A new AO objective design!?

[sailor_ed]

I purchased an AO 40/.66 plan achro on ebay. Cat # worn off. It has an odd dimple right in the center of the first lens. I had assumed that it was damage but now I have seen another lens advertised that seems to show the same thing. Does anyone have any thoughts about this? It is not an odd reflection, it is an actual depression but it's edges are quite polished looking unlike a fracture.
Thanks
Ed

[apochronaut]

There is nothing odd about this at all. Using a concave front lens is a common design for many later generation plan objectives. Most of the better designs of the big manufacturers use it. I'm guessing the objective is an 1128.

Of the bright field transmitted light objectives, American Optical originally issued the cat.# 1023 objective in the later 60's. It had a planoconvex front element. Around 1980, they released the cat.# 1309 advanced planachro. It had a concave convex front element , which is beneficial for improved planarity. Additionally, the 1309 has superior colour correction. Later in the decade the 1128 came out. It had a concave convex front element as well.
Although the 1128 was released later than the 1309, it entered the market more of as an improvement on the 1023 as well but of a cheaper to manufacture design. Although the 1309 outperforms the 1128, the latter has the benefit of having a longer working distance, so it is useful with a haemocytometer and a slightly better choice in that than the 1023.

viewtopic.php?f=5&t=4420&hilit=objective+comparison

[sailor_ed]

So everyone except me probably knows about this. Well, not only me but the seller too.

Thanks apochronaut!

[ChrisR]

Concave yes, but I'd never noticed any dimples!

[apochronaut]

Yes there are dimples, or more accurately stipples. The surrounding glass abutting the concave lens is blackened with a coating and it sometimes has a stippled appearance. It is really just there, similar to the ground glass around the exit lens of many condensers to block light that is too far off axis and might cause flare. I'm not really sure why the word " dimple " came up but the concave front element does look like a dimple.

[MichaelG.]

There is nothing odd about this at all. Using a concave front lens is a common design for many later generation plan objectives. Most of the better designs of the big manufacturers use it. I'm guessing the objective is an 1128. [ ... ]

Please forgive my innocence regarding A.O. designs, but I remain confused by the photo in the opening post: It appears to show a relatively large diameter front element, with a small concavity ground into it [in which case the surrounding area would need to be masked] ... Is that actually the construction ?

A link to the patent, or any other source for the design, would be very interesting.

MichaelG.
.
Edit: I note that you have already confirmed the 'blackening'

[MichaelG.]

... It has an odd dimple right in the center of the first lens. [ ... ] It is not an odd reflection, it is an actual depression ...

So: is this ^^^ the optical illusion to beat all optical illusions ??

MichaelG.

[apochronaut]

The front element has a concave front surface but a relatively narrow diameter. This allows the rear surface to have a gentler curvature and therefore broader diameter than a .66 lens would normally require but still attain a .66 N.A. plus a wide image circle. Thus the actual lens element is much wider than the front surface indicates. The surrounding unused front portion of glass has a stippled light block, in order to reduce reflection and block rays from entering and scattering. I had one of those that had the surround worn off and the lens had reduced contrast as a result.

In the 1309 objective, the front element also has a concave front surface but the surround is a disk that both blocks incoming light but is also anti-reflecting. The 1128 objective is a less expensive planachro, made for routine lab and university use. The coating of the front element's surround is a less expensive way of accomplishing the same result. The 1309 is quite close to an apo in performance and it's price and w.d. also so.

[75RR]

It appears to show a relatively large diameter front element, with a small concavity ground into it [in which case the surrounding area would need to be masked] ... Is that actually the construction ?

The Zeiss F 40x/0.65 would appear to have a similar design.

[MichaelG.]

Thanks, both

MichaelG.

[sailor_ed]

So I guess I must examine my lens more carefully to see if the surrounding mask is still intact?

[apochronaut]

It looks o.k. to me. Nice and black. If it is gone or partially so, you see a ground glass surface.

[viktor j nilsson]

I have a Nikon CF Fluor 10x 0.50 objective where quite a bit of the black paint has come off due to poor packaging by an eBay seller. I got a partial refund, but kept it as the image is really good despite this. If you look through it at an angle, the damage is very obvious. But viewed straight through, all the damage is outside the exit pupil.

It probably does kill contrast a bit; I might add a dab of flat black paint some day.

[apochronaut]

Yes, paint it. I did that with the AO one that had totally lost it's mask and it completely cured the contrast issue.