By lens working distance, I am referring to the distance from the end of the lens (or the end of the lens hood if in place) to the subject. This is the amount of space you have to work in.
While lens working distance matters little to most of my shooting, there are situations where knowing the distance in front of the lens matters. Macro photography at MFD (Minimum Focus Distance) is typically the situation where working distance matters the most and other than physical obstruction (such as a lens hood bumping into part of the subject), frightening away the subject is usually the biggest issue.
Lens manufacturers always include the MFD (Minimum Focus Distance) in their list of specs, but the MFD spec is based on the distance from the subject to the imaging sensor, not from the end of the lens or lens hood to the subject. This is no problem, as data from the site's Lens Specifications and Measurements tool along with a simple calculation will provide the needed from-the-end-of-the-lens working distance.
Simply take the "measured" LL (Lens Length – with or without hood) and add 1.4" (36.4mm) to account for the Canon EF and EF-S lens mount imaging sensor-to-electrical-contact distance (ISCD). The Nikon F lens mount ISCD value is 1.6" (40.5mm). Subtracting the total imaging sensor to end-of-lens distance from the MFD (you can use this site's measurement or the manufacturer spec) provides the WD (Working Distance) for the lens at MFD. I'll make this a formula:
WD = MFD - LL - ISCD
I just completed the Sigma 105mm f/2.8 EX DG OS HSM Macro Lens review, so I'll use that Canon-mount-equipped lens for an example. From this lens' specs, we see that the MFD is 12.3" (312mm), the total measured lens length is 5.3" (134mm) and 7.2" (182mm) with the hood installed.
Using the above formula to determine the without-hood working distance:
WD = 12.3" - 5.3" - 1.4" or WD = 312mm - 134mm - 36.4mm
Using this calculation shows that the Sigma 105 OS lens' working distance is about 5.6" (141.6mm). Install the hood and the minimal working distance goes down to 3.7" (93.6mm).
If you would like to rely on the manufacturer-provided lens length, the formula must be adjusted slightly. Most lens manufacturers (including Canon, Nikon, Sigma, Tamron and Zeiss) provide lens length specs that exclude the distance from the rear of the lens mount to the protruding electrical contacts. This usually explains the discrepancy that you see between the manufacturer specs and the actual measurements shown in the specs and measurements tool.
To use the manufacturer-provided lens length in the above formula, the electrical contact extension length from the lens mount must be added to the imaging sensor-to-contact distance (ISCD) in the formula. This distance from the sensor to the rear of the lens mount is called the flange focal distance (FFD) and is what now gets plugged into the formula. The distance from the back of the Canon EF and EF-S lens mount to the contacts is .3" (7.6mm), giving it an FFD of 1.7" (44mm). The distance from the back of the Nikon F lens mount to the contacts is .24" (6mm), giving it an FFD of 1.8" (46.5mm).
The new formula using flange length is:
WD = MFD - mfgLL - FFD
Shooting with a format other than Canon or Nikon? More flange focal distances can be found on Wikipedia.
To determine the working distance at a focus distance other than the MFD, simply plug your focus distance into the MFD value in the formula.
While easy to calculate, remembering the calculation is not always easy. Keep this page in your physical or mental bookmarks for the next time you need to calculate lens working distance.