An important feature of any macro lens is the distance it allows between its front element and the photographic subject. This distance at full magnification is called the working distance. If it is too short, your lens may not only scare away your subject, but also cast a shadow on the scene.
The working distance w of a lens is determined by its minimum focus distance (MFD) dmin between sensor and subject, the camera’s flange focal distance dflange between sensor and lens mount and the length of the lens barrel dbarrel between lens mount and front element as
|w||=||dmin – (dflange + dbarrel)||(W1)|
Note that if your lens extends for macro photography, you will have to use its maximum length here. If you are using a lens extension tube, you will have to subtract its length as well.
For example, consider the AF macro lenses in the Micro Four Thirds system (rounded to full mm):
|lens||MFD||flange focal distance||barrel length||working distance|
|Olympus 30 mm f/3.5||95 mm||19 mm||60 mm||16 mm|
|Lumix 30 mm f/2.8||105 mm||19 mm||63 mm||23 mm|
|Leica 45 mm f/2.8||150 mm||19 mm||62 mm||69 mm|
|Olympus 60 mm f/2.8||190 mm||19 mm||82 mm||89 mm|
Obviously, a longer focal length allows for a much longer working distance. So does a longer focal length make a better macro lens?
If you are after dragonflies in the wild or want to take photos of coins with natural light, it is certainly a good idea to use a longer focal length. Note that a telephoto lens with an extension tube may also work well in these cases.
On the other hand, if you want to take photos of the underside of mushrooms in their natural habitat or other things in tight spaces, a shorter focal length may be the only chance to get the image. So as with other photographic subjects, it depends on what you want to do.