Minimum focus distance is an important lens specification, but this number does not indicate the working distance, 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 for lens selection. Macro photography at MFD (Minimum Focus Distance) is typically the scenario where working distance matters the most, and along with physical obstruction (such as a lens hood bumping into part of the subject), frightening away the subject can be a problem. You would run too if it seemed that something 5x or 10x your height was going to bump into you.
The manufacturer provided MFD (Minimum Focus Distance) or MOD (Minimum Object Distance) spec is the distance from the imaging sensor 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 working distance.
The manufacturer-specified lens length mfgLL must be subtracted from the minimum focus distance.
This distance from the sensor to the rear of the lens mount is called the flange focal distance (FFD). 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). Here is a table of flange focal distances.
The working distance formula using flange focal distance is:
WD = MFD - mfgLL - FFD
Shooting with a lens mount other than listed in the chart? 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.
If you would like to rely on our measured lens length, the formula must be adjusted slightly. Most lens manufacturers (including Canon, Nikon, Sigma, Sony, 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 measured lens length in the above formula, the electrical contact protrusion length from the lens mount must be subtracted from the flange focal distance to get the imaging sensor-to-contact distance (ISCD) shown in the new formula below. 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 Canon RF mount ISCD is about 0.51" (13mm). The Nikon F lens mount ISCD value is 1.6" (40.5mm). The Sont E mount ISCD is about 0.57" (14.5mm).
WD = MFD - LL - ISCD
Just prior to creating this page, I 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's 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's working distance is about 5.6" (141.6mm). Install the hood and the minimal working distance goes down to 3.7" (93.6mm).
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.