Zeiss finite objectives 20x to 32x?

[PeteM]

I've recently assembled several Zeiss scopes for our Micronaut program and have noticed that Zeiss 20x and 25x objectives are hard to find. Worse, four of five Zeiss 25x objectives acquired were delaminated and had to be sent back.

The 16x and 40x ones, including Plan and Neofluar versions, are widely available and almost always good. Even two 40x apos arrived in good shape. Zeiss also made some 20x finite versions - hard to find - and 32x long working distance ones (easier to find, but usually intended for inverted scopes).

So, I have three questions for those who know Zeiss:

1) Are the problems with 25x Zeiss known and common? Or just my bad luck?

2) Is there a reason Zeiss objectives don't show up all that often between the widely-available 16x and 40x magnifications? I do know that Zeiss didn't even list a 25x (or 20x or 32x) DIC slider for its last finite DIC version. Maybe they figured the 16x had a high enough numerical aperture to obviate the need for something in that range? Maybe the 25x couldn't be easily made strain-free? Or??

3) Is there another finite objective choice closely matching Zeiss corrections (and phase annuli) in the 20-25x range? Especially for protists and phase contrast - I'd like to be able to equip several scopes with that magnification.

[imkap]

I have a 25 and it isn't delaminated... I know nothing more of it

[Hobbyst46]

PeteM, my experience, from my small arsenal, is that the (no. of delaminated/total) ratio is 0/2 (10X0.22 or 0.30), 1/3 (16X0.40 or 0.32), 0/2 (25X0.45), 1/3 (40X0.75, dry), 1/2 (63X1.25 or 1.40), 0/1 (100X1.30). So, no indication for a specific problem with the 25X, relative to other objectives. None of them is strain-free or DIC, though.
Naturally I inquire the sellers about delamination, but that is obvious and trivial. I probably discovered some of the delaminations only lately.

[apochronaut]

Just a general de-lamination comment.
I have handled multiples of a lot of different objectives,, mostly Spencer, AO/Reichert , B & L , Zeiss West, Nikon and Leitz. There are patterns . It is almost guaranteed for instance that a B & L 20X .50 flat field phase objective will be de-laminated by now, if it has been used., and it is always the same element. Certain other objectives have similar stories, with a disproportionate number of failures in objectives that have been used and more often than not in the same doublet. New old stock, not so much.
It seems that de-lamination in certain instances is caused by the action of light on the cement and not just poor cement chemistry.

[Adam Long]

1) I must have about 30 Zeiss objectives now. Only two show any delamination, only one of which, 16x PH2 Neofluar, seriously degrades the image (fine detail is doubled). The other, Planapo 25x, shows multiple planes of delamination but limited to the periphery so still gives good images. Eyepieces and telan lenses are a different matter, at least half are seriously delaminated, but thankfully repair is easier.

I also have a F-LD 20x, in good condition,
a Plan PH2 25x, this is in good condition
two 16x Neofluars, one phase and one not, both are good. I think you have been unlucky, but I only have two 25x so perhaps too small a sample.

I've think I've only seen the 25x Neofluar once on ebay, seems very rare, and the Plan version is not exactly common. But the 25x Planapo seems to be at least as common as any other planapos.

2) Apart from the rare 7-hole Phomi/ Uni nosepiece, Zeiss standards are limited to 4 or 5-hole nosepieces. That doesn't leave much room for low and high power if you have too many mid-power options. I rarely have 16x, 25x, 40x, on at the same time, usually I have 4x, 10x, 16x, 25x, 63x corr as my dry set but with 5 you end up with a big jump somewhere however you arrange. An Optovar helps of course.

[PeteM]

. . . Eyepieces and telan lenses are a different matter, at least half are seriously delaminated, but thankfully repair is easier. . .

Adam - - curious -- have you found a way to repair delamination in the intermediate pieces (finite adjusting telan lenses)? I've heard of some folks soaking them in balsam oil, but only success if the delamination is around the edges rather than the sort of oil-slick-looking delamination more to the middle.

[apochronaut]

Synthetic cements can sometimes be repaired with heat. I have had partial success loosely clamping doublets between two cork jaws, at around 250°F. I don't know what the cement was.

For balsam, I have had better success again clamping between cork jaws and using ethanol. The critical thing is the timing. You want to just barely dissolve and reset the lenses , not dissolve and remove material. Once reset, the ethanol evaporates quickly, so the balsam rehardens quickly. Often there is still some peripheral unevenness but it is usually outside the f.o.v. Plans are harder to do than conventional objectives.
With balsam oil, the oil takes a long time and it is difficult to determine the end point. The oil is not very volatile , so once overdone, you can't take it away: the cement is too soft.

[Adam Long]

. . . Eyepieces and telan lenses are a different matter, at least half are seriously delaminated, but thankfully repair is easier. . .

Adam - - curious -- have you found a way to repair delamination in the intermediate pieces (finite adjusting telan lenses)? I've heard of some folks soaking them in balsam oil, but only success if the delamination is around the edges rather than the sort of oil-slick-looking delamination more to the middle.

Sorry for the late reply - totally missed this. Yes, I've successfully repaired three telan lenses so far - two from the single Epi heads and one from the widefield revolver 0.63. In all cases it was the top doublet that failed with an oil slick look. The widefield lens was more peripheral but was clearly affecting detail - out of focus highlights were not circular but the shape of the delamination front.

IMG_5424D.jpg (99.6 KiB) Viewed 944 times

The difficult part is removing them from the brass mount which on one side is turned to a very thin edge that is pressed tightly against the glass. The lens is freed by carefully bending the brass out with the tip of a scalpel under a stereo microscope, but you must be very careful not to chip the edges of the glass. With the three I only made one chip big enough to be seen with the naked eye. A a hard plastic tool would be safer but it is hard to make one sharp enough.

Once the lens is out, mark the up/down orientation then soak the joint in immersion oil. This can take a week or more, but I find the oil front typically penetrates half the remaining gap each day so you soon have an idea if it is working. If there are areas where it is not penetrating scrape any excess cement out of the joint.

The three lenses are all now functioning well with no delamination visible.