There is a difference between focus distance and working distance of a camera lens. The focus distance is the distance between the two focal planes of the lens, i.e., the distance between subject and film or sensor. The working distance, on the other hand, is the distance between the subject and the front mount of the lens, i.e., in practice, the smallest gap between camera equipment and subject. The working distance is particularly relevant in close-up and macro photography, because in these conditions the distance between camera and subject are very small, and this may make it difficult to position the camera and light sources appropriately.
The focus distance is a function of the focal length of the lens and the reproduction ratio. In an ideal lens (in practice, a thin lens consisting of a single optical element), at a reproduction ratio of 1:1, the focus distance if 4 times the focal length. Thus, with an ideal lens of focal length equal to 105 mm, the focus distance at 1:1 is 420 mm. The focus distance increases at both lower and higher reproduction ratios than 1:1.
Lenses of the same focal length used at the same reproduction ratio, on the other hand, may have quite different working distances. The working distance depends on the optical, as well as mechanical design of the lens.
Modern macro lenses usually change focal length when focusing at close distances, and therefore their focus distance and working distance cannot be predicted without experimenting with the lens (see also below).
A further factor that complicates this matter is that lenses often have two nodal points. In a simple lens (i.e., a lens consisting of a single, thin and rather flat optical element), the nodal point is located in the geometric centre of the lens. The distance between the nodal point and the focal plane is equal to the focal length of the lens when this is focused at infinity. At 1:1, the distance between nodal point and subject is twice the focal length, and so is the distance between nodal point and film/sensor plane. Modern camera lenses, on the other hand, consist of several optical elements, and the nodal point relative to the film/sensor plane usually is not the same as the one relative to the subject. For instance, in modern wide-angle lenses, the distance between the back element and the film/sensor (normally 40-50 mm) is substantially higher than the focal length of the lens (which can be as low as 8 mm). Therefore, the back nodal point is located in the air at the back of the lens. The front nodal point, on the other other hand, is usually located within the lens in a wide-angle. The situation is reversed in telephoto lenses, which usually have a physical length significantly lower than their focal length. Therefore, the back nodal point of a telephoto lens usually is located in the air in front of the lens itself.
These factors make it difficult or impossible to work out the focus distance and working distance of camera lenses without having access to detailed data (which usually is not made available by manufacturers). Therefore, it is much easier to measure these characteristics by experimenting with lenses.
At first, the exactness of the magnification ratios printed on the focusing scale of the lens should be verified. This can be done by taking pictures of a ruler or millimetre paper at different reproduction ratios. Once the effective reproduction ratios at different settings are known, the focus distance and working distance can be measured for each reproduction ratio. Most camera bodies have a marking that shows the exact position of the film/sensor plane, which is necessary to take accurate measurements of the focus distance
As an example, the following table reports the working distance (WD) (measured from the subject to the plane of the front filter mount of the lens, without lens shade) and focus distance (FD) in mm, measured at a few reproduction ratios with three macro lenses. The last column at the right reports the effective focal length (f) at 1:1, computed as FD/4. The latter assumes that the two nodal points of each lens coincide, and therefore is only an approximation (albeit a useful one for practical purposes). In the table below, I used the reproduction ratios as marked on the focusing scale of each lens, which, for practical purposes, I know to be sufficiently precise. If reproduction ratios are measured with precision, it is possible to compute the distance between nodal planes, but this procedure is not described here.
The above table invites several considerations. On the one hand, it is good that modern macro lenses use floating elements to correct optical aberrations, because this ensures a very good optical quality of images. Internal focusing is also good, because it prevents a lens of long focal length from becoming excessively heavy and expensive. On the other hand, lenses of medium focal length (e.g., the Micro Nikkor 60 mm) keep their focal length almost constant throughout the focusing range, while those of long focal lengths decrease it considerably when focusing at close range. Therefore, in the close-up and macro range, the differences in focal length among macro lenses are much lesser than suggested by their focal length at infinity (which is the only one normally specified by the manufacturer).
The working distance is of immediate usefulness when choosing a macro lens for a particular situation. For instance, when photographing living animals that can see well and are wary of large moving objects, a lens with a high working distance is a natural choice.
In this context, a factor to consider is whether the lens needs a lens shade for a particular shot. For instance, the Micro Nikkor 60 mm has a front element that is sufficiently recessed at all reproduction ratios to render a lens shade superfluous. The front element of the Micro Nikkor 105 mm (non-VR) is well recessed at low reproduction ratios, but at 1:1 may need a lens shade when used in full sunlight. The Sigma 180 mm needs a lens shade when used in sunlight at all reproduction ratios. Of course, a lens shade further reduces the effective working distance.
Based on the above considerations, is it worth owning more than one macro lens? In my practical experience, owning the three lenses discussed above is justified, because close-up and macro photography account for virtually all my professional production. I find myself switching among these lenses quite often, rather than re-positioning my camera when changing the reproduction ratio. Would it be useful for me to have additional macro lenses, like the Tamron 90 mm and Sigma 150 mm? Probably not, because the actual differences among focal lengths would become too small to be significant.
If you do macro photography only occasionally, a single macro lens is probably sufficient. In this case, generally I recommend a 105 to 150 mm. A 180 or 200 mm is useful for living subjects that are likely to flee, but is awkward to use as a general-purpose close-up lens because of the very long working distance. If you must do repro work (which is essentially close-up), or small-product photography, a shorter lens (50-60 mm) is probably better, because its shorter working distance allows you to stay within arms' reach of the subject. This is handy to adjust subjects, light sources and reflectors, and to change subjects when you must shoot several in succession.