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Objective definitions

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#1 Hugh Peck

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Posted 23 October 2019 - 09:33 AM

Can anyone give me an idea of what these mean in the real world? Since I'm not likely to be using any in a metallurgical sense I'm sure I can scratch that off. Probably fluorescence as well. Am presuming an iris will help with dof and possibly contrast and sharpness.

Brightfield (44)
Darkfield (42)
Fluorescence (5)
Metallurgical (7)

With Iris (3)
Without Iris (45)

Achromatic (9)
Fluor (5)
Plan (54)
Infinity (22)



#2 Blind as an Eagle

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Posted 23 October 2019 - 07:54 PM

http://micro.magnet....my/anatomy.html



#3 Blind as an Eagle

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Posted 23 October 2019 - 07:59 PM

PIC

objectivespecificationsfigure1.jpg


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#4 Hugh Peck

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Posted 23 October 2019 - 08:40 PM

Thank you. Rather informative picture. Also, found this site while surfing the interwebs (getting my keyboard all wet):

 

https://www.microsco...cope-objectives



#5 Microscopy

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Posted 03 November 2019 - 06:04 AM

I'll give it a try

 

Can anyone give me an idea of what these mean in the real world? Since I'm not likely to be using any in a metallurgical sense I'm sure I can scratch that off. Probably fluorescence as well. Am presuming an iris will help with dof and possibly contrast and sharpness.

Brightfield (44)
Darkfield (42)
Fluorescence (5)
Metallurgical (7)

With Iris (3)
Without Iris (45)

Achromatic (9)
Fluor (5)
Plan (54)
Infinity (22)

I'll give it a try, limiting it to finite objectives. Consider this part one of my comment as the issue is a large library in itself, starting with:

 

"Brightfield (44)": The usual objectives, found on biological microscopes, intended to be used with transmitted light, mostly (but not exclusively) for biological applications. They come in all kinds of build lengths, ranging between 32 and 45 mm's. 

 

What they all have in common is their screw tread, which is standard RMS screw thread = Withworth 0.8" x 36 tpi.

 

Some have only basic information on their mounts, some have half a book written on it...

 

"40/0.65 - 160/0.17" is pretty much everything a user needs to know: magnification: 40x, numerical aperture: 0.65, tube length: 160mm, coverslip thickness: 0.17 mm.

Low magnifications (2.5x, 3.5x, 4x ...) can be used with or without coverslip and those usually have something written on them like "4/0.10 - 160/-". 

 

"Brightfield" objectives (like all microscope objectives) come in several classes regarding optical correction.

It's important to realise that there are no standards cut out in stone regarding optical correction! Every manufacturer does and did as he pleases.

They only agreed pretty much on one thing: the point of optimal sensitivity for the human eye being around 529 nm (that's yellow-green). Up to this day, that ad hoc standard remains in place.

 

Regarding optical correction, "brightfield" objectives can be divided in:

  • achromats
  • (semi)(plan)achromats
  • fluorites
  • (plan)apochromats

 

Let's discuss those briefly:

 

  • achromats: designed so that 2 wavelength's always have a common focal point. What wavelengths is the manufacturers decision. No correction for spherical aberrations
  • (semi)(plan)achromats: chromatic aberrations corrected as in achromats. No correction for spherical aberrations. Curvature of field corrected to a more or lesser extend. No standards there
  • fluorites: designed so that 2 or 3 wavelength's always have a common focal point. What wavelengths is the manufacturers decision. Correction for spherical aberrations for 1 or 2 wavelengths
  • (plan)apochromats: designed so that at least 3 wavelength's always have a common focal point. What wavelengths is the manufacturers decision. Correction for spherical aberrations for at least 2 wavelengths. Curvature of field corrected to a more or lesser extend. No standards there.

Notice in all the above: very often corrections being divided between objective and eyepiece, hence the need to use matching objectives/eyepieces.

 

 

 

The important thing for people who don't have large amounts of money to throw away is this: "brightfield objectives" can be used for brightfield, dark field (up to a certain extend), polarisation microscopy (with limitations). They can't be used for techniques, involving "something to be put in the objective", like phase contrast, anoptral contrast, differential interference contrast ("DIC") and so on. 


Edited by Microscopy, 03 November 2019 - 06:34 AM.

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#6 Hugh Peck

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Posted 03 November 2019 - 09:47 AM

Thank you. That provides a lot more detail than I've found. It seem most place give you the absolute minimum or assume the reader already knows a lot. 



#7 j.gardavsky

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Posted 08 November 2019 - 11:54 AM

Very good infos as above!

 

When you are sticking with Olympus, then there is a good introductory tutorial

https://www.olympus-...omy/objectives/

 

The old microscope objectives have been designed and optimized for some specific distance between the objective and eyepiece, like 160mm, or 170mm (tube lengths in plain language).

When you mount a 160 objective on a wrong tube length microscope, then your EP  will eventually not achieve sharp focus with high contrast.

 

Best,

JG


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