We are talking here about true Macro lenses, not the marketing hype Macros that are really only a close focussing lens. True Macro photography does not start until you reach the reproduction ratio of 1:1. That means that the image on the film/sensor is the same size as the object the image relates to.Macro photography continues to be called just that until you reach reproduction ratios of around 10:1, after which it becomes Microphotography.
So, why so many focal lengths to do the same job? Well there are a number of reasons, including angle of view, working distance and depth of field.
For example, at the two extremes, the 50mm f/2.8 EX DG Macro lens gives an angle of view of 46.8° and a minimum focussing distance of 18.9cm whereas the 180mm f/3.5 EX DG HSM Macro lens gives an angle of view of 13.7° and a closest focus distance of 46cm, but both give a magnification ratio of 1:1.
Now imagine you need to photograph small items repeatedly over a long period of time. The easiest way to achieve this would be with a suitable set-up as seen here where the camera is fixed on a stand and the objects are placed on a table beneath it. Now to use the 180mm lens in this situation would mean a stand with a post over a metre long, not a sensible proposition, whereas a 50mm lens fits the bill perfectly.
Other disciplines require longer working distances, as with shooting insects, butterflies and some smaller reptiles where it is not so wise or desirable to approach too closely. Here, the longer focal lengths come into their own. In between these two extremes are the others that have various advantages and disadvantages.
Traditionally, the 105mm f/2.8 EX DG has been the most popular, not only as a compromise between the extremes, but doubling up as an excellent short telephoto suitable for portrait work. The 70mm f/2.8 EX DG, as the newest of the bunch, has been introduced to fit in the same area on APS-C sized sensors but, sensibly, retains the ability to be used on full frame and 35mm film cameras too.
The 150mm f/2.8 EX DG HSM occupies the large gap between the 105mm and the 180mm and inherits the internal focussing of the longer lens along with the sonic motor. This does make things like bug hunting a little easier, as the autofocus is somewhat quicker. However, fast autofocus is not the prime domain of True Macro lenses because of the need for extreme accuracy in the focussing. Because of this, Macro lenses tend to have finer pitched focussing threads that need more turning to achieve focus.
Depth of field is a subject in its own right but suffice to say that it is measured in millimetres in Macro photography. DOF also decreases, as focal length gets longer, introducing another reason for having multiple choices. Apparent DOF also changes with sensor size, smaller sensors giving apparently greater depth and introducing another variable.
Another point that should be remembered here is that quoted closest focussing distances are measured from the film/sensor plane. This means that most of the camera body, along with all of the lens, is included in the distance given. This introduces another problem in some disciplines, that of getting light onto the subject without shadows being cast by the camera/lens combo. Over the next few articles we will look at these variable areas in more depth.