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Many microscopes utilize backlight illumination compared to traditional direct light illumination because the latter usually over-saturates the object under inspection. A specific type of backlight illumination used in microscopy applications is Koehler illumination. In Koehler illumination, incident light from an illumination source, such as a light bulb, floods the object under inspection with light from behind (Figure 2). It employs two convex lenses: the collector lens and the condenser lens. It is designed to provide bright and even illumination on the object plane and on the image plane where the image produced from the objective is then reimaged through the eyepiece. This is important because it ensures the user is not imaging the filament of the light bulb. Since backlight illumination floods the object with light from behind, it is also referred to as brightfield illumination.

The perfect time to use a polarising filter in Iceland is when the lighting is bright or intense – for example, during the middle of the day in any season. The brightness of daylight can mean that the colours of a scene are at risk of being washed out, so using a polariser for your landscape photography will result in deeper, more saturated colours, particularly if you are photographing the luscious green and earthy tones of the Icelandic Highlands.

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As you are learning to use your polarising filter, make sure to practice under different conditions with reflections, water and the sky. Take a number of photographs of the same scene at varying levels of polarisation, from zero to maximum, in order to compare the effects. The differences should be dramatic.

By using a polarising filter, you can dramatically improve your landscape photography to create images that are vibrant, dynamic and which stand out from the rest.

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Another situation in which you might consider using a polariser is to add definition to bright clouds. This technique can really make the clouds seem to pop out of the image. You can also use a polarising filter to deepen the blueness of the sky and to minimise haze from fog or temperature inversion. This will result in a much clearer photo with enhanced details.

Put simply, it reduces the amount of reflected light that goes through your lens and into your camera’s image sensor, cutting out the haze while allowing your camera to capture a clearer scene. This effect is unique to a polarising filter and is not something that can be replicated in post-processing, so to make the most of it, you’d best start learning how to use a polarising filter in-field!

Linear polarising filters are designed to be used with manual focus film cameras. While they can polarise light more dramatically than a circular polariser, they can throw off your autofocus system and even cause metering issues. Use of a linear polarising filter may also cause dimming and colour shifts through the viewfinder of your DSLR, though the final image that you capture won’t be affected in any way. It just means that taking photographs will be a little bit more difficult, as what you see through your LCD may not be true to how it’s actually been captured.

When placed in front of the camera lens, a polarising filter has the power to transform a flat, lifeless image into one with deep vibrancy and contrast. It works by darkening bright blue skies, increasing the saturation of foliage such as green Icelandic moss, and even allows you to manage reflections or reduce glare from the surface of water.

Also, think about how you can use one at places like Jökulsárlón Glacier Lagoon, Lake Mývatn, Mount Vestrahorn and the almighty Goðafoss. At Jökulsárlón, you can use the polariser to bring out the colours and contrast in the pieces of ice floating about in the lagoon, whereas Vestrahorn is a great place to experiment with reflections in the water. You can either cut out the reflections altogether or use the polariser to create a mirrored effect.

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When viewing fluid materials such as bacteria, cell cultures, blood, etc, it is necessary to use a coverslip in order to protect the object under inspection and microscope components from contamination. A coverslip, or glass microscope slide, changes the way light refracts from the object into the objective. As a result, the objective needs to make proper optical corrections to produce the best quality image. This is why objectives denote a range of coverslip thicknesses for which they are optimized. Typically, this is listed after the infinity symbol (which denotes that an objective is an infinite conjugate, or infinity corrected design) and ranges from zero (no coverslip correction) to 0.17mm.

Most microscope objective specifications are listed on the body of the objective itself: the objective design/standard, magnification, numerical aperture, working distance, lens to image distance, and cover slip thickness correction. Figure 7 shows how to read microscope objective specifications. Since the specifications are located directly on the body of the objective, it is easy to know exactly what one has, a very important fact when incorporating multiple objectives into an application. Any remaining specifications, such as focal length, FOV, and design wavelength, can easily be calculated or found in the specifications provided by the vendor or manufacturer.

There are two main types of polarising filters on the market – those which are warming and those that are neutral in tone. A warming polariser is excellent for shooting hazy scenes. It adds a subtle yellowish or reddish tone, which cuts through the haze to create clearer photos with a more natural appearance.

On the other hand, a neutral polariser will produce a slightly cooler tone. These kinds of polarisers allow you to maintain colour fidelity whilst highlighting the natural tones and colours of the scene.

Reflective objectives utilize a reflective, or mirror-based design. They are often overlooked in comparison to their refractive counterparts, though they can correct for many issues present in the latter. Reflective objectives consist of a primary and secondary mirror system (Figure 6) to magnify and relay the image of the object under inspection. Edmund Optics® utilizes the popular Schwarzschild design, though other designs are available. Since light is reflected by metallic surfaces and not refracted by glass surfaces, reflective objectives do not suffer from the same aberrations as refractive objectives and, thus, do not need the additional designs to compensate for these aberrations. Reflective objectives can produce higher light efficiency as well as better resolving power for fine detail imaging because the system is primarily dependent upon the mirror coating instead of upon the glass substrate being used. Another benefit of reflective objectives is the possibility of working deeper into either the ultra-violet (UV) or infrared (IR) spectral regions due to the use of mirrors compared to conventional refractive optics.

If the objective follows a simple microscope standard (such as DIN or JIS) then it is listed on the body to show what required specifications must be present within the system. Most compound microscopes employ the Deutsche Industrie Norm, or DIN, standard. The DIN standard has a 160mm distance from the objective flange to the eyepiece flange (Figure 8). The other available standard is the Japanese Industrial Standard, or JIS. The JIS standard has a 170mm distance from objective flange to eyepiece flange (Figure 9). Paying attention to these two distances is necessary when choosing the proper objective and eyepiece in order to make sure that the image projected from the former is properly imaged through the latter. Though the image distances are different for DIN and JIS, there is no difference in optical performance; they are equal in quality. Similarly, each standard utilizes the same RMS mounting thread of 0.7965" x 36TPI.

Another situation in which you might want to avoid using a polarising filter in Iceland is when the light has intense colour, such as during the Midnight Sun season or during sunrise or sunset.

Although you can turn the polariser in a direction that enhances the colours of the rainbow, the filter can sometimes create a ripple or unrealistic effect, especially with a wide angle lens as we mentioned before. Turned the other way, the polariser can minimise the visibility of the rainbow, making it seem dull and less vibrant. So to keep rainbows colourful and looking great in all of your photographs, experiment by having the polarising filter both on and off, as this is a situation in which the decision to use a polariser can swing either way.

Eyepieces and objectives both have magnification that each contribute to the overall system magnification. Magnification is usually denoted by an X next to a numeric value. Most objectives contain a colored band around the entire circumference of the body that indicates their magnification (Figure 7). For example, a yellow band denotes a 10X magnification.

In general, it is also not a good idea to use a polarising filter when shooting images that you will stitch together later on to create panoramas. As each image will have a different polarising effect, the final picture won’t have the polarisation applied evenly across the frame. This means that you can end up with increased banding in the sky that is difficult to fix in post-processing. So to keep your sky clear and coloured evenly, remove the polariser when you are taking panoramas.

Objectives allow microscopes to provide magnified, real images and are, perhaps, the most complex component in a microscope system because of their multi-element design. Objectives are available with magnifications ranging from 2X – 200X. They are classified into two main categories: the traditional refractive type and reflective. Each category is further divided into types: finite conjugate and infinite conjugate (infinity corrected). In order to choose the correct objective, it is important to know the benefits of one category and type from another.

Of course, if you want to highlight wet surfaces or reflections, then you will also probably want to remove your polarising filter.

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In order to understand how the components of a microscope can be integrated with various optical, imaging, and photonics products, consider the following optical microscopy applications: fluorescence microscopy and laser ablation. Each utilizes its own unique setup in order to work with components from a microscope.

With darkfield illumination, direct rays of light are not sent into the objective but instead strike the object at an oblique angle. It is important to keep in mind that these rays still illuminate the object in the object plane. The resultant darkfield illumination image produces high-contrast between the transparent object and the light source. When used in a microscopy setup, darkfield illumination produces a light source that forms an inverted cone of light blocking the central rays of light but still allowing the oblique rays to light the object. Figure 3 illustrates a sample darkfield illumination setup where the hollow cone of light is the numerical aperture of the objective. By comparison, no rays are blocked in a brightfield illumination setup. The design of darkfield illumination forces the light to illuminate the object under inspection, but not to enter the optical system, making it better for a transparent object.

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Next, you’ll want to adjust the filter in small increments, which will allow you to see how the alterations to the filter’s position can change the overall scene. A circular polarising filter has a ring that you can rotate. To use it, simply twist the ring to activate the polarising effect. Make sure to look at your image through the viewfinder or LCD as you are turning the filter ring. You will notice that as you rotate the polariser, the level of glare and saturation in your photo will change.

Once you become comfortable with using a polarising filter, you will find that they are useful even when there is not sky, water or reflections in your image. Keep reading, as we’ll explain a little bit below about when you can use a polarising filter in Iceland to create photos that really pop!

Aside from these scenarios, you can use a polarising filter when you are trying to achieve a longer shutter speed, as these filters will naturally reduce light entering your lens by a few stops. This can help you to get that gorgeous dreamy effect on running water, like in waterfalls.

A fluorophore (or fluorescent dye) is used to mark proteins, tissues, and cells for examination or study. Fluorophores can absorb light of one wavelength and emit (fluoresce) light of another wavelength. In a typical fluorescence microscopy setup three filters are used: an excitation filter, an emission filter and a dichroic filter. Each fluorophore has a specific absorption or excitation wavelength band, the excitation filter will transmit only that specific range of wavelengths. The fluorophore, once excited, will emit a different range of wavelengths. The emission filter transmits only the emission wavelengths. A dichroic filter that is specifically designed to reflect the emission wavelengths and transmit the excitation wavelengths is used to separate the excitation and emission channels. Figure 13 illustrates a typical fluorescence imaging setup. For additional information on fluorescence microscopy, view Fluorophores and Optical Filters for Fluorescence Microscopy.

A third type of illumination used in microscopy is epi-Illumination. Epi-illumination produces light above the objective. As a result, the objective and epi-illumination source substitute for a Koehler illumination setup. Using the objective for a large section of the illumination makes epi-illumination very compact – a major benefit of this design. Figure 4 illustrates an epi-illumination setup that is used frequently in fluorescence applications. For more information on fluorescence microscopy, view Fluorophores and Optical Filters for Fluorescence Microscopy.

Furthermore, polarisers don’t tend to work that well with extreme wide-angle lenses. This is because wide-angle or fisheye lenses can cover an extraordinarily wide field of view. Therefore, if a lot of your frame includes the sky, then there’s a good chance that you’ll end up with bands of lighter and darker shades of blue.

Illumination within a microscope is just as important as selecting the proper eyepiece or objective. It is crucial to choose the correct illumination in order to obtain the most conclusive results. Before deciding on the type of illumination setup to work with, consider the application setup, object under inspection, and desired results.

If you have multiple lenses with different filter sizes, just purchase a circular polarising filter that fits the lens with the largest thread. Then you’ll be able to use a step-down ring so that the filter fits the thread of your other lenses. If you are affixing the polariser to a filter holder, then you won’t have this issue as you will be able to use it across all of your lenses, provided that you have adapters to fit the filter holder onto lenses with different threads.

Bear in mind that a polarising filter will reduce the amount of light reaching your camera’s sensor by as much as two or three f-stops, so you should avoid using one in low light situations. This includes during blue hour, when shooting at night, or when photographing the Aurora Borealis. If you want to reduce reflections in dim light or to maximise the clouds at sunset, then you will need to make adjustments by choosing a slower shutter speed, mounting your camera on a tripod, using a lower f-stop, or physically adding more light to the scene.

Although polarising filters can make your photos all the more brilliant, there are also times when they will be not very much help at all, or will make your photo look worse. Adding an extra layer of glass to your lens increases the chances of lens flare.

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The most commonly used category of objectives is refractive. In a refractive design light passing through the system is refracted, or bent, by the optical elements. Each optical element is typically anti-reflection coated to reduce back reflections and improve overall light throughput. Refractive objectives are often used in machine vision applications that require resolution of extremely fine details. There are multiple refractive objective designs each utilizing different optical configurations. The designs can range from two elements in basic achromatic objectives (an achromatic lens and a meniscus lens) to fifteen elements in plan-apochromatic objectives (Figure 5). Plan-apochromatic objectives are the most complex, high-end objective design with chromatic and flat field correction done within the objective itself.

An important thing to note is that once your filter is attached, it is best to always set your focus manually before rotating the polariser. This is because the front ring of the lens that the polariser is attached to can rotate while you are focusing, which can result in an accidental change in the polarising effect.

In a finite conjugate optical design, light from a source (not at infinity) is focused down to a spot (Figure 11). In the case of a microscope, the image of the object under inspection is magnified and projected onto the eyepiece, or sensor if using a camera. The particular distance through the system is characterized by either the DIN or JIS standard; all finite conjugate microscopes are either one of these two standards. These types of objectives account for the majority of basic microscopes. Finite conjugate designs are used in applications where cost and ease of design are major concerns.

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Plan, also known as planar, semi-plan, semi-planar, or microplan, objectives correct for field curvature. Field curvature is a type of aberration present when the off-axis image cannot be brought to focus in a flat image plane, resulting in a blurred image as it deviates from the optical axis. Figure 10 illustrates field flatness measured radially from the center in achromatic, semi-plan, and plan objective designs. Achromatic objectives have a flat field in the center 65% of the image. Plan objectives correct best overall and display better than 90% of the field flat and in focus. Semi-plan objectives are intermediate to the other two types with 80% of the field appearing flat.

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When the lighting is colourful or golden, removing the polariser can sometimes help you to take a better shot. It allows the colour to be reflected off wet surfaces, meaning that the colour in the atmosphere will be enhanced and your photo will appear to be more dramatic.

The polarising effect of your filter is at its greatest when you shoot at a 90-degree angle to the sun, so it is perfect to use for compositions with side-lighting. When the sun is behind you at 180-degrees, the polarising effect will be non-existent. Between these two points, the amount of polarisation will vary. You will know once you have achieved maximum polarisation, as reflections will disappear and the contrast between the blue sky and structure of the clouds will be more prominent.

DIN and JIS have historically been used when considering a classic compound microscope. Some microscope manufacturers prefer to list the tube lens length by the optical properties instead of the mechanical. For a DIN standard objective, this changes the tube lens length to 150mm because the eyepiece is imaging the intermediate image plane (Figure 8). Lastly, there is a dimension typically listed for objectives to allow the user to consistently know what length it is: the parfocal distance (PD). The parfocal distance is the distance from the flange of the objective to the object under inspection. For DIN objectives this distance is a standard 45mm and for JIS is it 36mm (Figures 8 and 9).

Fluorite objectives further correct for aberrations using advanced glass types containing fluorspar or other synthetic substitutes. Just like achromatic objectives, fluorite objectives are designed to correct for chromatic aberrations for red and blue wavelengths. However, fluorite objectives are designed to correct for spherical aberration at two or three wavelengths instead of just green, typically have a higher NA, and feature a better resolving power and higher degree of contrast.

Field of view or FOV is the area of the object that is imaged by a microscope system. The size of the FOV is determined by the objective magnification. When using an eyepiece-objective system, the FOV from the objective is magnified by the eyepiece for viewing. In a camera-objective system, that FOV is relayed onto a camera sensor. The sensor on a camera is rectangular and therefore can only image a portion of the full circular FOV from the objective. In contrast, the retina in your eye can image a circular area and captures the full FOV. This is why the FOV produced by a camera-microscope system is typically slightly smaller than that of an eyepiece-microscope system. Equations 3 and 4 can be used to calculate the FOV in the aforementioned systems. In Equations 3 and 4, $ \small{H_{\small{\text{Camera Sensor}}}} $ is the sensor size of the camera and $ \small{H_{\small{\text{Eyepiece Field Stop}}}} $ is the field stop of the eyepiece.

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Knowledge Center/ Application Notes/ Microscopy Application Notes/ Understanding Microscopes and Objectives

A polarising filter is an essential item in the camera kit of any serious landscape photographer who wants to take beautiful photos of Iceland. This useful tool changes the way that your camera sees and captures light, particularly in relation to reflections and glare.

Polarising filters are not cheap, so when purchasing one, keep quality at the forefront of your mind and remember that the sharpest photos are produced by high quality glass. In short, you should put the same amount of effort into purchasing a polarising filter as you would into buying a new lens.

A compound microscope is one that contains multiple lens elements. It works similar to a simple magnifier which utilizes a single lens to magnify a small object in order for the human eye to discern its details. With a simple magnifier, the object is placed within the focal length of the single lens. This produces a magnified, virtual image. With a microscope, a relay lens system replaces the single lens; an objective and an eyepiece work in tandem to project the image of the object onto the eye, or a sensor – depending upon the application. There are two parts to a microscope that increase the overall system magnification: the objective and the eyepiece. The objective, located closest to the object, relays a real image of the object to the eyepiece. This part of the microscope is needed to produce the base magnification. The eyepiece, located closest to the eye or sensor, projects and magnifies this real image and yields a virtual image of the object. Eyepieces typically produce an additional 10X magnification, but this can vary from 1X – 30X. Figure 1 illustrates the components of a compound microscope. Additionally, Equation 1 demonstrates how to calculate the overall system magnification. In Equation 1, m is magnification.

Experimenting with a polarising filter in these locations may also allow you to create an interesting foreground where there wasn’t one before, by cutting out glare in the water, letting your viewer see into the depths and to catch a glimpse of what lurks beneath.

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In an infinite conjugate, or infinity corrected, optical design, light from a source placed at infinity is focused down to a small spot. In an objective, the spot is the object under inspection and infinity points toward the eyepiece, or sensor if using a camera (Figure 12). This type of modern design utilizes an additional tube lens between the object and eyepiece in order to produce an image. Though this design is much more complicated than its finite conjugate counterpart, it allows for the introduction of optical components such as filters, polarizers, and beamsplitters into the optical path. As a result, additional image analysis and extrapolation can be performed in complex systems. For example, adding a filter between the objective and the tube lens allows one to view specific wavelengths of light or to block unwanted wavelengths that would otherwise interfere with the setup. Fluorescence microscopy applications utilize this type of design. Another benefit of using an infinite conjugate design is the ability to vary magnification according to specific application needs. Since the objective magnification is the ratio of the tube lens focal length $ ( \small{f_{\small{\text{Tube Lens}}}} ) $ to the objective focal length $ (\small{f_{\small{\text{Objective}}}} ) $ (Equation 5), increasing or decreasing the tube lens focal length changes the objective magnification. Typically, the tube lens is an achromatic lens with a focal length of 200mm, but other focal lengths can be substituted as well, thereby customizing a microscope system's total magnification. If an objective is infinite conjugate, there will be an infinity symbol located on the body of the objective.

Microscope and objectives are complex optical systems with many uses. They are no longer used solely for biological setups (e.g. looking at cheek cells in an introductory biology class); rather, they can be used to study the emission wavelength of a flourophore, to analyze a 5μm defect on a machined part, to oversee the ablation of material off a base, and within a host of other applications in the optics, imaging, and photonics industries. Understanding the importance of each constituent part of a microscope and their specifications enables any user to choose the best system and achieve the best results.

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To get you started using a polarising filter, let’s explore the ins and outs of what they are, including what they actually do, the different effects that they have, which one to choose for landscape photos, how to use them, as well as when it is suitable to use one in Iceland to enhance your own photography.

A microscope is an optical device used to image an object onto the human eye or a video device. The earliest microscopes, consisting of two elements, simply produced a larger image of an object under inspection than what the human eye could observe. The design has evolved over the microscope's history to now incorporate multiple lenses, filters, polarizers, beamsplitters, sensors, illumination sources, and a host of other components. To understand these complex optical devices, consider a microscope's components, key concepts and specifications, and applications.

When microscopes were first invented, eyepieces played a major role in their design since they were the only means to actually see the object under inspection. Today, analog or digital cameras are used to project an image of the object onto a monitor or a screen. Microscope eyepieces generally consist of a field lens and an eye lens, though multiple designs exist that each creates a larger field of view (FOV) than a single element design. For a simple guide on selecting the right design, view Choosing the Correct Eyepiece.

Brightfield illumination requires a change in opacity throughout the object. Without this change, the illumination creates a dark blur around the object. The end result is an image of relative contrast between parts of the object and the light source. In most cases, unless the object is extremely transparent, the resulting image allows the user to see each part of the object with some clarity or resolution. In cases where an object's transparency makes it difficult to distinguish features using brightfield illumination, darkfield illumination can be used.

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Now that you’ve got the lowdown on using a polarising filter for your landscape photography, don’t be afraid to get it out and to use it in Iceland! This essential tool will ensure that you create dramatic photos that are vibrant and dynamic, enhancing your portfolio with truly great images.

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The final scenario in which you might consider removing your polariser is when you are shooting rainbows at iconic locations, such as the waterfalls in Iceland. Waterfalls are known for creating beautiful rainbows on sunny days, as they produce so much spray! If you are lucky, you may even catch a double rainbow.

On the other hand, circular polarisers have been developed for use with autofocus lenses and complex electronics. When purchasing one, make sure that it says ‘circular polarising filter’ on it, otherwise you might end up with one that is linear.

About the author:  Serena Dzenis is a landscape photographer based in Iceland. You can find more of her work on her website or by following her on Facebook and Instagram.

Using a polarising filter on our tours to the Snæfellsnes Peninsula, the Westfjords, Vatnajökull National Park or the Highlands will help you to reduce glare on the surface of the water at some very iconic locations.

The Numerical Aperture (NA) of an objective is a function of the focal length and the entrance pupil diameter. Large NA objectives sometimes require the use of immersion oils between the object under inspection and the front of the objective. This is because the highest NA that can be achieved within air is an NA of 1 (corresponding to 90° angle of light). To get a larger angle and increase the amount of light entering the objective (Equation 2), it is necessary to use immersion oil (index of refraction typically = 1.5) to change the refractive index between the object and the objective. High NA objectives in conjunction with immersion oil are a simple alternative to changing objectives, a move that may be costly.

Two common uses of lasers are to (1) heat material onto a base or (2) to ablate material off of a base. Laser ablation systems require microscope components because of the precision beam manipulation (i.e. focusing, bending, scattering reduction, etc) required. A laser ablation setup typically contains custom optics, rather than off-the-shelf components, with the laser precisely designed into the system (Figure 14). The laser is oriented in an epi-illumination design to utilize the microscope objective's ability to focus light at the object plane, and to produce extremely small spot sizes with minimal aberrations. Also, an eyepiece allows the user to see where the laser is located and to make sure everything is working properly. Filters are necessary to block the laser from causing damage to the user's eye. Laser ablation setups are used in medical and biological applications because they offer higher precision than conventional surgical methods.

The quality of an objective and eyepiece determine how well the system performs. In addition to choosing the magnification and complexity of the design, understanding correct quality correction is extremely important when deciding on the type of objective to use. Quality correction (i.e. achromatic, apochromatic, plan, semi-plan) is denoted on the objective itself to allow the user to easily see the design of the objective in question. There are typically two levels of chromatic aberration correction: achromatic and apochromatic. Achromatic objectives are among the simplest and least expensive of objectives. They are designed to correct for chromatic aberration in the red and blue wavelengths, in addition to being corrected for spherical aberration in a green wavelength. Limited correction for chromatic aberration and lack of a flat FOV reduce objective performance. Apochromatic objectives, by contrast, provide higher precision and are chromatically corrected for red, blue, and yellow. They also provide spherical aberration correction for a broad spectral range and generally have a long working distance given the extremely high numerical apertures (NA) that this optical design offers. Apochromatic objectives are ideal for white light applications, whereas achromatic objectives are best suited for monochromatic. Both objective designs, however, suffer significantly from distortion and field curvature, which worsen as objective magnification increases. Therefore, it is always important to focus on the complete system performance, rather than just objective performance alone.