Making full use of the aperture.

Everything relating to microscopy hardware: Objectives, eyepieces, lamps and more.
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apochronaut
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Making full use of the aperture.

#1 Post by apochronaut » Wed Nov 22, 2017 3:53 pm

Average quality 4 or 5 objective microscopes, almost always have a standard complement of features. There is a good reason for some of these features being standard. A 40x .65 dry objective for instance, offers a good balance between resolution and w.d. They have a degree of versatility too. In a 4 or even 5 objective complement, the addition of a pair of 15 or 16X eyepieces , can be used to fill in that large higher magnification gap, with 600 or 640X and still fall within acceptable resolution parameters.

There is not a good reason for some of the other features that have become default convention, though. For what reason, is the standard, as common as a yolk in an egg , 100X 1.25 oil immersion achromat objective, screwed into every nosepiece? Why not, 1.1 or 1.3? In the past, 1.30 or 1.32 achromat objectives were fairly common and as well, differing oil immersion magnifications.
It seems that 1.25, might be the best balance of maximum aperture X economic production and this became doubly so, as the convention moved forward in time and customer expectations dictated an economy of scale. How many people are going to buy a microscope with a 1.2 aperture over one with 1.25, even if the system cannot use it? So, 100X 1.25 oil became the default but then, so did 10X W.F. eyepieces!
The problem is, that unless you are using magnifications of about 1000 X the aperture, you are not using the aperture to it's full extent. If you are using a 10X eyepiece, which has become a marketing default, a 100X oil immersion objective need not have an N.A. any higher than 1.0. In the past, manufacturers produced really good 12.5X eyepieces, which when mated with the 1.25 achromats, could increase the resolution to that, commensurate with a 1.25 N.A. In other words, a 1.25 objective, should give full resolution up to 1250X but that magnification is seldom used.

One of the ways manufacturers have previously manipulated this N.A./magnification ratio to their customers advantage, is to offer oil immersion objectives in an array of magnifications. Spencer and , I think every other maker pre-1960, offered apochromats of 90X or thereabouts and 1.30 N.A, and sometimes also 1.4 . 15X compensating eyepieces were usually made with a wider apparent field than the 10X and became the default in apochromat systems. 90 x 15, yields 1350X, or a perfect peak resolution capability for an apochromat, which is slightly higher at around 1050, than the 1000 x N.A. ,used for achromats. At one time Spencer had an 82X 1.30 apochromat and a 16x compens . Other manufacturers had similar combinations available with which to maximize resolution.

It would be nice if the default eyepieces tossed into common microscope kits were in fact W.F. 12.5X but the choice is now, probably one to do with convention and marketing to customers expectations. People are suspicious of something they do not normally come across, especially when they know little about the subject.
For any one looking to up their magnification a bit and can utilize a short tube objective( 35.7mm, I believe on this one), here is a rare gem, that should go pretty cheaply; a B & L 80X 1.25 oil objective, clearly designed to be used with the very nice 15X W.F. oculars, that they made during the 50's.

https://www.ebay.com/itm/ANTIQUE-VINTAG ... Ciid%253A1
Last edited by apochronaut on Wed Nov 22, 2017 7:43 pm, edited 1 time in total.

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mrsonchus
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Re: Making full use of the aperture.

#2 Post by mrsonchus » Wed Nov 22, 2017 7:28 pm

Hi apo' - that's a very interesting and useful post my friend - you've got me thinking now for sure.
Would the my beautiful Water-immersion x50 n.a. 1.0 give it's potential with for example a x20 eyepiece? Almost seems a sheer waste now to use it with the, as you say now de facto std x10 eyepieces.
Similarly I have a superb x40 Plan-Apo 0.75 that maybe fine used with x15 or even x20 eyepieces - I'm going to give this a go I think.

Thanks for this interesting post apo'.

John B. :)
John B

apochronaut
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Re: Making full use of the aperture.

#3 Post by apochronaut » Wed Nov 22, 2017 9:10 pm

What I notice with pushing the magnification to the limit, is that more
detail is revealed but the process does also reveal the true nature of
the optic. Numerical Aperture , while very important in determining the resolving
power of an optic , is after all just a measure of an angle. Two optics
of the same N.A. should have very similar resolving power but in practice,
so many other factors, enter into an objective's quality that while
detail may be observed at the limit of resolution, it can be obscured by
lack of contrast, poor colour correction, etc.

case in point here, distilled from an earlier post. The objectives are in
fact being pushed beyond their theoretical limit of resolution in this
case but there is a surprising difference in the clarity of detail at the magnification they are bearing.

three objectives, of post 1985 production. 63x .80, 60x .85 and 60x .80. 1) a semi-plan achromat. 2) a strain free achromat and 3) a planachromat
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mrsonchus
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Re: Making full use of the aperture.

#4 Post by mrsonchus » Wed Nov 22, 2017 10:56 pm

An interesting and pretty revealing or at very least suggestive result. The greater contrast of the semi-plan achromat is to my eyes giving more detail at the expense it seems of the frustule's edges which are 'blacked-out' somewhat. The strain-free is a little lacking comparatively at least, whilst the planachromat is indeed plan but relatively low in contrast.
The semi-plan seems almost to have a touch of the oblique to it with a hint of 3D.

Interesting comparison, thanks. :)

John B. :)
John B

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