An important feature of any macro lens is the distance it allows between its front element and the photographic subject at full magnification. This distance 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 dwork 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 lens front element, as
| dwork | = | 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 or a teleconverter, you will have to subtract its length as well.
For example, consider the AF macro lenses available in the Micro Four Thirds system (all figures rounded to full mm):
| lens | MFD | flange focal distance | barrel length | working distance |
|---|---|---|---|---|
| dmin | dflange | dbarrel | dwork | |
| Olympus 30 mm f/3.5 | 95 mm* | 19 mm | 60 mm | 16 mm |
| Lumix 30 mm f/2.8 | 105 mm | 19 mm | 64 mm | 22 mm |
| Leica 45 mm f/2.8 | 150 mm | 19 mm | 63 mm | 68 mm |
| Olympus 60 mm f/2.8 | 190 mm | 19 mm | 82 mm | 89 mm |
| Olympus 90 mm f/3.5 Pro | 224 mm** | 19 mm | 136 mm | 69 mm |
| Olympus 90 mm f/3.5 Pro | 250 mm | 19 mm | 136 mm | 95 mm |
*at 1.25× magnification, **at 2× magnification, others at 1× magnification
8.1 Focal length and working distance
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 a lens extension tube may also work well in the latter case.
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.