In the usual configuration a positive lens is made of crown glass. This glass has a low dispersion, spreading the colors out to a lesser extent. Then a flint glass element, with a higher dispersion, is used as a negative element. Chosen correctly, this results in the red and blue wavelengths coinciding at the same focus. The green is still slightly different, but the difference is relatively small. (In the diagram above, the lens on the left is the positive crown, the second lens is the negative flint.)

The first step in determining the FOV is to find the field number on the objective lens. This number gives the diameter of the microscope field, assuming no other eyepieces or magnifiers are added.

In a triplet, red, green, and blue light is brought to a single focus. This leaves other colors such as violet out of focus, so there is residual color as in an achromat, but the amount is significantly less. The greater number of lens elements also means better correction of other aberrations such as coma and spherochromatism.

Once the field number and magnifying power of each lens in use are known, a simple equation exists to help observers find the total field size of the image in view. If the objective lens is the only magnifier in use, the field size is equal to the field number divided by the objective magnification of the lens, or:

This section details the optical design and inherent aberrations of refracting telescopes. For a more basic overview of this design please see theRefractors page. For a review of the optical design terms, see the Optical Aberrations and Optical Design sections.

Depth offocusdefinition

The field number can be easily located on a microscope, as it's typically written on the side of each lens, next to its magnifying power. For example, if the numbers on a microscope's objective lenses are 10X/15, 40X/20, and 100X/25, the objective magnifications would be 10X, 40X, and 100X, while the field numbers would be 15, 20, and 25, respectively.

Defined as the minimum distance at which an observer can distinguish two adjacent points, resolution increases with magnification. That means especially detailed objects may require the superior resolution that comes with higher magnification, even if the FOV is reduced.

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Alternate designs are possible, such as the Petzval four-element design popularized by TeleVue. This design uses a long-focal-ratio (over f/10), low-dispersion doublet in combination with two elements farther down the optical path which speed up the focal ratio and flatten the field. The high number of degrees of freedom allow this design to be very well corrected, even at speeds as fast as f/5. This makes it an ideal photographic instrument.

The field observation window also decreases quite rapidly when auxiliary lenses are added. A single eyepiece with a magnification power of 10X reduces the field of view by a factor of 10, so observers adding extra lenses should know just how much width they'll lose.

Depth offieldmicroscopeformula

FS = FN/(ME*MO), where  FS = field size, FN = field number, ME = eyepiece magnification, and MO = objective magnification

If an eyepiece is added, the field size is equal to the field number divided by the total magnifying power of the entire assembly. Since magnifying power compounds geometrically, the total magnifying power is the product of the microscope eyepiece and microscope objective lens. In this case, the field size can be expressed as:

The tradeoff between FOV and magnification means that observers have several factors to take into consideration when they choose how much power is right for their project. Two of these factors are resolution and depth of field.

The lower the dispersion of the crown glass, the less the residual chromatic aberration (the difference between green and red/blue focus). Many telescopes now use extra-low dispersion (ED) glasses to minimize chromatic aberration. These are sometimes called apochromatic refractors, but do not adhere to the strict definition of an apochromat as outlined below.

The 3D images that stereo microscopes provide also require consideration of depth of field, or DOF. The calculations aren't as straightforward as those for FOV, but a higher magnification is directly proportional to a greater depth of field. As with resolution, if observers need to distinguish between multiple layers of their specimen, they may need higher magnification at the expense of FOV. However, stereo microscopes typically have a larger default FOV than conventional compound microscopes, so the tradeoff is less with this type of scope.

Depth of view microscope definitionpdf

FOV is simply a measurement of the diameter of the image that the viewer can see. It's typically expressed as the diameter in millimeters of the observer's field of vision, and is used to quantify the size of the object under inspection.

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From the life sciences and medicine to semiconductors and jewelry, microscopes have become essential tools for many industries. They bring details to light that the naked eye could never see, but the insights viewers gain can be skewed if they don't know how large the image is. That's what field of view expresses.

The two lenses together make up the telescope objective. There are two basic configurations for the objective: cemented and air-spaced. A cemented objective has the same curvature on the inside surfaces of the lenses (the second and third optical surfaces). This eliminates possible degrees of freedom from the design. This type of objective will normally have more coma than an air-spaced objective and is usually only seen on designs with very small apertures and slow focal ratios. An air-spaced objective gives two additional degrees of freedom (another surface curvature and the thickness of the air space), allowing better control of coma.

Whether you're examining a cell with an optical microscope or soldering components onto a printed circuit board (PCB), knowing the size of the object in view helps you complete your work more effectively. For example, knowing the size of a microscope field can help the observer determine how large an organelle within a microorganism is, and could also give other insights about important image features.

Some residual color exists in the wavelengths outside the visual range. This may result in some violet halos with less-than-ideal designs when used for imaging, but visually a true apochromat should show no significant color aberrations. Coma is significantly reduced, in the neighborhood of 3 to 4 times less than a comparable achromat. Field curvature in a triplet is comparable to that in a similar achromat. Field flatteners are usually available to eliminate curvature for photographic applications. Designs like the Petzval eliminate field curvature.

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Fieldof view microscope definition

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In the above equation, note that field of view is inversely proportional to magnification power; a lower total magnification allows for a larger field, and a higher magnification results in a smaller field. This is to be expected, because the closer an observer zooms into an object, the smaller the window they view.

Calculating a microscope’s field of view differs somewhat for compound and stereo microscopes. Both involve a calculation based on the total magnification of the microscope and a parameter on the lens, but stereo microscopes have other factors to consider.

A simple lens focuses different wavelengths of light to different points. Specifically, red light is focused farther from the lens than blue light. This is longitudinal chromatic aberration. If focused for the green light to which the human eye is most sensitive, the red and blue colors will appear out of focus, yielding a blurry image.

The primary aberration in an achromatic refractor is longitudinal chromatic aberration. Lateral color tends not to be an issue. Both coma and off-axis astigmatism are present and can be problematic for wide-field imaging. An air-spaced objective has less coma than a cemented one. Doublet objectives suffer from slight spherochromatism, where spherical aberration is eliminated in green light but there is slight undercorrection in red and a slight overcorrection in blue. Field curvature exists, so wide field photography may be difficult without a corrector lens, although the limiting factor with an achromat is almost always the chromatic aberration. As with most telescopes, distortion is negligible.

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A measurement of the diameter of the visible image, field of view (FOV) clarifies how much of the object the viewer is actually observing. This allows the viewer to analyze the specimen more clearly, perform any needed operations with greater precision, and have a better understanding of what they see. In this article, we'll show you how to calculate field of view, what it is, and some FOV differences for compound and stereo microscopes. Here's a deeper look.

As an example, a microscope having an objective lens with 100X magnification power and a field number of 25 mm would have a field size of 25 mm/100, or 0.25 mm. Similarly, if the same microscope added an eyepiece having a magnification power of 20X, the field size would be 25 mm/(100*20), or 0.0125 mm. Because the field size can be quite small, instead of expressing field size in terms of millimeters, some observers may multiply their value by 1,000 to convert their units to micrometers

The strict definition of apochromatism is having three wavelengths of light focusing to the same point. This normally requires a third lens element in the objective. The normal configuration is a positive, low-dispersion crown, combined with two high-dispersion flints, one negative and one positive. The lenses can be cemented, air-spaced, or a combination thereof. Oil-spaced triplet lenses are sometimes seen as well, where the air space is filled with an immersion oil. Spaced triplets, like spaced doublets, have more degrees of freedom, allowing better correction of aberrations.