Re: Darkfield condenser


"justbeats" <steve_beats@xxxxxxxxxxx> wrote in message
news:1126517553.936322.209240@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>I just got a second-hand Zeiss darkfield condenser. IMHO it's the
> best bang for the buck of any accessory I've ever bought for my Zeiss
> inverted! For diatom imaging at high mag (and high NA), the improvement
> in contrast compared to other techniques (e.g. BF, PC, COL or DIC) is
> massive. I'd already tried low NA darkfield with excellent results, but
> I didn't expect the hi-NA version to be quite so much better again.
> Wow!
>
> It's the one accessory I don't mind having to use immersion oil
> for! Good job too as it's wholly non-functional if not oiled to the
> back of the slide :-)
>
> Couple of questions...
>
> The condenser is marked "1.2/1.4". Obviously this is the NA, but
> why two numbers? Is it related to immersion medium (i.e water/oil or
> glycerine/oil in this case)?
>
> I have an old set of prepared slides containing many species of
> Coscinodiscus, including some extremely rare varieties and two marked
> "new sp?". Unfortunately, the slides are very thick and I can JUST
> get the illumination onto the specimen but the condenser is touching
> the slide (except for a very thin smear of oil between the two
> surfaces). I guess thick slides preclude the use of a hi-NA darkfield
> condenser? Shame, as it was those that I most wanted to use this DF
> technique on. Any way to solve this limitation?
>
> Cheers
> Beats
>
Justbeats, I'm glad you like the condenser. To be honest with you there
aren't a lot of them in use. Darkfield gives you lots of contrast and quite
a bit less resolution. With immersion you are lucky to get 1.0 NA.

The dual na markings are for the dual na's of the condenser, one for inside
and one for outside. Remember this condenser does not give a arch of
illumination, only diffracted rays are brought to the lens. The lack of
working distance is part and parcel of how an oil cardiod condenser works.
They haven't improved with age since there are not a lot of purchasers.

Don't let me talk you out of using this interesting piece. Use the heck out
of it!

Kevin Cunningham
SMS


.

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