If you see an unknown objective....

[Barry]

Have done a quick search but so many results turned up, that I cannot find the answer....

Looking around for microscope objectives for use on bellows (no microscope here), I know one prime feature to pay attention to is if it is infinity corrected or designed for a tube of a certain length. OK. And NA ofcourse, OK.
But what if an objective is designed for brightfield, or darkfield? Is this a feature to pay attention to? And in what respect?

Thanks,
Barry

[ChrisLilley]

Search is good, but as you say turns up a lot of results in this case. The FAQ section is a good place to start, in particular:

FAQ: which lenses are good at more than 1X in a bench setup?
[ur=http://www.photomacrography.net/forum/v ... php?t=8279]Bellows[/url]

It looks as if we don't have a FAQ that specifically covers your question (finites vs. infinites, correction in the objective vs. some correction in eyepieces or tube lens, brightfield vs darkfield/cpol/fluor/etc). It may well be time to start one, as that info is covered in multiple places. One useful recent discussion on the topic:

Objective Lenses - What Do The Numbers Mean - What's Best

[rjlittlefield]

But what if an objective is designed for brightfield, or darkfield? Is this a feature to pay attention to?

At the magnifications you probably care about it doesn't matter. At high magnifications like (40X and above), objectives designed for darkfield may have smaller apertures and/or slightly adjustable apertures. These make it easier for a darkfield condenser to create the required hollow cone of illumination in which no direct rays enter the objective.

--Rik

[Barry]

Thanks guys for the informative answers!

One additional question. Am I right if I say that it doesnt matter if you have an plan apochromat or an apochromat (non-plan) because stacking will solve the difference between the two?

Thanks,
Barry

[rjlittlefield]

Am I right if I say that it doesnt matter if you have an plan apochromat or an apochromat (non-plan) because stacking will solve the difference between the two?

Yes, stacking will fix the difference between plan and non-plan. With stacking, you can work even with a very curved field. See HERE for an extreme example.

--Rik

[Pau]

Of course Rik is right, but perhaps the answer is not so simple, because apochromats are usualy old desings and its field of view may be smaller that most planapos. (I'm not aware of any apo not plan modern objective, and modern mean for me from the 70-80s to now)
Also with a flat subject like a microscope slide the stack must be deeper than with plan objectives.

[rjlittlefield]

Pau is of course correct also. I debated mentioning those issues, but decided (in contrast to my usual style) to go with a simple answer. In retrospect, that was probably an error.

I'm not aware of any apo not plan modern objective

It's a confusing aspect, but in the current line of Nikon objectives shown at http://objectives.nikoninstruments.com/, 19 are labeled as being "plan apochromats" while 8 more are labeled as being just "apochromats" (not plan). All of these non-plan apos are very wide aperture immersion objectives, quite expensive and almost certainly outside Barry's interests.

I agree that with ordinary objectives, seeing "apo" without "plan" would indicate either an older design or a labeling error.

--Rik

[Pau]

...while 8 more are labeled as being just "apochromats" (not plan). All of these non-plan apos are very wide aperture immersion objectives, quite expensive and almost certainly outside Barry's interests.

Rick, again you are right , some very specialized lenses (infrared, UV, very high NA for fluorescence..) are apos, not only in the Nikon line but also in Zeiss an likely other manufacturers.