Why a 50x OIL objective?

[Scoper]

I have noticed that 50x OIL objectives exist.

What are they used for?

Thanks

[Scarodactyl]

40x/1.4s and similar are obvious enough, 50x/0.90 presumably gives a wider FoV and higher dof to act as a finder for a 100x/1.40 oil objective without having to remove the oil.

[Tom Jones]

Hematology. Also gram stains in clinical microbiology. And higher N.A. Going from 40x dry to 100x and back, you will likely cover the tip of the 40x with oil necessitating cleaning before further use. With a 40x oil, or more commonly 50x oil, you can go back and forth while doing a diff without getting oil on the lower power objective. You need to be able to move quickly between magnifications to be sure you are examining all the appropriate parts of the slide.

The 50x objectives are more popular and common as the magnification is higher. I once replaced a normal, non-fluorite, non-apo, plan 50x oil Nikon lens that had leaked oil with a Plan Apo 40x oil of my own, temporarily, while waiting for a replacement objective. I got a lot of whining from the Hemo techs because the cells were smaller! Better resolution with the Apo, but size mattered!

[apochronaut]

Sometimes the folks who decided such things at product development meetings, delegated 40X objectives for BF , and the 50X as an oil immersion alternative for special purpose, paired with the 100X. Over time many of them sprouted an iris, and became fine optics for DF and fluorescence . Having a 50X with high N.A. allows for the use of 15X eyepieces with which to get a nice spread of magnification : 500X, 750X and 1000X and if the 100X N.A. is a 1.4, 1500X is not too out of the question with an apo and all under immersion. 15X W.F. eyepieces will have a slightly wider apparent field than the 10X in the same eyepiece series too.

Reichert was one of the few that did not make a 50X for their 45MM parfocal infinity diascopic system. Instead they produced an exceptional 40X 1.0 planapo oil with iris.

[Underthescopes]

They are amazing for clinical hematology, larger field to do a differential in.