Parafocal distance

I think I understand what parafocal distance is. But I am looking for objective lenses to order, and cannot find it mentioned in any of the written descriptions. From the Openflexure building instructions, it seems that the common choice is 45 mm parafocal distance with the printed plastic spacer. How do I ensure that two objectives are both suitable for this configuration?

The parfocal distance will usually be part of the details that you see when you purchase an objective lens. To fit the same version of an Openflexure microscope two lenses need to be either both infinity corrected or both 160mm tube length, and preferably the same par-focal distance. 45mm is pretty much the most common parfocal distance, but a different distance ‘just’ means moving the optics module nearer to or further from the stage. There is limited adjustment range for this in the Openflexure microscope. On version 6 of the microscope a 35mm parfocal distance can be accommodated easily by using the microscope body without the sample riser. On v7.0.0-alpha the thickness of the sample riser is built in (for added strength and to be able to use embedded nuts). Infinity or 160mm tube length are both common, they will each need a different version of the optics module.

Thank you. I have ordered a 40x objective for 160mm that I think is parafocal 45mm. I believe I have printed the right versions of the parts. In the meantime I will dig out my arduino stuff that I have not looked at for years.

Often the cheap microscope objectives don’t mention it explicitly, but they might mention the sample-to-image distance which specifies the same thing; a “195mm” objective means 45mm parfocal (plus 150mm from the shoulder to the image) while “185” is 35mm parfocal. Generally plan corrected or better objectives will be 45mm, and only the very cheapest (or oldest) ones are 35mm.