Working Distance
W.D. (Working distance) refers to the clearance in mm between the front tip of the objective and the surface of the specimen focused.
N.A. (Numerical Aperture) and W.D. are not directly related. But most of the time, for lens elements of a similar size, increasing the WD result in a lower NA.
So there’s usually a trade-off between one or the other.
Long working distance (LWD) and ultra long working distance (ULWD) objectives will have lower NA than the normal version of the same objective.
Here a table with different Olympus objectives to make comparisons between WD and NA.
| Objective | W.D. | Focal Length | ||||
|---|---|---|---|---|---|---|
| MDPlan 20x/0.40 | 1.00mm | 9.00mm | ||||
| NeoDPlan 20x/0.40 | 1.00mm | 8.99mm | ||||
| NeoSPlan 20x/0.46 | 3.00mm | 9.00mm | ||||
| MSPlan 20x/0.46 | 3.10mm | 9.00mm | ||||
| ULWD MSPlan 20x/0.40 | 11.00mm | 8.99mm | ||||
| ULWD NeoSPlan 20x/0.40 | 11.00mm | 8.99mm | ||||
| SLMPLN 20X/0.25 | 25.00mm | 9.00mm | ||||
| MSPlanApo 50x/0.95 | 0.24mm | 3.60mm | ||||
| NeoDPlan 50x/0.75 | 0.34mm | 3.60mm | ||||
| MDPlan 50x/0.75 | 0.38mm | 3.60mm | ||||
| MSPlan 50x/0.80 | 0.47mm | 3.60mm | ||||
| NeoSPlan 50x/0.80 | 0.50mm | 3.60mm | ||||
| LWD MSPlan 50x/0.60 | ?3.20mm | 3.60mm | ||||
| ULWD MSPlan 50x/0.55 | 8.10mm | 3.60mm | ||||
| ULWD NeoSPlan 50x/0.55 | 8.10mm | 3.60mm | ||||
| SLMPLN 50x/0.35 | 18.10mm | 3.60mm | ||||
| NeoDPlan 80x/0.90 | 0.18mm | 2.25mm | ||||
| MDPlan 80x/0.90 | 0.20mm | 2.25mm | ||||
| ULWD MSPlan 80x/0.75 | 4.10mm | 2.25mm | ||||
| ULWD NeoSPlan 80x/0.75 | 4.10mm | 2.25mm | ||||
| NeoMSPlan 100x/0.90 | 3.00mm | 1.80mm | ||||
| ULWD MSPlan 100x/0.80 | 3.20mm | 1.80mm | ||||
| ULWD NeoSPlan 100x/0.80 | 3.20mm | 1.80mm | ||||
| SLMPLN 100x/0.60 | 7.60mm | 1.80mm |
Mitutoyo are well known for their long working distance objectives.
| Objective | W.D. | Focal Length | ||||
|---|---|---|---|---|---|---|
| Plan Apo LWD 1x/0.025 | 11.0mm | 200.00mm | ||||
| Plan Apo LWD 2x/0.055 | 34.0mm | 100.00mm | ||||
| Plan Apo LWD 5x/0.14 | 34.0mm | 40.00mm | ||||
| Plan Apo HR 5x/0.21 | 25.5mm | 40.00mm | ||||
| Plan Apo LWD 7.5x/0.21 | 35.0mm | 26.67mm | ||||
| Plan Apo LWD 10x/0.28 | 34.0mm | 20.00mm | ||||
| Plan Apo HR 10x/0.42 | 15.0mm | 20.00mm | ||||
| Plan Apo SL 20x/0.28 | 30.5mm | 10.00mm | ||||
| Plan Apo LWD 20x/0.42 | 20.0mm | 10.00mm | ||||
| Plan Apo SL 50x/0.42 | 20.5mm | 4.00mm | ||||
| Plan Apo LWD 50x/0.55 | 13.0mm | 4.00mm | ||||
| Plan Apo HR 50x/0.75 | 5.2mm | 4.00mm | ||||
| Plan Apo SL 100x/0.55 | 13.0mm | 2.00mm | ||||
| Plan Apo LWD 100x/0.70 | 6.0mm | 2.00mm | ||||
| Plan Apo HR 100x/0.90 | 1.3mm | 2.00mm |