Howto mount an infinity corrected microscope objective on a camera

While I’m looking for a new microscope for taking good pictures of water mites I’ve read a lot of forum post on a rather simple montage which allow the use of a microscope objective (the infinite type) on a camera with a macro bellow.

Why using microscope objectives without microscope for photomicrography.

Here are some reason why trying to follow this path.

• It is much cheaper than buying a good modern microscope.
• You are not limited by the type of objective that induced the ownership of a particular microscope. When you have the proper bellow and tube lens, every kind of microscope objective is a question of cheap adapter.
• You can choose very modern objectives.
• You can choose objectives with interesting feature like super wide field or long working distance.
• The setup can be more lighter and easier to transport than a normal microscope.
• The micro stand can be made with an old enlarger, some quick release clamp and a precision slide (like a Velmex – the more expensive part of the lot).
• For my project of getting good pictures of water mites, it appear that I won’t need the 100x objectives common with microscopes and that I will mainly need epi illumination and no transmission light.
• No trouble and headache with the question of the adaptation of a SLR on the trinocular head of a microscope.
• You get a 360° access to your subject for lighting (a Charles Krebs point).
• This kind of DIY project is very funny and rewarding, a least in the end you will have improved your knowledge in basic optic.

Some drawbacks of not using a microscope.

• The setup can be less handy than a microscope for the focusing stage.
• For long observations, a binocular microscope is better.
• You can’t use bright fields illumination with Köhler illumination.
• Doing microscopy without microscope sometime looks like reinventing the wheel.

The infinity corrected (∞) objectives.

• They are generally modern objectives.
• They generally don’t need chromatic correction in the lens barrel or in the microscope eyepiece.
• As modern objectives, they get all the improvement made in lens coating made during the last 50 years.
• They are recognized by a sign ∞ on the objective barrel.
• The beams issued by the objective are parallel.
• They need a tube lens (between the objective and the camera sensor).
• The focal lens of the tube lens vary according to the manufacturer (see microscopyu)
• According to the same source, for lower magnification (lower than 10x) , it’s better to use objectives with larger thread size and longer parfocal length (hence the choice made by Nikon of 25mm thread and 60mm parfocal distance).
• The distance between the tube lens and the camera sensor is critical in this assembly (not sure, is this distance important for the size of the image on the camera sensor ?)

The best (?) infinity corrected objectives.

There a hierarchy in the quality of microscope objectives.

• Achromat
• Plan Achromat
• Plan Fluorite
• Plan Apochromat

The tube lens to be used with an infinity corrected microscope objective.

A tube lens is used in microscope between the objective and the eyepiece or the camera sensor. One side effect of the tube lens is that it reverse the resulted image.

Theoretical calculation for the position of the tube lens.

• A priori, the focal length of the tube lens determine the distance between the tube lens and the camera sensor and the distance between the objective and the tube lens is not important (parallel beams of infinity designed objectives). But this animation shows that the infinity space between the tube lens and the objective vary and is not the same for all the focal length of the tube lens.
• The above mentioned animation shows also that the greater the « infinity space » the lower (and better) the angle of the oblique light rays. I wonder what these « oblique light rays » are in an infinity objective  with parallel beams ? This a concern that I will have to try to check with my future setting.
• According to this other animation on a Nikon website, the infinity space seams to be really infinite.

Example of size found on microscopes.

• Nikon Eclipse E600 – CFI 160 objectives : The size is not given in their brochure but their is roughly 290mm between the base of the objective (nosepiece) and the camera sensor. As the tube lens if of 200mm of focal length it remain roughly 290mm-200mm=90mm of space between the nose piece and the tube lens. This space is called « Afocal space ».

My tube lens.

Here is the one I’ve purchased at SurplusShed : A. JAEGERS 31MM DIA X 172MM FL ACHROMATIC OBJECTIVE

Test with different positions of the tube lens.

I have tested my 172mm tube lens in different position and it appear that the approximate respect of the tube lens focal lens give the best results. Here are some result obtained with my Leica 4x objecttive. The magnification is the magnification measured on the sensor.

• Tube lens 172mm – 170mm between tube lens and sensor – 50mm between tube lens and nose piece => magnification 3.75 and approximate working distance 28mm.
• Tube lens 172mm – 290mm between tube lens and sensor – 50mm between tube lens and nose piece => magnification 5.80 and approximate working distance 20mm.

Without checking the quality of the image obtained (I wait for a micrometer slide to try to do fine focus) it seems that the first setting is the best as is allow to get a magnification very near the 4X magnification for which the objective has been designed.

Test without tube lens.

Is was possible to get focused images without tube lens and even without macro bellow with a 4x objective. I will write a specific article on this subject.

My initial setting

List of the items from the camera to the objective.

1. Camera Panasonic GH1.
2. Adapter for Exakta lens mount to micro four third camera.
3. Ihagee Macro Bellow.
4. Adapter from Exakta camera to M42 lens (in fact 1 adapter from Exakta to 49mm, 1 step up ring from 49mm to 52mm and one step down ring from 52mm to 42mm).
5. A set of 3 M42 macro tube from Paragon.
6. Inside the precedent : a tube lens Jaegers 172mm of focal length, achromatic. Fixed with the help of some scotch tape and of a milk bottle cap.
7. M42 to M25 adaptor.
8. Leica C Plan 4X.

Infinity microscope objective on camera

Links.

• Charles Krebs wrote a complete and precise article on this subject.
• A rather complete discussion on this infinity corrected lens on the forum lenaturaliste (in french)
• An other forum thread on the same subject on photomacrography.
• A Nikon page on the CFI60 system.