A Buyers Guide to Compound Microscopes for Mycology
Part 2: Microscope Lenses
In this installment, I’m going to discuss magnification and resolution, then focus on several of the most important components of a compound scope, the objective lenses, the condenser, and the eyepiece lenses. Knowing about these components is very helpful, not only in buying new parts for an existing scope, but in evaluating the features of a new scope.
Numerical aperture and resolution versus magnification
Numerical aperture, or NA for short, is a rather technical sounding term, but an important one. For reasons of space, I won’t go into the gory details of the concept here (though for those who are interested, I’ll post an explanation on my blog, at the URL given at the end of this article), except to note that it correlates directly with the degree of resolution of which the lens system is capable, that is, how small of an object or detail you can distinguish before it blends into a neighboring point. To put it even more simply, numerical aperture is your friend, and, generally speaking, you want to maximize your effective NA at a given magnification so that you can view smaller objects and more detail.
It’s also very important to make a distinction between magnification and resolution, because this is a great source of confusion for people buying microscopes and lenses, and something some microscope sellers take advantage of to make their scopes sound better than they actually are. Magnification, quite simply, is making a given image larger. Resolution is how small of an object or detail you can distinguish in the image. An increase in magnification without increase in resolution is called “empty magnification,” that is, the image is larger, but no more details are revealed. It is analogous to taking a 10x10 pixel jpeg and blowing it up to 100x100—the image may be bigger, but obviously, additional details are totally lacking.
Many microscope dealers will advertise that their scopes are capable of “1600X magnification.” This is meaningless— typically, it means that the scope has been fitted with 16X power eyepiece lenses rather than 10X ones, but since the 100X objective lens has the same numerical aperture regardless of the eyepiece, there is no greater resolution from the additional magnification. With these concepts introduced, we can discuss the lens system of a compound scope.
The objective lenses, or “objectives” for short, are the characteristic lenses on the nose of a compound scope, the ones you rotate into position to change magnification. A set of objectives will differ in magnification and also resolution and NA, with these numbers written next to each other on the barrel of the lens. Because the effective numerical aperture in air cannot be greater than about 1.0, any lens with a higher numerical aperture must be immersed in lens immersion oil to realize its full resolution and even to focus properly. Virtually all 100X objectives have an NA of 1.25 or greater and are oil immersion lenses (or, more rarely, water immersion lenses).
Objectives come in a number of types with increasing degrees of correction for various kinds of optical aberrations. The most basic type are what are called achromatic lenses, or “achromats,” which partially correct for what is called chromatic aberration, the tendency for an image to be slightly unfocused when the different wavelengths of the light spectrum don’t quite focus on the same plane. Achromats are also typically corrected to give reasonable flatness of field, that is points on a single plane appear flat rather than distorted like a fish-eye lens, though points toward the edge of the lens may appear slightly out of focus. “Plan” lenses have a greater degree of correction for flatness of field, typically having good focus through the entire field of view. Lenses that are “fluor” have a higher degree of correction for chromatic aberration, and a resultingly higher NA, than achromats, and apochromatic lenses have the best NA and chromatic correction of all. Plan apochromat lenses are some of the most corrected lenses you can buy, and are correspondingly very expensive – I’ve seen such lenses sell used for over $900! The objective set sold as part of a larger microscope are typically achromat or occasionally plan achromat lenses. A scope sold with a set of plan lenses is worth paying some extra money for, since they can be expensive when sold individually.
Objective lenses should have the magnification factor and NA written on the barrel of the lens, along with other critical information. The barrel length that the objective is made for will also be included, typically 160 or 170 (millimeters), or 8 (infinity corrected, the newest generation of compound scopes)—note that if you have a 160 mm barrel-length scope, you should not use 170 or 8 objectives, and vice versa. The thickness of cover slip (in millimeters) the objective calls for is also given, either as a number, like 0.17, or a "–" symbol, indicating it can take any cover slip of 0.17 or less, or no cover slip at all. (There are also the rare "O" lenses, which indicate that they must be used be used with no cover slip—be wary of these for mycology, because one typically needs a cover slip to prepare slides of fungal tissue.
It comes as a surprise to many beginners that the condenser is an absolutely critical component of the microscope and plays as important a role in determining resolution as the objectives. The condenser is the lens apparatus found immediately below the stage; its function is to focus light on the specimen. A condenser has a set maximum NA, though its NA can be varied up to this point by adjusting the condenser diaphragm. Matching the NA of the condenser with the NA of the objective is a critical step in realizing the full resolution of which the objective is capable, and is an important part of Kohler illumination.
The relationship between the components in getting resolution is a bit complex—if the condenser NA is greater than that of the objective, the objective NA represents the greatest possible resolution. If the objective resolution is greater, than resolution comes out as an average between the NAs of the condenser and objective. A complicating factor for condensers is that, like objectives, achieving an NA of greater than 0.9 requires an oil immersion layer between the top condenser lens and the slide, using a very thick high-viscosity immersion oil. In practice, this requires a lot of cleaning every time one sets up a slide, and is not typically used for most microscopy, except in situations where one wants to achieve the highest resolution possible, such as publication-quality microphotography.
A “dry” condenser is one with an NA of 0.9 or less, which can therefore be used without oil immersion. A 0.9 NA dry condenser will give you the best resolution possible without oil immersion – better than with higher NA condensers, in fact. Higher NA condensers are of two kinds “dry/oil” condensers that get their best resolution and focus under oil immersion, but focus reasonably well without oil, and dedicated oil condensers that absolutely require oil immersion for proper focus.
Like objectives, condensers come with various degrees of correction. The simplest and most common is the Abbe-type condenser, which has some simple correction for spheric and chromatic aberration. There are also achromatic condensers, more fully corrected for chromatic aberration, and aplanatic condensers, with a greater degree of correction for spherical aberration, and achromatic/ aplanatic condensers, with both corrections. For simple achromat and plan achromat objectives, an Abbe condenser should be fine, though more highly improved objectives should be matched with more highly improved condensers.
The ocular, or eyepiece, lenses are the final component in the lens system, adding the final magnification, focusing the image from the objective on the eye, and making it visible. The total magnification of a scope is found by simply multiplying the objective lens magnification by the ocular lens magnification (typically 10X). Note that the microscope head (or other components) may have an additional magnification factor as well (typically 1.25X) which is also factored into the total magnification.
Good oculars are “widefield,” which means that they have a wide field of view, and “high point,” which means the focal point is high enough above the lens surface that the viewer does not have to press their face right up against the eyepieces, and the image can be viewed even by someone wearing glasses. Plan objective lenses typically require ocular lenses with matching correction in order to get an image with full flatness of field.
Some oculars are made with an internal resting flange, where one can add a measuring reticle or pointer disc. (I’ll discuss measuring reticles more in an upcoming article.) In upcoming articles in next season’s Mycena News, I’ll discuss other components and features of a compound scope, as well as the accessories one needs for mycological work and for cleaning the scope.
As I did in the last article, I’ve put more information and links on my blog. The article can be found here: