Spring wildflowers are wonderful and intricate little things. Many are no larger than a quarter in size, some even smaller. For many beginning photographers when they come across this sort of subject they automatically reach for their macro lens in the unfortunate belief that if they just fill the frame with the blossom they will make a great photograph.
My best wildflower photographs are made at about 1/8 to 1/2 life-size, and frequently the flower blossom only occupies a portion of the image frame.
This range can be outside of many zoom lenses magnification capabilities but larger than the 1:1 capabilities of a macro lens. The 1/8 to 1/2 life-size range typically allows the photographer to get close enough to the wildflower bloom to see the intricate detail while also allowing for the inclusion of other elements like a plant leaf or another blossom. Many times this makes for a more interesting photograph because it places the blossom in context of its surroundings. One of my favorite lens combinations for shooting these wildflower scenes is my 70-200mm lens with 25mm of extension tube.
An extension tube is placed between the lens and the camera body. It acts like a bellows and allows the lens to focus closer than normal when working at minimal or close to the minimal focus distance of the lens. Extension tubes do not work with wide angle lenses (they vignette) nor do they work with lenses if you are attempting to focus on something that is far away. They are intended for close-up work only. While they are not a replacement for a macro lens they can add an extra dimension to your camera bag and give you a great tool for shooting wildflower close-ups.
The primary advantage of the extension tube is its simplicity. They contain no optical elements inside. They are simply a spacer designed to fit between your lens and the camera body; as long as the electronic sensors match your camera you are in business. This means that you can purchase off-brand varieties and they will function perfectly well. Kenko brand extension tubes cost about ½ as much as the camera name brand and have served me well over the years. A set of Kenko extension tubes (12mm, 24mm and 36mm) cost about $150.00; much less than a macro lens.
Another advantage of using a 70 to 200mm zoom lens with extension tubes instead of a 90 or 100 macro is that the zoom lens at 200mm has a narrower degree of view. This results in better background control; what is behind your flower blossom yet still included in your image. The longer focal length lens will also give you more working distance between the lens and your subject. Working distance between your lens and subject is beneficial for using reflectors, scrims, or subject comfort if you are photographing little critters that might spook easily.
Different zoom lenses will react differently to the effects of adding an extension tube, my best advice is to experiment with your lenses and a set of extension tubes of varying length.