extender or an extension tube it allows you to get even closer. You can also combine the extender and the extension tube. At the time of this writing, I have not yet had the time to test this lens on dragonflies (when I bought it, dragonfly season was almost over in Germany) but I hope to use this a lot next year.
The 4/300 has a much narrower angle of view than a 180mm lens and therefore will help you to get calmer backgrounds. I highly recommend a 4/300 for any photographer seriously interested in close-up photography of flowers, large insects or similar subjects. A 2.8/300 can also be used, but due to it's much higher weight is not so well suited for close-ups. Also the 2.8/300 lenses often do not focus as closely as the 4/300 lenses.

Super Telephoto Lenses

At first it may seem strange to use a super telephoto lens like a 4/500 for close-ups. The magnification ratio of my Canon EF 4/500L IS is only 1:8, which does not make this lens very suitable for close-ups. But when you do not want to get too close that lens can offer some interesting possibilites. Also keep in mind that you can add an extension tube and an extender to the lens which increases the magnification ratio. The 4/500L can be interesting for very shy insects like some dragonfly species. Also you can not always got as close to a flower as you want to. There maybe be an obstacle between you and the flower (like a deep creek). Or imagine you are walking on a trail and see a flower several meters away from the trails. You don't want to leave the trail because you do not want to trample down other flowers to get the shot (The well being of other flowers should always be more important than a good photograph). In some areas (like some national parks) it may also be forbidden to leave the trails. In those cases a 4/500 may save the day. Due to it's very narrow angle of view, a super telephoto lens also makes it very easy to isolate a subject against a calm background.
I do not suggest you should spend thousands of Euros or Dollars for a big lens just to photograph flowers. That would be overkill, but if you already have such a lens (maybe because you are shooting birds), think about using it from time to time for flowers and insects.

Summary

So what to do ? It depends all on your needs (and on how much money you want to spend). I use almost all the options above (except the 100-400 which I also used, but have sold now) and this works very well for me.
If you get only one lens I suggest to invest in a 180mm (or 200mm) macro lens (Sigma also offers a 150mm lens which is quite interesting). If you want to buy more lenses an interesting combination would be a 100mm macro and a 4/300 telephoto lens (consider adding an extension tube and 1.4x extender).
In the end you must decide on your own what is best for you. I hope that article could give you some interesting information about the various options and you can now make a more informed choice.