Uses of very low power objectives

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NCBot
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Uses of very low power objectives

#1 Post by NCBot » Fri Sep 24, 2021 9:09 am

Probably a dim question but what are the typical uses of a very lower power objective, e.g. 1x or 2x, on a compound microscope?

Scarodactyl
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Re: Uses of very low power objectives

#2 Post by Scarodactyl » Fri Sep 24, 2021 9:21 am

Scanning large FoVs or examining larger subjects, though the shorter working distance will limit the latter to large flattish subjects. For higher quality imaging or examination of larger subjects other classes of lenses are better (sonce they aren't forced to be parfocal with higher mag objectives).

Adam Long
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Re: Uses of very low power objectives

#3 Post by Adam Long » Fri Sep 24, 2021 10:33 am

I mostly use my 2.5x for quickly finding fast-moving micro-organisms. For example I've got a slide at the moment with a single Planarian. At 25x (with 10x eps) the view covers somewhere between 1/4 and 1/9 of the 22mm x 22mm area under the coverslip, so I can scan the whole area in a few passes. Planarians are fast when they want to be, and with my next lowest power objective (6.3x) it can become a game of cat and mouse as you slowly scan the slide bit by bit while the critter relocates to the area you've already swept.

25x also gives me a nice overlap with my stereo microscope which does 7-45x. My Universal Epi setup only goes down to 50x (4x & 1.25x Optovar) which is quite limiting.

The normally illuminated area of the Zeiss condensers is limited to the area covered by the 6.3x, but by swinging the top condenser lens aside, or the aux lens in, you get good illumination in brightfield and adequate in darkfield.

apochronaut
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Re: Uses of very low power objectives

#4 Post by apochronaut » Fri Sep 24, 2021 1:07 pm

That would depend on the actual objective.
It is quite a simple matter to make a low magnification objective for scanning . Some are little more than small magnifying glasses with a single achromat element. Most companies made various magnifications like that from 2X up to about 4X. They normally are not fully plan nor are they perfectly corrected to the field stop but some of them are quite flat over a limited field and decently corrected. In some cases the working distance is extreme and they are hard pressed to focus on a normal stand. Anything below about 3 X was usually designed for something special. Beck (U.K.) made a 2X apochromat in the 50's with a huge w.d. It makes an excellent photographic lens.
Since the more average achromat types of low power objectives have no coverslip requirement, they are very good for viewing objects under vertical illumination. Better resolution than many stereos and quite cheap to buy.
Since the 80's though, an increasing number of really sophisticated low power objectives have come along as dedicated low power , plan , well corrected objectives for very low power observations at magnifications no greater than a stereo microscope can provide. The difference is that these modern objectives have N.A.'s that give far better resolution than all but a very expensive stereo can give but at a cost. Making a 2X or 1X well corrected objective requires more technical design expertise than making a well corrected 10X , so they are really expensive but less than a stereo that can achieve the same resolution.

Adam Long
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Re: Uses of very low power objectives

#5 Post by Adam Long » Fri Sep 24, 2021 1:18 pm

The difference is that these modern objectives have N.A.'s that give far better resolution than all but a very expensive stereo can give but at a cost.
Given that with 10x eyepieces we are rarely troubling the resolution limits of even fairly pedestrian low-power objectives, presumably this is mainly an advantage for photography?

apochronaut
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Re: Uses of very low power objectives

#6 Post by apochronaut » Fri Sep 24, 2021 1:46 pm

That is definitely true and a major drawing card for them, since also, many of them are built to superior colour correction standards.
A lot of the modern ones are plan fluorite and plan apo designs which takes them to another level of colour correction, so despite there being little empty magnification with older achromats , resolution is naturally improved through lack of ca and sa with the more complicated designs. The more complex designs also allow for more normal working distances and parfocality. Parfocality with those older 1 to 3X objectives didn't exist. Sometimes the working distance was over 50mm.
The modern designs can be dialled as a normal objective but do need a wider field condenser .

Spencer made an odd ball 3.5X way back in the 30's, so not quite low enough to be in the discussion but interesting nonetheless. They catalogued both a 30.2mm f.l., 3.5X .09 with a w.d. of 24.1mm and a 24mm f.l., 3.5X .08 with a w.d. of 4.2mm. The latter objective can be dialled right in parfocal with a 10X objective. The way it was done was to modify a standard shorter focal length and thus higher magnification objective with a negative lens on an extension, lengthening the objective and reducing it's magnification. It is also quite flat. Their standard 25mm f.l. objective was 5.1X. Kind of a harbinger of things to come in low power objective design, as a greater diversity of glass formulas came along.

MicroBob
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Re: Uses of very low power objectives

#7 Post by MicroBob » Fri Sep 24, 2021 3:20 pm

These high quality low power objectives are nice when one simply has such a big object, like a cross section through a plant or an embryo skull - one photo, all in, nothing to mix up, no stitching...

What I found though is that it is difficult to illuminate such a large area in a way that the image is not only highly resolved but also colourfull. The common method to remove the condenser gives very subdued colours. Just the base lens of the condenser might work, perhaps with a difuser on top of it. Zeiss West made a set of "Brillenglas" condensers, simple single lenses with a stop to limit the light exit to a center portion of the lens. But even with these my impression is that I get better colours from a stitched image from a higher power objective. Probably the colours depend on the angle of the light cone in a way. A good resolution is easy to acheive, any illumination works more or less.

So more a special objective, especially below 2,5:1, but very useful if one has the objects to photograph.

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