Thinking about a new objective or two

[Crater Eddie]

In setting up my micro work area again for the winter, I am reminded that when I packed everything away I was considering changing my lineup of objectives for my LOMO Biolam microscope.
I rarely use the 100x objective anymore, was thinking about replacing it with a 60x. I also rarely use the 4x, have been thinking of getting a 20x.
This would make the final line-up 10x, 20x, 40x, 60x.
I usually observe "pond life" and occasionally observe human blood cells (stained), and urine sediments.
Thoughts?

[75RR]

I observe pond life as well.
I find a low objective very useful to see the forest.
With the high objectives one tends to get lost among the trees.
It is also useful in finding runaway subjects.
Just today I had to switch from a 25x to a 2.5x in order to run down a very active Nautilus.
A 10x would still have had too narrow a view.

I would keep the 4x and trade the 100x if you have to let one go.

[gekko]

Hi Cater Eddie,
I take it your microscope is fitted with a 4-objective turret? Is the turret "user replaceable" or is it basically fixed?

[Crater Eddie]

75RR: I do take your points, I just almost never find myself using the 4x.

Gekko: Yes, unfortunately the turret is only 4 position. It looks like it comes off with one screw, though I have never had mine off. I see them on Ebay occasionally. Would love to replace it with a 5 position turret, but I don't know if that is possible, have never seen one that looked like it would fit. LOMO made a more up-scale microscope called the Laboroscope which had a 5 position turret. I never see the parts pop up on Ebay though. I doubt that they would be interchangeable anyway.

[gekko]

If the turret is easily removable (that is, you can remove it and replace it with a slide on the stage), then perhaps you can buy another 4-postion one that you can fit with one or more objectives that complement the ones you have no the microscope. If it is secured by a thumb screw, it should be easily removable. However if you need a screwdriver then I would not try to remove it (it is not meant to be "user" removable, and it might need alignment if it is disturbed). At least that is what I think.

[JimT]

My two cents; don't get another turret. You will spend too much time switching the turrets. Instead, replace the 100x OI with a 60x dry. I did and use it a lot more than I used the 100x. You can always reinstall the 100x if you want to get really close.

My set up is 4x, 10x, 40x, and 60x with a 10x eyepiece. No Complaints.

JimT

[Crater Eddie]

JimT: Yes, that is my thought. Since I don't use the 100x oil, the 60x dry would be a nice compliment to the 40x, especially when observing blood cells.

Gekko: The turret is mounted with a regular screw not a thumbscrew, so is not really meant to be changed. If I could find a 5 position one that would fit I would jump on it, but as that seems unlikely, I'm stuck.

I think I'll concentrate on finding a nice 60x and run with the 4x, 10x, 40x, 60x lineup for a while. My eyepieces are 10x, and as some might recall, the LOMO binocular head has an internal 1.5x corrector lens built in.

[gekko]

Just an idea that you may want to think about: would it be worth considering a 40x apochromat? It should have a NA (hence resolution) comparable to a 60x achromat. LOMO apos appear to sell at very reasonable prices, but I don't remember if the ones I've seen are 45 mm parfocal distance or 30+ mm. and I don't know which is compatible with your microscope.

Edit: Another idea (as Oliver suggested): again, if my memory serves, LOMO make a 40x (and perhaps 60x, I don't remember) water immersion objectives that are also reasonably priced, and may be worth looking at. I think the 40x water-immersion achromat has a NA of 0.75. Again, subject to being corrected, I think some water immersion objectives are meant to be used with a cover glass, and otheres without, so one needs to know about the particular objective.

[Crater Eddie]

Gekko, you make some good points. The objectives that came with the Biolam are acromats, I would certainly be interested in upgrading to one or more apochromats if it is likely to make a significant improvement in image quality.
The stock acromats have a parfocal distance of 33mm. If the apos are parfocal at 45 it wouldn't really hurt anything would it? I can deal with having to refocus. The Q-PLAN objectives that I got a few years ago have a parfocal distance of 45mm. Too bad they are only a very slight improvement over the acromats.

The LOMO catalog page talks about the acromats and Plan acromats a little here: http://www.lomoplc.com/multiscopeDataPage.htm
Just for giggles, you can also see the catalog page for the Laboroscope here: http://www.lomoplc.com/laboroscopeDataPage.htm

It's odd that these pages are still active even though the microscopes were discontinued years ago.

There are 2 or 3 LOMO water immersion objectives, yes, but I don't know much about them. I have seen them pop up on Ebay now and then.

[gekko]

I don't know what others think, but my thinking is that a 40x apochromat can be used in place of a 60x achromat with comparable NA and you'll get comparable resolution (hence images can be displayed at the same final image size while taking advantage of the better optical corrections of the apo). I think that you'll also get similar disadvantages: with both the 40x apo and 60x achro you will need to use a minimum (ideally no) water layer between object and cover glass (for the reasons Oliver outlined above), and you will have a very small working distance and shallow depth of field.
Using 45-mm objectives on a microscope designed for 33-mm (objective specs being otherwise identical) would degrade the image (in the form of increased spherical aberration) because the actual tube length will be longer than designed. The degradation will be more pronounced with larger NA objectives (higher power, or apos of same power). The reverse (using 30+ mm objectives on microscopes designed for 45 mm objectives) will work fine if a spacer is used to make them parfocal at 45 mm.
That is what I think, for what it's worth.
Thanks for the links-- I'll check them out later today.

[Crater Eddie]

I was planning on using the 60x mainly as a "high dry" for observing stained blood cells, I rarely go higher than 40x on pond life (maybe I would with better glass!) but I take your points.
Thanks for your information re the parfocality distance. I thought that as long as the objectives were designed for the same length tube (160mm in this case) everything was good. Perhaps this mismatch is why the Q-Plan objectives were a disappointment.
I admit that I have a lot to learn about optics, I appreciate the input of the more knowledgeable folks here on the forum.