Dave Walker in his article on the Zeiss 1x objective mentioned his desire to one day own a Leica M420 macroscope. I am fortunate enough to have such a scope, and thought an article on this instrument might be of interest to readers of Micscape. I acquired an M420 about 3 years ago from an internet seller I have not regretted the investment - this is one spectacular scope.
The Leica M420 macroscope is an optical precision instrument for studying objects in their entirety in conjunction with large working distances and large fields of view. The multiple-coated optical system involves a single vertical beam path which, for purposes of comfortable observation in the binocular tube, is then split into two identical parts (Figure 1). The vertical observation mode ensures parallax-free imaging and is ideal for faithful documentation, precise measuring, and investigations involving polarized light. While this design maximizes image quality, NA and working distance (4 inches!) it does not provide true stereo as both eyes view the same image (hence the name "macroscope").
Leica offered the M420 macroscope for all tasks relating to documentation and investigation in which highest accuracy, optimum image quality and maximum information content were paramount. Leica touted the M420 (supplied with either the achromatic 1:5 or apochromatic 1:6 zoom objectives) as being ideal instrument for faithful photography, which it is. The macroscope allows for very convenient imaging as various cameras can be inserted into the built-in phototube. According to Leica each instrument is assembled and adjusted to extreme tolerances using environmentally-acceptable procedures, and is then subjected to exhaustive inspections and tests under extreme conditions. The build and optical performance of my unit attest to the validity of these claims.
The low binocular tube, which introduces an additional magnification factor of 1.25×, promotes comfortable viewing. So do the distortion-free "humongous" wide-field 10x/21B eyepieces for spectacle wearers (Figure 2). Their adjustable eyecups and long exit pupils permit viewing either with or without spectacles. Parfocality ensures that the visual and photographic images remain in focus when the magnification is changed. As figure 1 indicates there is an iris diaphragm located inside the body just above the top lens of the objective (the ideal location). The iris allows one to increase contrast and depth of field. In general use it is best to keep the iris opening at more then 50% in order to prevent noticeable degradation in image quality.
I purchased my M420 with a large boom stand. Leica also offers an upright stand, a more practical arrangement for general use. Unfortunately the likelihood of such a stand appearing on the used market is pretty much nil. In addition my macroscope was the second generation model - it looks exactly like the earlier Wild version but is branded with the Leica name. Shortly after acquiring Wild, Leica retooled the carrier system for a different stand geometry and this version is the one now in production. Consequently the current upright stands will not readily accommodate older carriers. For these reasons I designed a new support system and had it made by up by a machinist friend (Figure 3) starting with an old Nikon stereo microscope stand. There are two support columns, one large and one small one, to keep the unit aligned when the carrier assembly is move it up or down (Figure 4). The macroscope focusing mechanism has both coarse and fine movements - very handy when shooting images for stacking or getting the focus bang on.
My scope is equipped with the 1:6 apochromatic objective (Figure 5). The intrinsic zoom range of this objective is from 5.8× to 35× (however, due to the 1.25x corrective lens, the viewing head yields a zoom range of 7.3× to 43.8×) Because of its excellent chromatic correction for three spectral colours and for all intermediate tones, the "Apozoom" is the objective of choice if requirements stipulate maximum sharpness, highest resolution and maximum contrast rendition. Leica offers 0.4× and 2.0× lens for Apozoom objective. I recently purchase the 0.4× reducing lens. It doubles the working distances to an astounding 8 inches. (With the reducer in place I have to stand up to use the viewing head!)
I place objects being observed or photographed on a large rotating stage to which I have added a square mechanical stage. The mechanical stage can be rotated 360 degrees. I use a Schott-Fostec fibre optic system for illumination of stationary subjects and electronic flash for imaging a moving subjects (although I have begun to use the flash for static objects as well). Reflected light is supplied by a very large ringlight (8 inches) mounted concentric to the objective (Figure 6). For subjects requiring transmitted light (including darkfield) I use a small (1 inch) ringlight located below the stage. The subject is placed on a translucent white plate for brightfield (Figure 7), a clear glass for darkfield and a flat black plate for reflected light.
To capture an image I use a Nikon Coolpix 4500 and an achromatic 10x relay lens (the M420 upright tube has an adapter designed for the 23mm eyepieces). Parfocality is easily achieved by slight adjustment of the focusing rings on each eyepiece. The viewing head receives 50% of the light, the remaining 50% goes to the camera.
I have uploaded a page of images taken with this microscope re: images. Hopefully these serve to illustrate the amazing quality of this landmark instrument. Email me if you would like an electronic copy of the sales brochure.
Comments to the author are welcome (firstname.lastname@example.org).
Published in the July 2005 edition of Micscape.
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