For those of you who own a similar objective, how would it compare to more modern 40x finite objectives? Say, for example, an Olympus 40x SPlan at .70NA?? Or a Nikon 40x Fluar at .75NA? I know the the newer objectives would have a much wider fields (around 26mm). Much higher contrast as well? Does the extra .95NA of this antique Spencer add significant detail if focus stacking? Or is it more the cool factor of having such a fine if venerable objective that would make it desirable?
Apochronaut, our resident expert or authority on all things Spencer and American Optical would be the logical one to whom to pose such questions, but good luck finding him around here on a weekend.
I own an example of that objective, as well as its big brother the 3 mm. I also own a later version of it. I'm certainly no authority, and I have never used any of the other lenses you mention, but I'm happy to give you my comments.
Firstly that's an apochromat objective - just about as fully corrected as it was possible to make at the time. If you use it with a fully corrected or achromatic condenser and one of Spencer's compensating eyepieces, you will enjoy images without any color fringes. You must use the compensating eyepieces with it because part of the correction for the objective is in the eyepiece.
As has been said, the field will not be as wide as in more modern lenses, but also it will not be quite as flat overall as a plan either. And the later versions of that lens which came coated seem to give a brighter image to my old eyes. Still, due especially to the correction collar and the high numerical aperture you are likely to get just about as good a visual image, if you have everything adjusted correctly. It takes a bit of practice to use the collar properly while the lens is mounted on the scope. As for photographic work, I really couldn't say since i have no experience there. I will note, however, that at the time that lens was manufactured, apochromats were considered the best for color photography under the microscope.
When I first got mine, I didn't have any compensating eyepieces for it, and all I had was the standard two element Abbe condenser that came on my Spencer series 13. I couldn't see much improvement over an ordinary achromat objective. Then it occured to me to dig out the pair of Spencer Planoscopic eyepieces I had purchased earlier and use them (an old book from the early forties recommended such eyepieces for higher powers and na.s.) Even though not compensating, they were quite an improvement. Better condensers later on helped even more.
I use that lens on both my Research models #3 and #5 - they are completely set up for apocrhomats, with compensating eyepieces and fully corrected condensers etc. So I get pretty nice results from that lens, even though it isn't par focal with most of my other objectives which are newer vintage.
That is indeed kinda pricey for such an old lens, and to me it would not be a very a good buy at that price if I didn't already have the eyepieces for it, but this seems to be an exquisite example of such judging from the seller's photos. I have purchased several items from him in the past, too, and they are always as good as they appear to be.