Which Microscope Should I Buy?
Should I Buy a New or a Used Microscope?
I know that many dyed-in-the-wool microscopy hobbyists would call the following statement blasphemy, but if you are new to the microscopy hobby and would like to buy a scope that will allow you to dive right in and get started immediately, you should either buy yourself a new microscope, or get a good vintage scope that has been professionally refurbished and is ready to go. I say this because I understand that nothing is more frustrating to the typical newcomer than wasting time and money on a microscope that is not ready to be put to use. To be fair, those who would call this advice blasphemy know that a new microscope of the type most amateurs can afford will almost certainly be significantly inferior to a high-quality clinical or research microscope made back when quality was something manufacturers strived for. For many of these people, taking apart and servicing a vintage microscope is an enjoyable aspect of their hobby. The newcomer should be aware that an Olympus BH2 from the 1980s, while indeed a very high-quality scope, may need a fair amount of up-front work before it is really usable. It is not at all uncommon for the grease in these older scopes to have solidified over the decades, leaving the focus mechanism very stiff or even completely seized, or for various accessory components on these scopes to be incorrect, damaged, or even outright missing.
So all of this is why the Empire of Dirt Workshop exists. It can be summed up in three points: 1) I want to provide those who do not want to become a microscope technician with a way to get their hands on a nice Olympus BH2 scope that’s ready to be put to use. 2) I want to help those willing to take on the challenge of servicing their own BH2 scopes, by providing assistance in the form of the PDFs and videos linked here, which were produced by the Empire of Dirt Workshop. 3) I am one of those people who thoroughly enjoys taking apart and servicing quality vintage microscopes, and this is an excellent way for me to be able to do that.
Repair services and replacement parts for BH2 scopes are no longer available from the manufacturer, leaving used, surplus, and third-party suppliers to fill the void. But before you make a decision between a new scope or a vintage gem, it is important to understand that the parts situation for most new, low-cost, re-branded Chinese scopes is no better than that which exists for vintage scopes. Outside of a return or exchange during the initial warranty period, good luck finding a knob, an internal gear, or any other mechanical part needed to repair these commodity microscopes. You may be able to buy eyepieces, objectives, and perhaps even darkfield condensers from the original resellers, but most do not offer a bench repair service nor repair parts for their equipment. These scopes are very much a product of our modern throw-away society and as such were designed to a low-cost price point with no provision for long-term serviceability. For these low-cost microscopes, it is simply more economical to replace the entire scope rather than to pay a skilled technician repair it!
Are the Microscopes Made In China Any Good?
So, having said all that, are the microscopes made in China any good? There are some very high-quality microscopes manufactured in China today. Some of the best microscopes available from the Big Four manufacturers (Leica, Nikon, Olympus, and Zeiss) are all, or in-part, manufactured in China. The flip side of this is that there are also microscopes of abysmal quality manufactured in China. I’m talking scopes with a quality of workmanship and construction that would a Big-Four microscope owner’s skin crawl. These scopes tend to be the no-name, rebranded scopes sold by shady dealers who use wildly exaggerated performance claims such as 2000X, 2500X, or even 3000X (!) magnification to sell their microscopes. These scopes are offered at prices that seem too good to be true, and those who take the bait will likely be disappointed. As it turns out, any price, however low, is too high for a piece of junk!
Between these two extremes, there are of course microscopes of mid-range quality made in China as well. So be careful and do your research before buying a microscope, regardless of the country of origin. Whether manufactured in private factories owned by the reseller or in independent factories on contract with the reseller, all microscopes are built to a specific price point to meet the specifications of the reseller. It is not so much the country of origin that matters, but rather the reseller’s dedication to quality and customer support. If you buy a no-name, rebranded scope from a shady dealer who markets their wares using misleading performance claims, then don’t be surprised if the scope you receive is horrible, and don’t be surprised when the dealer either refuses to answer, or cannot answer, any questions you may have after your microscope arrives.
Should I Spend the Extra Money for an Infinity Microscope?
If you have decided to buy a new microscope, and are now looking to find a quality scope at an affordable price, you will no doubt be faced with the question of whether you should buy in infinity scope or a finite-conjugate scope. To answer this, there are a few things you should know about the infinity-vs-finite debate. The optical designs of compound light microscopes fall into one of two basic types: 1) finite-conjugate optics, and 2) infinity optics. The difference between these two design types lies in how the objective lens produces the intermediate image that is further magnified by the eyepiece lenses. When selecting a microscope, it is critical to have at least a basic understanding of the differences between finite-conjugate and infinity optics, since each type has its own unique set of advantages and disadvantages, and since the optics of one type cannot be used on microscopes made to accept the other type.
In general, microscopes with infinity optics are more expensive than finite microscopes with 160mm tube-length optics. Except for a very few specific cases, there are no inherent performance differences between the two optical types. The cost difference between finite and infinity optics is irrelevant if you plan to buy a new scope from one of the Big Four manufacturers (i.e., Leica, Nikon, Olympus, or Zeiss), since all they offer these days are infinity scopes and you will therefore have no choice in the matter. But cost aside, there are a few things to consider before making the decision of whether to invest in a microscope with infinity optics over one with DIN-compliant finite-conjugate optics.
The primary advantage of an infinity scope is that the length of the infinity space within the optical tube (i.e., the space between the objective lens and the tube lens) is not critical for the performance of the optics. This allows manufacturers of infinity scopes the freedom to provide intermediate optics of modular design, which can easily be installed into the infinity space of the microscope stand, to provide additional functionality without upsetting the optical performance of the scope. Contrast this with a finite-conjugate scope, wherein the introduction of modular intermediate optics will upset the critical tube length, necessitating the use of supplemental optics in the intermediate attachment to compensate for the increase in tube length. Depending on your specific interests and budget, this may or may not be a significant concern. Unless you plan on adding such things as reflected illumination or differential interference contrast to your scope, this aspect of infinity optics probably doesn’t matter much, especially since both of these functions are quite expensive and are out of reach of most amateurs.
A second advantage to infinity optics is in photomicrography. Microscopes with infinity optics do not typically require compensating eyepieces (i.e., eyepieces that provide correction for residual chromatic aberration not removed by the objectives), but those with finite-conjugate optics frequently do. The reason for this is that all of the correction for chromatic aberration in infinity scopes typically occurs either completely within the objectives, or if not, is split between the objectives and the tube lens built into the infinity microscope’s frame or viewing head. Regardless of which way this is accomplished, this means that the intermediate real image presented to the eyepieces is fully compensated (i.e., has no significant chromatic aberration or field curvature) and can be picked up for digital imaging with a simple camera that replaces one of the eyepieces. This is a cheap and simple solution, although not the best performing solution.
Contrast this with the scheme used in many finite-conjugate designs, such as the Olympus BH2. In these finite designs, the intermediate real image often contains some degree of uncorrected chromatic aberration (and perhaps field curvature in other manufacturers’ scopes). In these scopes, matching compensating eyepieces must be used to remove the residual aberrations in the intermediate image. Due to this reliance on compensating eyepieces to remove residual aberrations from the final image, the simple technique of replacing one of the eyepieces with a digital camera cannot be used. Instead, either matching compensating eyepieces or a matching compensating photo eyepiece (i.e., photo projection lens) must always be present in the optical pathway, making this solution a bit more complex and expensive for the user. This method tends to provide significantly better performance than the simple eyepiece camera.
The list of potential disadvantages with infinity optics includes poor optical and mechanical interchangeability. Finite objectives built to the DIN standard can be freely swapped among DIN-compatible finite scopes from the various manufacturers, with the only caveat being that the proper eyepieces and photo eyepieces will be necessary to provide optimal performance. This is not necessarily true for infinity scopes, for a few reasons. First is the matter of optical compensation. Infinity objectives which rely on the tube lens to provide some of the necessary optical compensation could reasonably be expected to perform poorly in a scope from another manufacturer, since they would quite possibly be used with a tube lens with different characteristics than what was originally intended. Whether or not this difference is objectionable, or whether there even is a visible difference, is difficult to predict without knowing the specifics of the two manufacturers’ optical designs. The second reason is that the mechanical design of infinity objectives can make them physically incompatible. While some infinity objectives use the same RMS threads as DIN-compliant finite objectives, many do not. Whether or not a given infinity objective would provide acceptable images on another manufacturer’s scope is irrelevant if the objectives are not mechanically compatible with that scope.
What Magnifications Do I Need?
The correct answer, of course, is that it all depends on what you intend to look at. But without knowing the specifics, a good recommendation is to just get yourself a microscope with the standard line-up of optics, which includes dry 4X, 10X and 40X objectives, a 100X oil-immersion objective, and a pair of 10X widefield eyepieces. Whatever it is that you intend to look at, it’s safe to say you should absolutely ignore any claims made by microscope sellers of magnifications exceeding 1500X. Curiously, if you look at the marketing brochures from the Big Four microscope manufacturers (i.e., Olympus, Nikon, Zeiss, and Leica), you will find that none of their brochures even mention the magnification that their scopes can provide. Amazingly, even the brochures for their top-of-the-line, obscenely expensive research microscopes say nothing of magnification, and do not include the letters “2000X”, in bold text, anywhere!
The reason for this is that any compound light microscope can be fitted with a 100X oil-immersion objective (the 100x oil-immersion provides the highest resolving power of any of the objectives commonly available for routine biological applications) and a pair of ultra-cheap 20X eyepieces. When used together, you will indeed get 2000X magnification. Never mind the fact that the laws of physics, especially those pesky bits concerning the wave nature of light and diffraction, preclude you from ever getting good or even decent images at 2000X magnification, you will nonetheless see an image magnified 2000X in all of its blurry and disappointing glory. That’s all these guys are doing. They’re throwing in a cheap pair of 20x eyepieces, so you will think their scope is good to 2000x.
By the latter part of the 19th century, the science of optics had advanced to the point where microscope designers were building scopes that performed right up to the diffraction limit of visible light. The diffraction limit, or diffraction barrier, as it is sometime called, imposes a limit to the resolving power of a conventional light microscope caused by the diffraction effects of light waves passing through the specimen under observation. These early microscopes provided useful magnifications of up to 1400X or so, and guess what? The same laws of physics apply today, as then, and because of this you still cannot buy a microscope that can see beyond the diffraction limit. To be sure, there have been many significant improvements made to the design of microscope optics since the 19th century, and there will no doubt be more in the future, but none of these improvements will allow even the best 100X oil-immersion objective to reach beyond the diffraction limit and provide 2000X magnification to any but the most naïve of microscopists.
If you want the highest useful total magnification possible, go out and get yourself a microscope with a top-tier 100X oil-immersion objective boasting an apochromatic design with a numerical aperture of 1.40, and get yourself a good pair of 15X eyepieces and an aplanatic / achromatic NA 1.40 condenser to go with it. In terms of useful magnification, you’ll have the best performing conventional light microscope available at any price. These rock-star lenses, with their rock-star price tag, will provide a maximum usable magnification of about 1000 times its rated N.A., just like the cheapest ones will do. It does a bit better in useful magnification than the cheaper ones, since its rated N.A. is a bit higher, but either way, 1500X is tops for it (1500X is actually a bit beyond “tops”, since 1400X is a more realistic upper limit for this objective). These high-dollar dream optics have the same upper limit of useful magnification as those made in the 19th century! In comparison to these rock-star optics, the cheaper 100X oil-immersion objectives available today are good to 1250X or so, since they sport a more modest N.A. of 1.25.
Just for the fun of it, go ahead and stick a pair of 20X eyepieces on your scope with your new rock-star objective, to raise the total magnification to 2000X (just like the Amscope and Omax folks are doing). What do you see? Ignore the annoyingly narrow field of view of these eyepieces. And sure, thanks to the extremely low eyepoint of these eyepieces, your eyelashes will rub the top elements of the eyepieces whenever you blink, which will smudge them with oil in no time, but go ahead and power through. What do you see? A blurry mess of an image that would look better, while showing the same level of detail, when viewed through your comfortable 10X widefield eyepieces is what you will see. So now that you’ve done this, and now that you’ve seen how the other half lives, treat yourself to the satisfying “cluck-clunk” sound that these 20X ergonomic disasters make as they hit the trash can. Now that feels good, doesn’t it?
Which Eyepiece Magnifications Do I Need?
Many microscopes sold today are advertised as providing 2000X, 2500X, or even 3000X magnification. Since a 100X objective provides the highest objective magnification routinely available for biological microscopes, this means that to achieve these stated magnifications, eyepieces of 20X, 25X, or even 30X would be needed, right? Technically speaking, yes. But the simple truth is that no conventional light microscope can provide usable magnifications of 3000X, 2500X, or even 2000X, so why worry about these eyepieces? All they will do is put you squarely in the camp of empty magnification, where the resulting images will be sadly disappointing.
Rather than ask why these eyepieces are included with low-end scopes, perhaps a better question would be to ask why they are not included with scopes from the legitimate, high-end manufacturers. Or why do the marketing brochures from legitimate high-end manufacturers not even talk about total magnification? I have always said that the best use for a pair of 20X eyepieces is to hear the satisfying “clunk-clunk” sound they make when they hit the trash can. All kidding aside, the 10X eyepieces supplied with your microscope are all that you will ever need. In some cases, 8X, 12.5X or 15X can be useful, but there is no reason to ever feel eyepiece envy if you “only” have 10X eyepieces. Believe me, if you buy a microscope with a pair of 20X, 25X, or God forbid, 30X eyepieces, they will spend their entire lives in a desk drawer somewhere, mocking you every time you open that drawer. You will wish you could throw them out, but of course you will never do that because they came with your microscope. But that “cluck-clunk” sound would be really satisfying, wouldn’t it?
If you’ve ever peeked into a pair of these 20X eyepieces, you know that the visual field is annoyingly narrow and the eye relief is horrible, making even the best of these damned things just about unusable. They are included solely to allow manufacturers can claim 2000X, 2500X, or 3000X (!) for their microscopes, to lure naïve and unsuspecting buyers. Do not be deceived, it is pure snake oil. Any claims for magnification above 1500X or so are simply deceptive advertising, and this fact alone should be sufficient justification for not buying scopes advertised in this way.
Which Objective Magnifications Do I Need?
A mid-range compound light microscope usually comes equipped with four standard achromatic objectives (4X, 10X, 40X, and 100X oil-immersion), which are perfectly fine to get you started in the microscopy hobby. It doesn’t take most hobbyists long to figure out that the 100X oil-immersion objective is inconvenient and messy to use. Couple this with the fact that they are used primarily to image bacteria (which are by far the least interesting living organisms you will ever observe), and you will likely find yourself rarely, if ever, using your 100X oil-immersion objective lens. That aside, if you’re like most people, as your knowledge and experience in the hobby grows, so will your desire for magnifications other than the basic four. Objectives of 20X and 60X are common additions to many nosepiece turrets, assuming there are open positions to accept them.
So do some soul searching, and if you decide you really don’t use your 100X oil-immersion objective all that often, take it off the turret, put it back into its protective canister, and tuck it safely away in that desk drawer, right next to those 20X eyepieces. That way, the 100X objective will be there for you when you need it, and more importantly, you’ve made room on the turret for either a 20X or 60X objective lens, which you will use way more often than that 100X oil-immersion objective in the drawer.
Should I Buy Plan Objectives For My Microscope?
If you do much photomicrography, or if you plan to do so in the future, you will likely want to upgrade from standard objectives to Plan (i.e., planar) objectives. Plan objectives will render a much larger area of the visual field in acceptable focus, providing photomicrographic images which are superior than those produced by standard non-plan objectives. If you already know that microscopy is going to be the hobby for you, and if you can afford it, buy a scope with Plan objectives right from the start. Although it will cost more up-front, it will save you money in the long run. That said, with the advent of modern focus stacking techniques, camera images taken with non-Plan objectives can be surprisingly good.
What Is a Mechanical Stage, and Do I Need One?
A mechanical stage is a mechanism on the microscope, either integral to the stage or an attachment to the stage, that allows the operator to easily and precisely position the specimen slide on the stage in order to observe specific areas of interest of the specimen. A typical mechanical stage consists of two knobs, usually coaxial, where one knob moves the specimen slide in the X (east-west) axis, and the other moves the slide in the Y (north-south) axis. A mechanical stage is especially valuable for those scope operators who have difficulty adapting to seeing the image through the eyepieces move in the opposite direction in which the specimen slide is maneuvered by hand.
Although a mechanical stage is not a strict necessity, it is a very nice feature to have on your scope. If your scope does not have a mechanical stage, and if for some reason you find yourself someday using somebody else’s scope which has one, then you will become immediately inflicted with a severe case of “mechanical stage envy”, and for this there is but one cure. Do yourself a favor and buy a scope with a mechanical stage right from the start.
Which Should I Get, a Microscope with a Monocular, Binocular, or Trinocular Head?
A binocular or trinocular viewing head is the best option for most people. The binocular images provided by these viewing heads can significantly reduce eyestrain during extended periods of observation, as compared to using a monocular scope (at least for those people who are able to use a binocular scope). For some people with certain vision disorder, such as strabismus or monofixation syndrome, it can be difficult or even impossible for their brains to fuse the images presented to their eyes by binocular eyepieces into a single, discernable image. Do not buy a monocular scope if you are able to successfully use a binocular microscope. Even those whose vision cannot fuse binocular images might want to consider a binocular scope if the scope will be used very often by others. Additionally, even if you cannot fuse binocular images, you may want to consider a trinocular scope, for convenience if the scope will be used for photomicrography, since this allows a camera to remain permanently affixed to the scope while it is also used for visual observations.
A trinocular head is essentially a binocular head fitted with an internal beam splitter, and with a camera port on top which allows you to permanently affix a camera for photomicrographic purposes while retaining full access to the eyepieces for visual observations. There is usually a shaft, with a knob on the end, sticking out the side of the head, which is attached to the internal slide-mounted beam splitter to allow the operator to position the beam splitter as necessary to accommodate the task at hand. If the light-path selection shaft is positioned such that the beam splitter is in the optical path, the available light will be split into two components, one of which is sent to the eyepieces and the other is sent to the camera port. The light-path selection shaft may also provide positions to send all of the available light to the eyepieces, for light-starved observations, or to send all of the available light to the camera port, for light-starved photomicrography. Some of the less expensive trinocular heads might have only one of the last two light-path selections described above, and some may not even have a light-path selection shaft at all, meaning that operation is only possible with the light split between the eyepieces and the camera port.
Which is Better, Halogen Lighting or LED Lighting?
Halogen lighting has enjoyed a long history in microscopy, going back to the 1970s. Halogen lighting operates at higher temperatures than standard incandescent lamps, which shifts the spectrum towards the blue, producing light with a higher color temperature and at a higher power efficiency. Halogen lamps are the only available option for producing black body radiation with a truly continuous spectrum, similar to that of the sun, making it the most suitable option for the eyes. Today, although there are still halogen-equipped scopes on the market, LED lighting is becoming much more the norm. The pros and cons of both are discussed below.
The filament of a halogen bulb, being a black body radiator, produces a continuous daylight balanced spectrum of light that renders all colors present in the specimen visible to the observer. In contrast, LED lighting is to some degree or another discontinuous, meaning that some wavelengths of light will be lacking in the spectrum, making the colors associated with those deficient wavelengths difficult to see. In this respect, halogen is the clear winner. But halogen has the inherent disadvantage that the apparent color temperature of the light changes as the lighting intensity is varied. At low levels of illumination, halogen lighting has a distinct yellowish, warm cast, which tends to go away as the intensity is increased.
For visual observations, this objectionable yellow cast can be removed by including a daylight blue filter in the illuminating path. For photomicrography, this aspect of halogen lighting means that the resulting exposures will have a different color cast, depending on the intensity with which the exposures were made, making it necessary for the operator to perform a custom color balance for each intensity setting to eliminate this effect. LED lighting does not suffer from this issue. The color temperature of LED lighting does not appear yellowish at all, at any intensity, so the daylight blue filter discussed above is not needed. In fact, if anything, LED lighting may appear a bit too cool to the eyes, with a slight bluish tint which some microscopists may find objectionable. However, after using an LED-equipped scope, most operators quickly adapt to the cool appearance of LED lighting. When LED lighting is used for photomicrography, a custom light balance for the camera needs to be performed only once. In this respect, LED lighting is the clear winner.
Halogen lighting creates a lot of heat in the housing for the halogen lamp and in the electronics driving the lamp. For halogen-equipped scopes whose lamp is located within the base, below the stage, this can be a real problem since the heat produced by the lamp causes the entire base to run hot, which can adversely affect the reliability of the electronics in the base, and to some degree, can adversely impact operator comfort. The designers of upper-end clinical and research microscopes generally put the halogen lamp external to the base, in a housing on the rear of the scope, in order to allow for a more comprehensive illumination system than could be placed within the base, and to allow for better thermal management. For these scopes, internal heat rise of the base is not really an issue. However, lower-cost microscopes which contain a halogen lamp integral to the base will have an inherently lower electrical reliability as compared to scopes with an externally mounted lamp, due to heating of the electronics within the base. The significance of this depends on the design of the microscope. A good design which properly addresses these thermal issues will have higher reliability than one which neglects thermal management.
Another issue that can be a problem with halogen lighting is heating of the specimen during extended observation at high light intensities. This can be a problem because of the relatively high level of infrared wavelengths present in the lighting spectrum produced by halogen lamps. This can be largely alleviated by the inclusion of an IR-blocking filter (i.e., adiabatic glass) in the illumination path, and many higher-cost research or clinical microscopes with halogen lighting contain an integral adiabatic glass filter or have provisions for placing one in the illuminating pathway. Lower-cost halogen-equipped scopes may not have such a filter, but these scopes tend to be the ones whose lighting is not powerful enough to pose a significant risk to the specimen in this way.
An additional factor that can cause overheating of the specimen with halogen lighting is the physical configuration of the lighting system. If the halogen lamp is included within the base, directly below the stage, as is true with many low-cost halogen scopes, waste heat from the halogen lamp will heat the stage above, and subsequently will heat the specimen. The better scopes that are equipped with halogen lighting have the lamp in an external lamphouse, typically on the rear of the scope, and these scopes will typically not exhibit specimen-heating problems. The lamphouse on these scopes is far enough away from the stage that the waste heat from the lamp does not affect specimen temperature.
Both halogen and LED lighting can produce significant amounts of short-wavelength UV radiation. In general, LED sources are worse than halogen, but this is not always the case. Since excessive UV radiation can cause cataracts and other health issues, the design of a microscope should include a UV-blocking filter in the lighting pathway to greatly reduce or eliminate the UV exposure of the operator. If you buy a microscope from one of the Big Four manufacturers (Nikon, Olympus, Zeiss, and Leica) you can be sure that this issue has been addressed and the microscope is safe to use. On the other hand, if you buy a no-name, rebranded commodity scope sourced through China, who knows?
The electrical efficiency of LED lighting is much greater than that of halogen lighting. This means that an LED-equipped scope will run significantly cooler and will require less electrical power to operate than a comparable halogen-equipped scope. This is not a significant issue for most applications, but the improved efficiency of LED lighting makes possible a design that can be operated on batteries, for convenient portable or field use. If ease of portability is important to you, make sure you buy one which includes the capability to operate off batteries, since many LED scopes are not so equipped.
A halogen lamp can be expected to provide anywhere from 100 to 2000 hours of service life (depending on the lamp type) when operated at full brightness. If you have a halogen-equipped scope, be sure to keep a spare bulb or two on-hand. In contrast, an LED should provide 20,000 or more hours of service life before failing. This means you probably won’t be replacing LEDs. On the other hand, if the LED ever does fail for some reason, the only way to repair the scope will likely be to obtain the necessary parts or service from the manufacturer. This can be an option if you have a scope from some of the better manufacturers but be aware that individual parts to repair lower-cost, rebranded commodity microscopes are generally not available.
If you end up with a scope that is equipped for halogen lighting, don’t be afraid to really crank up the lighting intensity if the viewing conditions call for it. Many people mistakenly believe that running their halogen scope at high brightness levels will reduce the life of the halogen lamp, when in fact, the opposite is true. Thanks to the halogen cycle, halogen lamps last longer when operated at high output.
Should I Get a Scope with Köhler Illumination?
If you intend to use your scope for photomicrography and are already planning to pay more for an upgraded model with a trinocular head and plan objectives, then do yourself a favor and find one that also includes Köhler illumination. The addition of Köhler illumination to a scope requires hardware not needed for conventional source-focused illumination, and this of course adds some cost, but in most cases the benefits are well worth the additional expense. Köhler illumination provides much better lighting uniformity than can be achieved with conventional source-focused illumination (aka Critical or Nelsonian illumination), and although you might be hard pressed to see the non-uniformity of source-focused illumination during routine visual observations, the difference will be much more noticeable in your photomicrographs.
Köhler illumination solves the problem of lighting non-uniformity by projecting perfectly defocused light onto the specimen plane, whereas conventional source-focused illumination projects a perfectly focused, and therefore visible, image of the light source (be that an LED emitter or a lamp filament) onto the specimen. Manufacturers of scopes using source-focused illumination almost always include a frosted glass diffuser in the lighting path to improve the lighting uniformity, but the results still do not match those achievable with Köhler illumination. An additional benefit of Köhler illumination is that it provides higher image contrast than conventional source-focused illumination, since the field diaphragm, which is present only in Köhler-equipped scopes, allows you to restrict the illumination to only the area of the specimen visible to the objective in use, thereby preventing glare from light that would otherwise fall outside the direct field of view of the objective lens. The design of the illumination system of scopes not equipped with a field diaphragm, by necessity, must illuminate the entire field visible to the lowest power objective (i.e. the objective with the widest field) on the scope, and when using the higher power objectives in these scopes, glare from the light beyond the visible field of the objective (and therefore beyond the field stop of the eyepieces) can cause a noticeable reduction in image contrast.