Microscope objective Leica C Plan – specifications & review

I’ve just found on eBay this very nice microscope objective.

Leica C Plan 4X/0.10

It’s a Leica C Plan 4x/0.10 ∞/-/↑ 506074.  Nicely built with engraved inscriptions. Unlike expected, not a lot of glass in this objective.

Signification of these hieroglyphs :

• C Plan : basic Leica series of objectives. Standard achromats.
• 4x : magnification factor
• 0.10 : NA = Numerical Aperture.
• ∞ : Mechanical tube length. The infinity corrected objectives generally need a tube lens (in the microscope) and they can’t be used on microscope with finite tube length.
• – : this mean that there is no correction for an eventual cover glass of 0.17mm. The objective can be used with or without coverglass.
• ↑ : This objective is to be used with transmitted light. In the DML instructions  it’s written : ‘The arrow label can be used for transmitted light ↑ (vacant position)« . I don’t know what is the meaning of vacant position.
• 506074 : reference number of Leica. This objective is also known as 11506074 (?) in the Leica database.

Why I have chosen this microscope objective ?

• It’s a modern objective made for infinity tube length. There is no need of chromatic correction in the eyepiece as in older objectives.
• With a micro four third camera as I have (Panasonic GH1), the tube lens may be useless.
• It was rather cheap on ebay. I may buy something more elaborated in the future but I wanted to makes some try as soon as possible.
• I prefer a Leica brand to some unknown Chinese brand even if it’s stupid as they may come from the same factory. I doubt Leica is still manufacturing it’s low end objectives in western countries.
• I wanted to begin with a 4x or 5x objective to take picture of water mites. It will be easier to crop the image than to join different files to get a whole water mite.
• With 4x or 5x objectives, you get more working distance than with greater magnification factor. This allow more possibilities for the lighting system.
• The low NA of this kind of objectives allow a much greater depth of field (55.5µm according to this chart) which means 1000/55.5 = 18 pictures for a complete stack for a water mite of 1mm of thickness.

Some concern about this Leica C Plan Objective.

• As it is a low end objective in the Leica system, will it be good enough for my usages ?
• Is the chromatic correction good enough ?
• The fact that it was designed for transmitted light may be a problem as I will mainly use it with lateral illumination (a kind of darkfield illumination). I don’t think so but will have to check.
• As an achromat objective it only provide correction for 3 wavelength chromatically. Fluorite or planfluorite are better with 4 wavelength corrected for photomicrography. Apochromat or planapochromat lenses are not better than planfluorite for photomicrography (from this page)

Specifications and calculations around the resolution of this objective

• With a magnification factor of 4x and a NA of 0.10, this page indicate that the limit of resolution is 2.75µm (it’s purely dependant of the NA). It may be good enough to get clear pictures of the setae, glandularia and acetabulae of the normal sized water mite.
• The depth of field is around 55.5µm and the depth of focus around 0.13mm (but I don’t understand what is « Depth of focus »).
• I don’t know the FOV (Field of view), it will probably depend on the way I mount it to my camera (focal length and position of the tube lens).
• This objective is designed to be used with an infinity correction tube lens of 200mm of focal length. Seen on this page.
• Focal length of this 4x objective : 200mm / 4x = 25mm
• M25 thread (not a common one)
• Working distance 26.2mm (seems a lots, even for a 4x objective).

Misc informations on this objective.

• Is sometimes a part of Leica AS LMD Microscope Laser Microdissection system ?
• Is sometimes a part of Leica DM LS ?

Todo.

• Show some pictures taken with this objective (I think not before the first December 2011, I’m still waiting some adapter to use it on my camera).
• Determine the theoretical optimum distance between the objective and the tube lens (see this page).

Links

• A nikon page with a lot  of explanations on the symbol found on the microscope objectives.
• Other explanation from Olympus.
• Page on Leica website which allow to find some basic informations on their objectives (the reference 506074 doesn’t give any result – maybe 11506074)