Spatial filter help, please... Will all 20X microscope objectives have the same focal length? If not...?

Question

I have recently taken delivery of a 20X microscope objective that I will be using to expand my reference beam for holography. At some point in the future (when budget allows), I will be incorporating said objective into a spatial filter. In determining pinhole size necessary, I will need to know the focal length of my objective.

Specs I know so far:

532nm
1.5mm beam
20X microscope objective

How do I determine the focal length of the lens in question? It is not marked with anything except 20/0.40...

Answer 1

They won't all have the same FL. In an ideal simple system they would, but the complexities of modern microscopes with different tube lengths, tube lenses and optionally "infinity-corrected" objectives makes things complicated. Ref. 1 is an example. Ref. 2 may help in understanding this.
You could find the actual FL by imaging through the lens a distant object (e.g., the sun) on a piece of paper and measuring the distance from the image to the lens. Exactly what part of the lens you measure to is something you'd have to find out.