Mineral High Resolution Photomicrography with Infinitive Optics and StackingLast Updated: 23rd Jul 2014
By Volker Betz
Mineral High Resolution Photomicrography with Infinitive Optics and Stacking
(Exploring the difficulties)
By Volker Betz, Taunusstein, Germany
Photographing very small crystals and much smaller details of crystals requires high resolution optics resolving in the 1 µm range. A look into the corresponding Internet fores (http://photomacrography.net) shows, that a suitable solution can be found by using infinitive plan apochromatic corrected microscope lenses in conjunction with telephoto lenses replacing the microscope body with the dedicated tube lens. Such systems outperform the conventional macrolens + bellows solution, which are limited by small working distance, lesser resolution and possible chromatic aberration, if the field of view (FOV ) is dropping below the 2 mm range. Of course such instrumentation is offered from industry as a ready to use product, but either to expensive for personal home use or – in the low cost range – of also low performance.
Mechanical parts and setup
The alternative home building has some challenging aspects. Stacking has to be done at levels below the DOF of the lens which are at an N/A of 0.30 only a few µm and any kind of vibrations must be minimized to a level not affecting the resolution. In order to fulfill the requirements the equipment was converted from the conventional vertical assembly to a horizontal setup. It is based on a strong aluminum profile plate which carries the stepper motor driven and pc-controlled micrometer translation stage, which also carries a optical lab jack for vertical manual (y-axis) adjustment. All the photographing parts are assembled on a commercial (Proxxon) manual x,y-cross table for manual adjustment in the coarse x- and z-direction. Other mechanical parts are custom cut aluminum profiles and the corresponding assembly screws and parts. Any mechanical work is limited to drilling some holes.
All movements to stack a series of pictures, are done at the object carrier by a trinamic stepper motor controlled by the Helicon Remote software. The how to do details to assemble this motor setup can be found on the Helicon Remote website. The y- position of the object is done manually with a lab jack also the coarse x and z position of the camera with the mechanical cross table which carries the photography assembly.. The minimal adjustment steps of the manual devices are sufficient to position the object in the FOV.
Primary intended to be used with infinite focusing microscope lenses, this setup can also be used with finite focusing lenses as shown in corresponding forum discussions: http://www.mikroskopie-forum.de/index.php?topic=12046 So a 25 mm luminar works well if mounted with an RMS to filter thread adapter. The resolution is about 20 % higher compared to bellows use. Performance can be tested for example by photographing an object micrometer, which als allows to calibrate the Field of View ( FOV). Own tests with different luminars and microscope lenses showed good results.
My personal focus was the use of lenses with long working distance and high aperture of the so called M Plan Apo class. Unfortunately this kind of lenses, frequently used for quality control of electronic chip production, are quite expensive as all plan apochromatic lenses. I aquired two used lenses (a Mitutoyo M Plan Apo SL20 with an N/A of 0.28 and a Optem M Plan Apo 10X with an N/A of 0.30 from ebay sources) and tested it with an object micrometer. Both worked fine with a good match to data sheet specifications. This must not be always the case with used lenses.
Numerical Aperture, Resolution and Depth of Focus
Some knowledge of this related parameters is essential to estimate the size of z-axis steps for stacking. Some data found in data sheets of Mitutoyo, Quioptic and Nikon have been compiled for the following diagram.
It is the nature of focus definition that the numerical figures have greater tolerances and so for 10 X lenses a DOF of ~ 3-6 µm and for 20 x lenses a DOF of ~ 2-4 µm can be assumed. Based on such weak figures only an approximate step size can be derived. If 5 µm steps are used and the target DOF ist 300 µm then ~60 steps are needed. This matches to the practical experience made.
LED-lights are now established as standard light source for microscopic work. I use for this setup six individual switchable LED´s on flexible holders with a magnetic foot, manufactured by E-mP ( www.stonemaster.eu ) for mineral photography. Color temperature is about 6400 K. In most cases softener screens are used and color balance is adjusted to shadow or to manually. Lights are carefully adjusted to avoid overexposed reflections and to make the mineral features visible. In general most pictures are taken rather underexposed, because this can be corrected very easy in post processing, while overexposed parts in the picture are very difficult to correct.
Finding the FOV
As the specimens are prepared under a stereo microscope and then transferred to the photographic equipment finding a small 1 mm FOV can be difficult. But a simple trick helps. With the DSLR camera switched off, a small LED pocket lamp can be used to adjust the position and coarse focus, just by manual attaching to the camera ocular. With the photo-lights switched off a small light spot is visible and and coarse x,y can be adjusted to the area of interest. Also a coarse z focus can be found by adjusting to the smallest light spot.
Cost of equipment
Cost of equipment is always in discussion. Some approximate € figures are given here for orientation, but real cost may be different.
Also neccesary is a computer with with corresponding software and a stereomicroscope.
Some useful links:
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