Buying a dark field compound microscope is necessary if viewing live, unstained organisms is a priority.  Dark field compound microscopes are excellent tools to use with living organisms and specimens which can be observed without altering their natural state.  These microscopes are commonly used for live blood cell analysis and observation of specimens that should not be stained with a fixative.  They offer excellent contrast and details and are ideal for observing specimens that would otherwise be difficult to distinguish using other types of optical microscopy such as brightfield illumination.  Here are some considerations when buying a dark field compound microscope:

Illumination
The primary difference between a dark field microscope and the more common brightfield microscope is the use of light.  In a bright field microscope, the specimen is illuminated by allowing the light to pass through it.  In a dark field microscope, light that should pass through the specimen is blocked and it is illuminated only from the sides, which makes the clear portions, including the background, appear dark.  Light is manipulated to produce a dark background through a dark field condenser.

The source of illumination is an important consideration.  Many of the newer dark field compound microscope models come with easy to detach electric bulbs so replacement should not be a problem.  When buying, find out what type of bulb is used and what wattage.  Generally, the higher the wattage, the brighter the illumination.  However, lower wattage bulb will usually perform as well as their higher wattage counterparts.

To ensure that you do get the kind of illumination you need, check the microscope yourself.  Depending on your requirement, a low wattage bulb will usually suffice.  A fluorescent bulb at 7 watts will often perform as well as a tungsten bulb at 20 watts for a monocular dark field microscope.  If you’re buying a binocular microscope, a halogen bulb may be a better choice because it offers stronger illumination.  Furthermore, find out if the bulb is replaceable and if it is, ask where the replacement parts may be purchased.

Optics
Other than the illumination, optics is the most important consideration when buying a good dark field compound microscope.  Look for quality lenses with a reliable focus system.   Objective lenses undergo standardization before they are released for distribution.  There are two kinds of this standardization – the DIN and the JIS.  DIN is the German standard for objective lens and is probably the more common while JIS is the Japanese standard.  Having lenses that pass either of these two standards will allow you to replace them with any unit from just about any brand or manufacturer in case lenses get damaged or lost.

Magnification
Dark field microscopes are best used for observing live organisms, so magnification power is also important.  If you’re looking for a compound type of dark field microscope, you must also understand that it is not recommended for use with bigger specimens since these microscopes are designed to have a small working space dividing the lens and the slide.  Furthermore, the magnification that many of these dark field microscopes come equipped with are designed for viewing small specimens with low contrast.

Dark field microscopes are capable of producing excellent contrast in specimens and it would be a waste if this is not put to good use by limited magnification.  Depending on the requirement, magnifications can range from a low 40X to a high of about 1500X.  Look for a good eyepiece and objective combination because these are the two components responsible for producing an excellent image of the specimen.  Try the microscope to see how well the coarse focus and fine focus work together.  At high magnifications, having a good fine focus will make a lot of difference.

Brand
If you prefer to shop by brand, there are excellent dark field microscope products from manufacturers such as Leica, Zeiss, Olympus and Nikon, although smaller companies such as Lomo and Meiji may also offer more affordable yet quality alternatives.

Warranty
The manufacturer’s warranty should not be an overlooked consideration when buying a dark field compound microscope.  This will help bring down the cost of parts replacement and labor in case the instrument does break down as a result of wear and tear.  Usually, manufacturer’s warranties are limited – meaning, they will not cover for damage inflicted upon the microscope deliberately.  Mishandling or improper usage, for example, will not be honored by the limited warranty.  Only parts and labor will be.

An alternative
If you already have a standard compound microscope and don’t want to spend for another unit with dark field microscopy capability, simply equip your current unit with a dark field condenser.  The condenser lens can manipulate light in the same way a dark field microscope works, thus producing dark field images effectively.  This special attachment will allow your standard microscope to function like a dark field microscope.