horus meaning - definition of horus by Mnemonic Dictionary - horus definition

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The principle behind this optical engineering feat is well known. The artangement of lens elements and groups bend the light path so the nodal point of the lens is moved towards the front of the lens. This known as a "reverse telephoto" design and with some ultra-wide angle lenses the nodal pount can actually be in front of the first element.

Telephotos are far easier to design than wide angle lenses and have a far simpler arrangement of elements and groups in the optical path. Wide angle zooms are even more complicated and harder to design.

Not great. You have the flange distance right but not the focal length. What's the "converging point" of a lens? The focal length is defined as the distance between the second principal point of a lens and the place where parallel rays would focus. The second principal point is a somewhat abstruse mathematical concept. It's the place where, if you put a pinhole, it would generate an image the same size that the lens is making.

During the first half of the twentieth century, darkfield microscopy (both compound and stereo) had a very strong following and a great deal of effort was expended in optimizing darkfield condenser systems and illuminators. This intense interest slowly began to fade with the emergence of more advanced contrast-enhancing techniques such as phase contrast, differential interference contrast, and Hoffman modulation contrast. Recently, new stereomicroscope illumination techniques, such as Nikon's oblique coherent contrast, which dramatically increase the contrast of transparent specimens, are being introduced and will ultimately probably displace a significant amount of interest in darkfield stereomicroscopy. However, a renewed interest in transmitted darkfield microscopy has arisen due to its advantage when used in combination with fluorescence microscopy.

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great. But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

Darkfield microscopy is a simple and popular method for rendering unstained and transparent specimens clearly visible. Good candidates for darkfield observation often have refractive indices very close in value to that of their surroundings and are difficult to image with conventional brightfield techniques. As an example, small aquatic organisms, oocytes, and cells in tissue culture have a refractive index ranging from 1.2 to 1.4, resulting in a negligible optical difference from the surrounding aqueous medium (refractive index of 1.3). These and similar specimens are ideal candidates for observation with darkfield illumination techniques.

The article you linked seemed to say that it's about bending the light to give a desired field of view equal and it has nothing to do with how far the center of the lens is to the sensor.

Specimens imaged under the proper conditions of darkfield illumination are quite spectacular in appearance (try, for instance, a drop of fresh blood). Often specimens containing very low inherent contrast in brightfield microscopy are readily observable in darkfield, and this type of illumination is ideal for revealing outlines, edges, boundaries, and refractive index gradients. Unfortunately, darkfield illumination is less useful for revealing internal details. Other types of specimens, including many that have been stained with dyes, also respond well to illumination under darkfield conditions. These include plant and tree thin sections (stained and unstained), diatoms, radiolarians, fossils, bone sections, embryos, and hair (both human and animal).

I know how simple optics work but I'll admit I don't know how the more complicated lenses work. But I am interested to learn so I will read the responses here from more knowledgeable people and the links they have posted.

The 10.5mm is not overly long but some other lenses carry this to more lengths. Take a look at the size of the new Sigma 18-35 ART.

Each SMZ stereo microscope from Nikon features industry-leading optics, large zoom ranges, and wide fields of view for bridging macro- to micro-imaging.

It is also difficult to design a very long focal length lens that is physically short. Telephoto lenses are shorter than their focal length, but not much less than half. A 400mm pancake lens is not practical.

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The digital images in Figure 4 illustrate the effects of darkfield and brightfield illumination on fibers in whole mount specimens prepared using Canada balsam and a microscope slide and coverslip. Figure 4(a) and 4(b) compare nylon fibers under conditions of brightfield (Figure 4(a)) and darkfield (Figure 4(b)) illumination. The fibers imaged with brightfield are seriously lacking in contrast and minute details are difficult to distinguish against the white background. In contrast, when the fibers are illuminated with darkfield techniques (Nikon SMZ1500 with a toroidal mirror illuminator), internal fiber detail is discernable to a higher degree and depth of field emphasis becomes more pronounced. A situation where fibers have too much contrast in brightfield is presented in Figure 4(c) for pineapple fibers, which are not transparent and almost opaque when visualized under brightfield illumination. Viewing the same pineapple fiber specimen with darkfield illumination reveals far more intricate detail (Figure 4(d)) and exposes longitudinal splits in the fibers that are not apparent in the brightfield image.

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Bright field dark field

As noted in Roger's web page, a lens with a given focal length can be very near or very far from the sensor. Optics is rather complex and counterintuitive in many ways.

Nevertheless: wide angle lenses on Single Lens Reflex cameras (including DSLRS) are by necessity reverse telephoto designs. For cameras that do not use a reflex viewing system ( mirrorless digital, rangefinders and view cameras) they do not need to be.

Darkfield microscopy is still an excellent tool for both biological and medical investigations. The technique can be effectively utilized to view a wide spectrum of biomedical and industrial specimens and can often reveal details that are not visible with other illumination methodology.

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I seemed to be under the impression that for example a 135mm lens had to be 135mm (about 5.3 inches) from the center point of the lens to the sensor. That's what basic diagrams like below seemed to imply.

Darkfield observation in stereomicroscopy requires a specialized stand containing a reflection mirror and light-shielding plate to direct an inverted hollow cone of illumination towards the specimen at oblique angles. The principal elements of darkfield illumination are the same for both stereomicroscopes and more conventional compound microscopes, which often are equipped with complex multi-lens condenser systems or condensers having specialized internal mirrors containing reflecting surfaces oriented at specific geometries.

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As I noted earlier, the focal length is the distance between the second principal plane and the focal plane when the lens is focusing a plane wave (like light coming from infinity). No nodes anywhere.

In practice, however, there is a limit. If you design a very short focal length lens for an SLR with a large flange distance, the lens will be very complicated and expensive.

If the rear of the objective in a stereomicroscope operating in darkfield illumination is viewed using a Bertrand lens or eyepiece telescope, it will appear filled with light. The faint diffracted light is reconstituted into a visible image at the plane of the eyepiece diaphragm with its contrast reversed to produce a bright image on a dark background. Because darkfield microscopy eliminates the bright, undiffracted zeroth order light, this form of illumination is very wasteful of light and thus demands a high intensity illumination source. Stereomicroscope illumination stands that are equipped for darkfield illumination take this factor into account, and high-intensity tungsten halogen bulbs are provided to produce sufficient light flux for the purpose.

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Ideal candidates for darkfield illumination in stereomicroscopy include minute living aquatic organisms, diatoms, small insects, bone, fibers, hair, unstained bacteria, yeast, and protozoa. Non-biological specimens include minerals, chemical crystals, colloidal particles, inclusions and porosity in glass, ceramics, polymer thin sections, and refractive index gradients. Care should be taken in preparing specimens for darkfield microscopy because features that lie above and below the plane of focus, especially fingerprints, dust, fibers, and cleaning residue, can also scatter light and contribute to image degradation. Specimen thickness and microscope slide thickness are also very important and, in general, a thin specimen is desirable to eliminate the possibility of diffraction artifacts that can interfere with image formation.

But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

Prime lenses 50mm and up to 100mm or more are usually simple lenses with most glass elements in the vicinity of the focal length. Obviously this doesn't work with short focal length lenses. Nikon's 10.5mm is going to be more complex, no way to get the glass elements that close to the sensor.

The nodal point is not relevant to this discussion. You measure focal length from the second principal point (or plane). The second principal point in a retrofocus lens is typically behind the rear element.

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I suggest that people who don't know about optics shouldn't talk about lens design. It's not all that important unless you're trying to design a lens. I design lenses so I have some background in the terms and concepts used in that discipline. But I still have to look them up.

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Sorry, i did not have the rigjt language at hand. I was trying to express in insufficient language the same idea you are expressing, and you clearly have more experience in this area than I do.

The configuration presented in Figure 1 illustrates a Nikon SMZ1500 stereomicroscope equipped with an advanced stand containing provisions for both brightfield and darkfield illumination through a clear glass stage mounted on the top of the stand. Also depicted is a digital Internet camera system (Nikon Dn100) capable of transferring images collected by the microscope to remote observers. Details of the darkfield illumination mechanism are discussed below. Many current Nikon stereomicroscopes are also compatible with Darkfield illumination.

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The stereomicroscope illustrated in Figure 1 produces an oblique cone of illumination using a specially-designed seven-sided toroidal mirror (Figure 2) that substantially reduces the stray light entering the large common main objective front lens. The toroidal mirror operates in a manner similar to high numerical aperture reflecting darkfield condensers that are equipped with internal mirror surfaces having a variety of curvature geometries.

As noted in Roger's web page, a lens with a given focal length can be very near or very far from the sensor. Optics is rather complex and counterintuitive in many ways.

One of the most popular darkfield condenser designs, heavily utilized for high magnification compound microscopy prior to the emergence of phase contrast, is the paraboloid condenser, which has a curved and mirrored cardioidal internal surface. Illuminating light passes through the condenser and reflects from a single surface that is made from a paraboloid truncated by a light stop oriented perpendicular to the condenser and microscope optical axis. This system is free from spherical, chromatic, and coma aberrations and produces a sharply focused cone of illumination for the specimen from all azimuths. Although the stereomicroscope toroidal mirror design illustrated in Figures 2 and 3 does not operate with the sophistication and precision of the paraboloid condenser, it is far more effective for illuminating specimens in darkfield than conventional reflection mirrors that have a cylindrical geometry. The diagrams in Figure 3 compare the toroidal mirror design with a more conventional cylindrical mirror found in a majority of stereomicroscopes. In addition to providing more even illumination from all azimuths, the toroidal condenser design substantially reduces the amount of stray light entering the objective front lens, which leads to a significant enhancement of contrast between the specimen and background.

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Illumination of specimens by darkfield requires blocking out of the central light rays along the optical axis of the microscope, which ordinarily pass through and around (surrounding) the specimen. Blocking these light rays allows only those oblique rays originating at large angles to strike the specimen positioned on the microscope stage. In a compound microscope equipped with a simple condenser system, the condenser (Abbe-style) top lens is spherically concave, enabling light rays emerging from the surface in all azimuths to form an inverted hollow cone of illumination having an apex centered in the specimen plane. If no specimen is present on the stage, and the numerical aperture of the condenser is greater than that of the objective, the oblique rays cross and miss entering the objective front lens because of their obliquity. The field of view will appear dark.

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A number of aftermarket products are currently available for retrofitting stereomicroscopes with transmitted darkfield illumination. In addition, many of the microscope manufacturers offer illumination accessories that can be conveniently utilized to achieve darkfield conditions for their stereo systems. Typical aftermarket darkfield illuminators are presented in Figures 5 and 6. The design illustrated in Figure 5 utilizes a fiber optic ring light to provide illumination for a specially crafted stage that contains an internal mirror system and an opaque light stop. Light from the ring light illuminator is reflected from the internal cylindrical mirror with the central (zeroth order) rays being blocked by the light stop to form an inverted cone of illumination. Specimens are placed directly onto a glass plate resting above the stage aperture and can then be visualized with darkfield illumination. The ring light is equipped with an external light source that contains a voltage supply and a high-intensity tungsten-halogen lamp. Another darkfield condenser design, which also contains provisions for brightfield illumination, is presented in Figure 6. This condenser system utilizes a slider to rotate between brightfield and darkfield illumination and also contains a light source coupled to the condenser by a fiber optic bundle.

Dark field microscopy

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Feb 23, 2014 — In conventional bright field illumination, your specimen is lit from a central light source (you can read more about bright field microscopy in ...

Well, there are simple lenses, like magnifying glasses or spectacle lenses. A simple lens focuses light from infinity just about one focal length from the lens. But if you put simple lenses together you can change all that because each lens in a train of lenses works with the image produced by the lens in front of it. So the focus can be anywhere or, for that matter, nowhere.

The article you linked seemed to say that it's about bending the light to give a desired field of view equal and it has nothing to do with how far the center of the lens is to the sensor.

In darkfield microscopy, contrast is greatly enhanced by the superposition of a brightly shining specimen on a dark background. Blocking of zeroth order light rays by an opaque stop enables only higher order light rays to bathe the specimen with illumination. Highly oblique light rays, diffracted by the specimen and yielding first, second, and higher diffracted orders at the rear focal plane of the objective, proceed onto the image plane where they interfere with one another to produce an image of the specimen.

Explore how mirror shape affects the amount of light entering the objective in darkfield stereoscopic microscopy. This tutorial demonstrates lightpath differences between conventional and toroidal mirrors.

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Specimens that have smooth reflective surfaces produce darkfield images that are primarily due to reflection of light into the objective. In situations where the specimen refractive index is different from the surrounding medium or where refractive index gradients occur (as in the edge of a membrane), light is refracted by the specimen. Both instances of reflection and refraction produce relatively small angular changes in the direction of light, enabling some rays to enter the objective. In contrast, some light striking the specimen is also diffracted, producing a 180-degree arc of light that passes through the entire numerical aperture range of the objective. The resolving power of the objective is the same in darkfield illumination as that achieved under brightfield conditions, but the optical character of the image (as discussed above) is not as accurately reproduced.

When a transparent specimen is placed on the glass microscope stage and observed under darkfield illumination, the oblique light rays cross the specimen and are diffracted, reflected, and/or refracted by optical discontinuities (such as the cell membrane, nucleus, and internal organelles) allowing these faint rays to enter the objective. The specimen then appears bright on an otherwise black background. In terms of Fourier optics, darkfield illumination removes the zeroth order (unscattered light) from the diffraction pattern formed at the rear focal plane of the objective. This results in an image formed exclusively from higher order diffraction intensities scattered by the specimen, and is also responsible for the main limitation of darkfield observation. Because the image is composed entirely from scattered light from the specimen, it is rich in glare and can even be distorted to varying degrees, so it cannot be considered a faithful geometrical reproduction of the specimen.

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horus meaning - definition of horus by Mnemonic Dictionary - horus definition

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Dark field microscopy

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