Back focal plane reference tables
Why the BFP is important
Image credits: phydemo
The principle of Köhler illumination is to focus the conjugate plane of the lamp into the back focal plane of the microscope objective in order to get a very uniform illumination.
In a transillumination setup this is very simple to achieve because you just shine a collimated beam through the front lens of the objective and it will focus into the BFP.
When using reflected light illumination, you have to focus the light into the BFP. This means an epi Köhler illumination is usually calibrated for a single BFP.
Modern MPlanFL objectives probably all have a BFP of -8mm for this reason.
UPlanFL all have a BFP of -19.1mm so we can assume this is for epi fluorescence and that the illumination for fluorescence is slightly different (to be confirmed).
Official data
Official data from EVIDENT. BFP is in mm.
| Reference | Name | BFP | |||||
|---|---|---|---|---|---|---|---|
| MPLFLN100X | MPlanFL N | 100x/0.90 | -8 | ||||
| MPLFLN50X | MPlanFL N | 50x/0.80 | -8 | ||||
| MPLFLN20X | MPlanFL N | 20x/0.45 | -8 | ||||
| MPLFLN10X | MPlanFL N | 10x/0.30 | -8 | ||||
| MPLFLN5X | MPlanFL N | 5x/0.15 | -8 | ||||
| MPLFLN100XBD | MPlanFL N | 100x/0.90 BD | -8 | ||||
| MPLFLN50XBD | MPlanFL N | 50x/0.80 BD | -8 | ||||
| MPLFLN20XBD | MPlanFL N | 20x/0.45 BD | -8 | ||||
| MPLFLN10XBD | MPlanFL N | 10x/0.30 BD | -8 | ||||
| MPLFLN5XBD2 | MPlanFL N | 5x/0.15 BD | -8 | ||||
| MPLFLN100XBDP | MPlanFL N | 100x/0.90 BD P | -8 | ||||
| MPLFLN50XBDP | MPlanFL N | 50x/0.75 BD P | -8 | ||||
| MPLFLN20XBDP | MPlanFL N | 20x/0.40 BD P | -8 | ||||
| MPLFLN10XBDP | MPlanFL N | 10x/0.25 BD P | -8 | ||||
| MPLFLN5XBDP | MPlanFL N | 5x/0.15 BD P | -8 | ||||
| UMPLFLN20XW | UMPlanFL N | 20x/0.50 W | -19.1 | ||||
| UMPLFLN10XW | UMPlanFL N | 10x/0.30 W | -19.1 | ||||
| MPLAPON100XO2 | MPlanApo N | 100x/1.45 Oil | -19.1 | ||||
| MPLAPON100X | MPlanApo N | 100x/0.95 | -8 | ||||
| MPLAPON60X | MPlanApo N | 60x/0.90 | -19.1 | ||||
| MPLAPON50X | MPlanApo N | 50x/0.95 | -8 | ||||
| MPLN100X | MPlan N | 100x/0.90 | -25.8 | ||||
| MPLN50X | MPlan N | 50x/0.75 | -18.6 | ||||
| MPLN20X-1-7 | MPlan N | 20x/0.40 | -20.6 | ||||
| MPLN10X | MPlan N | 10x/0.25 | -16.8 | ||||
| MPLN5X | MPlan N | 5x/0.10 | 6 | ||||
| MPLN20XBD | MPlan N | 20x/0.40 BD | -20.6 | ||||
| MPLN10XBD | MPlan N | 10x/0.25 BD | -16.8 | ||||
| MPLN5XBD | MPlan N | 5x/0.10 BD | 6 | ||||
| UPLFLN100XO2 | UPlanFL N | 100x/1.30 Oil | -19.1 | ||||
| UPLFLN60XOI | UPlanFL N | 60x/1.25 Oil | -19.1 | ||||
| UPLFLN60X | UPlanFL N | 60x/0.90 | -19.1 | ||||
| UPLFLN40X | UPlanFL N | 40x/0.75 | -19.1 | ||||
| UPLFLN20X | UPlanFL N | 20x/0.50 | -19.1 | ||||
| UPLFLN10X | UPlanFL N | 10x/0.30 | -19.1 | ||||
| UPLFLN4X | UPlanFL N | 4x/0.13 | -1.3 |
Measurements
Here are my own measurements made with rudimentary tools.
The lens is placed in direction of the sun and the circle formed on it’s back is measured at different distances to calculate the BFP.
Precision is not great but enough to get an idea of where is located the BFP.
| Name | BFP | |||||||
|---|---|---|---|---|---|---|---|---|
| Olympus | MSPlan | 5x/0.13 | ∞/- | -5 | ||||
| Olympus | MSPlan | 10x/0.30 | ∞/- | -30 | ||||
| Olympus | MSPlan | 20x/0.46 | ∞/0 | -25 | ||||
| Olympus | LWD | MSPlan | 50x/0.60 | ∞/0 | -19 | |||
| Nikon | CF Plan | 10x/0.30 | ∞/0 | -14.7 | ||||
| Nikon | Plan Apo | 4x/0.20 | 160/- | -5 |
Later I plan to make a better setup and to measure the BFP of all of my objectives.