Mineral photography - microscope objectives

Some people have taken to using objectives from transmitted light microscopes for macro photography. This leads to a whole pandoras box of nomenclature, which this article will attempt to at least partially unravel. It is worth taking the time to read various online tutorials (and there are a lot of them, and some good ones too) on how transmitted light microscopes work.

Some recommended links:

A "typical" objective

The diagram below shows a typical objective from a transmitted light microscope. This diagram was taken from a great little article on how microscopes work that I found at this link.

The markings (not all of which you are likely to find on any one actual objective) convey a vast amount of information.

Threads

The most common (but by no means the only) standard for microscope objective threads is what is known as the "RMS" thead (also known as the society thread). "RMS" stands for "Royal Microscope Society" and is a special 0.8"-36 Whitworth thread. Lots of adapters are available online for RMS thread to camera adapting. I'll not that one Plan-Apo objective I was considering was described as having 20mm threads which could be something different, or might turn out to be RMS after all. I see some literature describing the RMS thread as equivalent to the metric M20.25, so this objective probably does have RMS threads, but I always expect surprises. The "official" definition is in english, not metric units (see below).

The RMS thread is a standard Whitworth with 36 threads per inch. The dimensions are as follows: Male thread OD 0.7982/0.7952; root dia 0.7626/0.7596 Female thread Root Dia 0.7674/0.7644; top of thread 0.8030/0.8000. The Whitworth thread has both the crest and the root radiused with radius= 0.137329 x pitch and the included angle is 55 degrees.

Nikon and Leitz (Leica) microscopes do not use the RMS standard. Nikon uses 25 mm as the standard objective size making it non-interchangeable with other objectives.

Tube Length

The objective illustrated above specifies a tube length of infinity. Many objectives will specify a finite tube length like 160 or 200mm. There are two fundamentally different designs of microscope objectives, namely finite and infinite conjugate designs. Finite conjugate designs are designed to produce a real image at the distance specified. Infinite conjugate designs can work with any tube length, but require a relay lens in the tube (or eyepieces of a unique sort). Most modern microscopes use infinite conjugate objectives.

A defacto standard (finite conjugate) seems to be the "DIN" microscope:

A friend (JB) has been working with some Nikon finite conjugate objectives on a bellows and has found that he gets best results at the specified 160mm distance from the camera focal plane. Other distances produce flare, particularly with certain colors.

In a modern infinite conjugate microscope design, the distance between the objective and the tube lens is not critical and can vary. The tube lens does produce an image as some fixed distance, and different manufacturers make different choices:

Color Correction

As you probably (or should know) most materials have a different index of refraction for different wavelengths (colors) of light. This is how prisms work in their wavelength dispersing mode and is what led to the discovery that white light consists of all colors (wavelengths) mixed together (so to speak). The trouble is that for a lens, this causes each wavelength to come to focus at a different distance from the lens, and this is what is called chromatic aberration. There are a variety of ways to correct for this, and different schemes yield better correction than others. It is a question of trade-offs as in most engineering. The best corrected lenses have more lens elements and some elements are made of more exotic materials, some of which are hard to work with.

Don't get the two very similar words "achromat" and "apochromat" confused.

The Edmund catalog says: "An apochromatic objective lens is chromatic aberration corrected for red, blue, and yellow while a standard achromatic objective lens is only corrected for red and blue."

Plan-Apochromats are the most highly corrected objectives, have the most internal complexity, and cost the most too.

Numerical Aperture (NA)

With Camera lenses, you talk about F-number, with microscope objectives you talk about NA.

With Camera lenses and f-number, smaller numbers are better (and faster).
With Microscope objectives and NA, bigger numbers are better.

Objectives with higher NA are brighter, and can resolve finer detail.

The formula defining NA is A = n*sin(theta), where "A" is the numerical aperture, "n" is the refractive index of the media within which the lens is operating (typically 1.0 for air), and "theta" is the half angle of the cone of light entering or exiting the lens. The theoretical limit for NA (which is a dimensionless number by the way) is 1.0 for a lens operating in air, but a practical limit is more like 0.95 (for a half angle of 72 degrees). NA values greater than 1.0 are possible for lenses that operate in some fluid (water or oil), but these are virtually always higher power (such as 60x or above) objectives that are exceedingly unlikely to be used for photomacrography.

There is a direct relation between NA and f-number that can be worked out by simple geometry. For most lenses this is approximated by R = n / (2*A) where "R" is the f-number, "n" is the index of refraction of the media the lens operates in, and "A" is the numerical aperture.

The smallest feature that can be resolved by an objective of a known NA can be calculated by the formula d = lambda / (2 * A) where "d" is the size of the object resolved, in the same units as lambda, "lambda" is the wavelength of light being used to observe the object, and "A" is the numerical aperture of the objective. It should be noted that there are several versions of this equation and sometimes the value 1.64 is substituted for the 2.0 in the formula above. In truth the whole microscope system (including in particular the condensor in a transmitted light microscope) contribute to the perceived resolution.

The mid spectrum wavelength of light could be taken to be 550 nanometers, in which case a 10x objective of NA 0.30 would resolve 0.92 micron features. Blue light has a wavelength of about 380 nm where it transitions to ultraviolet, and red light has a wavelength of about 740 nm where it transitions to infrared. 550 nm light is green.

Nikon Achromatic Finite Conjugate Objective

Edmund Scientific sells several versions of these (at different powers). The 4x and 10x seem well suited for photomacrography, and in fact the 10x was well received on the photomacrography.net forums.

A friend recently received the 4x version and is already getting good results with it. Edmund sells these for decent prices ($50 for the 4x version). Note though that this is "just" an achromat, no truly fancy corrections for chromatic aberration going on here. The microscope objective itself has an "RMS" thread on the back end, which seems to be the defacto standard for these things.

A fellow named Doug Merson is using both the 4x and 10x for mineral photography, and getting what I would call excellent results. His photos and setup were originally described on the Mineral Forum website and many of his photos can be admired there.
He describes his setup (pictured below) as follows:

This is the camera set up I use for the micro mineral photography. It is Nikon D300 on a PB-6 bellows. Under the bellows set up is a Newport linear stage with a micrometer drive. The vernier on the micrometer reads in .001mm. Illumination is by a 150 watt halogen fiber optic light with a custom white balance set on the camera. The stage set up allows for course and fine adjustment of the specimen position. I have 2 Nikon achromatic finite conjugate objectives, 4X and 10X, that I use. They are designed for a 160mm tube length but I have used them both longer and shorter than 160mm. The images are stacked using Zerene Stacker. This photo was taken a our local mineral show in November. At home the set up is on a much sturdier bench.

Here is a photo of the 4x objective mounted on a conical RMS-to-M42 adapter (in this case also mounted on a M42-Nikon F adapter). Depending on what other adapters you might have, you might or might not find it useful to adapt this lens to M42. You also might or might not desire the extra extension this conical adapter would give you.

Note the markings:

Additional data on this objective, it has a 30.97mm effective focal length and a 4.5mm field of view, a working distance of 25mm.

Just for the record, the 10x version of this objective has an effective focal length of 16.6mm, a 1.8 mm field of view, 5.60 mm working distance, and a numerical aperture of 0.25. People using it say that the shape makes it reasonable to use at that short working distance and still get light on the subject.

M42 threads

M42 is somewhat of a "standard" thread mount, at least a well known one for some vintage and legendary camera lenses. Most importantly (to me at least) it was the thread used on old Pentax thread mount cameras and lenses (which I used to own). Apparently it was first used by Zeiss in 1938 for their Praktica cameras, and many other notable and historic cameras used it also.

The M42 spec is metric. Threads have a 42mm external diameter and a 41mm internal diameter, and a 1mm pitch. Also, the specifications for M42 threads allow for different classes of fit, and on cameras it's usually classed as a 'medium' fit and the full specification is M42 x 1.0 -6 (the higher the -X number the looser the fit).

There is also a closely related "T2" mount, which has finer threads (it has a 0.75mm pitch).

There is also another quite rare "T mount" lens standard with really fine threads (M42 x 0.38mm).

Mitutoyo M Plan APO

These are giant infinite conjugate objectives that have a good reputation for photomacrography. Being infinite conjugate lenses, they do require a relay lens. They are expensive. If you can get a good used one in the $500 price range, you should be pleased. They do not have RMS threads, they are much bigger.

Take a look at this Mitutoyo brochure. It describes their FS300 microscope, then has a section on the objectives at the end. Some likely candidates for photomacrography:

These guys had the 10x M Plan APO for sale for $490 (used) in October of 2011. They have the 5x for $375.

All stated magnifications are based on a tube lens focal length of 200mm. Mounting thread: 26mm x 0.706mm pitch (M26x0.706) (36 TPI).

There is a "M Plan Apo SL" series where "SL" stands for "super long working distance".


Feedback? Questions? Drop me a line!

Tom's Mineralogy Info / tom@mmto.org