DOH!!!

[GaryB]

I bought a Zeiss standard 16 a while ago and it was a bit of a mess, though it did clean up well enough and I've been happy with it. It came with a plan 10, a F-LD 20 and an F 40, all Zeiss. The 10 was good, the 40 was good, the 20, however, was bad. Non par focal and horrible image. I put this down to it being either just a rubbish lens because it has a NA of just .25, barely more than the 10x, or because it was a long working distance lens.

The one thing I never checked (for whatever reason) was it's cover slip thickness because they're all .17 right?.... you can probably guess where this is going...

2.0, not 0.17 but 2, two whole millimeters.

I thought to myself *what if I put a slide over my sample?* Lo and behold, a decent image! It suddenly dawned on me it's for looking up through the bottom of a petri dish or somesuch. I guess it's pretty useless on an upright, I just can't think why it took me so long to figure it out

[zzffnn]

GaryB,

You can use that F-LD 20 on an upright scope, for viewing petri dish (from top to bottom) filled with 1-2 mm of water. At NA 0.25-0.4, RI difference between water and glass does not matter that much.

At above NA 0.4 (I have tried NA 0.55), as long as you have a cover correction collar and trade 1.14mm water for 1.0mm of glass, you will still get decent image that way.

NA 0.25 is still a bit too low for 20x though, even considering 2mm thick cover (and long working distance? It must have a special application). Most 20x inverted objectives have NA of 0.4 and 30-45x inverted objectives have NA of 0.55-0.65.

I have an extra DIN plan PH3 20x NA 0.4 Bausch & Lomb inverted (1.2mm corrected) objective for trade or sale, at my original cost ($57.6), if you want.

[GaryB]

Hi Fan,
I've actually been having some fun with it. With it's low NA it has a pretty good depth of focus so while it's not the sharpest, it's still good for certain subjects. I looked it up in an old Zeiss catalog and it's good for 1 to 3mm cover sizes. I believe it's one of their cheapest objectives for rough and ready work. Once I have some spare cash I'll be getting one of the Lomo 20x objectives, hopefully an apo, but thanks for the offer