Since indirect backlight illumination is generally more effective than direct illumination, most microscopes do not include an internal light source.  Instead, they rely on daylight or on background illumination such as a lightbulb. In brightfield illumination, also known as Koehler illumination, two convex lenses saturate the specimen with external light admitted from behind. These two lenses, the collector lens and condenser lens, work together to provide a bright, even, and constant light throughout the system: on the image plane as well as on the object plane.  This system of illumination is used in many compound microscopes, including student microscopes and those found in many research labs.

where θ is the maximum 1/2 acceptance ray angle of the objective, and n is the index of refraction of the immersion medium. Figure 2 shows the ray angle θ of an infinity-corrected objective.

At Shanghai Optics, we design and manufacture custom objectives and imaging systems to support our customers’ needs in many industries, including medical, biomedical, machine version, scientific research, and metrology, etc. Taking the client’s budget and precision requirements into consideration, our experienced engineering team ensure that each design can be manufactured at a reasonable cost and the optical performance is being met based on fabrication, assembly, and alignment tolerance analysis.

Go up a Half Stop or a Third of a Stop... The "in between" partial stops are even less consistent mathematically than the whole stops. Between the largest apertures, you may have approximate half steps — f/1.4 to f/1.8 is not actually a half stop, but its close enough. At smaller apertures, rough 1/3 stops are common — between f/11 and f/16, you have f/13 – f/14. The steps are not actual 1/3 steps, but, again, its close enough. Photographers adjust freely up and down in fractional stops to improve their exposure — but they are usually not calculating aperture or percent change in illumination. They are just adding or removing exposure to lighten or darken the image — aperture adjustments become rather intuitive eventually, rather than being precisely calculated. Its more fun that way, after all.

Stops  f-stops and common fractional stops:  f/1 – 1.2 – f/1.4  - 1.8  - f/2.0 – 2.2 -  f/2.8 – 3.2 — 3.5 – f/4.0 – 4.5 –  5.0 — f/5.6 – 6.3 – 7.1 – f/8.0 – 9 – f/11 – 13 – 14 – f/16 –18 – 20 – f/22 – 28 – f/32 — f/45 — f/64 — f/90 — f/128

Add a Stop or Closing Down a Stop For the photographer, every time the f-stop increases by a factor of 1.4, the aperture is half as large — half as much light exposes the sensor. That is, if you change the f-stop from f/1.4 to f/2.0, you've added "a stop" — you've reduced your exposure by half. If you change from f/2.0 to f/2.8, you've added a stop. From f/5.6 to f/8 you've added a stop. Etc.

Microscope Objectives or Objective lenses are in many ways the heart of the microscope, and are typically mounted on a rotating nosepiece or turret to enable easy selection. Many microscopes will be equipped with a scanning objective (4x), a low power objective (10x), a high power objective (40x), and perhaps even an oil immersion objective lens.

That’s not fully explained…I know. The "up" and "down" jargon gets frankly confusing because f-stops increasing means aperture decreasing. Its just one of the hazards of photography.

Each microscope objective is itself a complex assembly of lenses, and besides contributing to the magnification, it is the objective lens which determines the resolution power of the microscope. An objective lens can also provide optical aberration corrections.  A reflective objective, for instance, includes two mirrors within the assembly. These mirrors can focus laser light as well as provide chromatic corrections.

The optical aberration correction determines the optical performance of an objective lens and plays a central role in the image quality and measurement accuracy of imaging or microscopy systems. According to the degrees of the aberration corrections, objective lenses are generally classified into five basic types: Achromat, Plan Achromat, Plan Fluorite (Plan Semi-Apochromat), Plan Apochromat, and Super Apochromat.

The ocular lens, or eyepiece, is also an optical assembly rather than a single lens, but it is typically more simple than the objective. Often it is composed of two lenses: a field lens and an eye lens. The design of the ocular lens determines the field of view of the microscope, as well as contributing to the total magnification of the system.

Field of View is the area of the object that can be imaged by a microscopy system. The size of the field of view is determined by the objective magnification or focal length of the tube lens for an infinite-corrected objective. In a camera system, the field of view of the objective is related to the sensor size.

"Diagram of decreasing apertures, that is, increasing f-numbers, in full-stop increments; each aperture has half the light gathering area of the previous one." (WikiP)

F-stops

A microscope objective is an important component of a microscopy or imaging system for a range of science research, biological, industrial, and general lab applications.. An objective lens determines the basic performance of an optical microscope or imaging systems and is designed for various performance needs and applications. It is located closest to the object and is an important component in imaging an object onto the human eye or an image sensor.

When purchasing lenses, the larger the aperture (the smaller f-stop) the better – and it will cost more. Most intro consumer zoom lenses have a maximum aperture around 4.0 or 4.5. A professional zoom lens may have a maximum aperture of 2.8 — such a lens will cost several times more than the consumer zoom lens. A decent 50mm prime less may have a maximum aperture of 1.8. A professional quality 50mm prime may have an aperture of 1.2 and cost a thousand dollars more than the f/1.8 lens. The size of the actual glass lenses in the f1.2 lens are much larger and are much more costly to manufacture.

Most objectives are designed to image specimens with air as the medium between the objective and the cover glass. However, for achieving higher working numerical apertures, some objectives are designed to image the specimen through another medium such as special oil with a refractive index of 1.51.

A simple magnifier (magnifying glass), works when the object to be examined is situated within focal length of the magnifier lens, enabling larger virtual image is produced. This type of magnifier is very limited in both resolution and magnification. A compound microscope, on the other hand, uses a relay lens system instead of the single lens, and since each lens component can contribute magnifying power, the result is greatly increased capability.

f-number

Add Two Stops... From f/5.6 to f/11 is two stops — your exposure has 1/4 as much light as before. Note that the number 11 is roughly 2x 5. From f/11 to f/22 is up two stops — 1/4 as much light. And, obviously, 22 is 2x 11.

Many objectives are designed to be used with a cover glass. Using an incorrect coverslip thickness can greatly reduce the optical performance of a microscopy system.

aperture中文

Two major lens components—the objective lens and the ocular lens, or eyepiece—work together to project the image of the specimen onto a sensor. This may be the human eye or a digital sensor, depending on the microscope setup.

At large apertures the f-stop is a small number and each successive stop is only a slightly larger number. As apertures get smaller, the f-stop number gets larger and the numeric gap between f-stops gets larger.

Pupilaperture

A microscope is a special optical device designed to magnify the image of an object. Depending on the type of microscope, it may project the image either onto a human eye or onto a recording or video device. As an example, consider the photographs of cells that can be found in a science textbook. These photographs have all been taken by a specialized microscope, and may be called micrographs.

f-stop vsaperture

Where do they get those particular numbers?!  f-stop values have to do with the area inside of a circle – the circle within your open aperture that allows light into the camera.  Note that every-other number is different by a factor of about 1.4. Note that 2.0 is approximately 1.4x1.4    So 1.414 is the square root of 2, which is related to the fact that when you open your aperture by a full stop, you double the amount of light allowed in.

This podcast discusses of the essence of aperture – one of the core technical concepts that controls exposure.  In the podcast, he compares aperture to turning on a faucet to fill a bathtub; the wider the aperture, the less time it takes to “fill” an image with light; the more open the faucet, the less time it takes to fill the tub with water. Filling the tub is like exposing an image until it becomes fully white. To be more accurate, each and every pixel in your camera's photo-sensor is such a bathtub or a bucket — if no water (light) gets in, that pixel is black. If the bucket fills with water (light), that pixel is white. Also, how aperture is related to depth of field is discussed (the depth of the region that is in focus).  The wider the aperture, the narrower your depth of field.  The smaller the aperture, the deeper the field of focus.

The ocular lens, located at the top of a standard microscope and close to the sensor (receiving eye) receives the real image from the ocular lens, magnifies the image received and relays a virtual image to the sensor. While most eyepieces magnify 10x, there are some which provide no magnification and others which magnify as much as 30x. The magnification power of the microscope can be calculated by multiplying the magnification power of the eyepiece, or ocular lens, by the magnification power of the objective lens. For example, an objective lens with a magnification of 10x used in combination with a standard eyepiece (magnification 10x) would project an image of the specimen magnified 100x.

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"An example of the use of f-numbers in photography is the sunny 16 rule: an approximately correct exposure will be obtained on a sunny day by using an aperture of f/16 and the shutter speed closest to the reciprocal of the ISO speed of the film; for example, using ISO 200 film, an aperture of f/16 and a shutter speed of 1/200 second. The f-number may then be adjusted downwards for situations with lower light." (WikiP)

(in the diagram, above, the max diameter of the lens is "D", which is similiar to the max aperture of the lens — though the aperture will always be somewhat smaller than the lens.)

Objective lenses can be classified based on the objective construction, field of use, microscopy method, performance (optical aberration corrections), and magnification. Many microscope objective manufacturers offer a wide range of objective designs, which provide various degrees of optical aberration corrections for supporting different needs. Mirrors or reflective elements are used in objective lenses for the applications that requires chromatic aberration over board spectral ranges. Most traditional microscopy systems use refractive objectives such as achromatic objectives (the cheaper objectives) for laboratory microscope applications and plan apochromats (expensive objectives) for biological and science research microscope applications.

f-stop是什么

While the simplest of microscopes is simply a magnifying glass with a single lens, compound microscopes used today are highly complex devices with a carefully designed series of lenses, filters, polarizers, beamsplitters, sensors, and perhaps even illumination sources. The exact combination of optical components used will depend on the application of the microscope; the wavelength of light with which it is intended to be used, and the resolution and magnification required in the final image.

Aperture

Note that each lens has its own max and min aperture (lowest f-stop and highest f-stop).  These aperture variations are one of the key distinctions in lens design, lens capability and, of course, lens price.

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Aperture is measured, or described in "f-stops". The smaller the "f-stop", the larger the aperture. Its odd at first, but that's the system.

Objectives are complex multi-element lenses. For any given application, careful consideration of the optical parameters and specifications is necessary. In many cases, custom-designed objective assemblies provide the best-fit solution for meeting all the requirements of a specialized application.  Custom parameters may include antireflection coatings, chromatic focus shift, working distance, image quality (MTF and spot size), lens mount, glass window thickness, and field of view, among others.

The parfocal length is the distance between the objective mounting plane and the specimen / object. This is another specification that can often vary by manufacturer.

Go Down a Stop or Open Up a Stop And, conversly, every time the f-stop decreases by a factor of 1.4, the aperture is twice as large — twice as much light exposes the sensor. If you change your aperture from f/16 to f/11, you've gone down a stop and double the amount of light reaching the sensor... your image will be lighter. If you adjust your apertuure from f/5.6 to f/4, you've opened up by a full stop; twice as much light gets through. From f/22 to f/16 is down a stop and double the aperture.

Aperture is one of the three corners of the "exposure triangle" — the three settings or controls that your camera offers to allow you control over exposure. (for the other two, see Shutter Speed and ISO)

Aperture literally means Opening or Hole. A camera's aperture is a hole of varying size that influences how much light exposes an image — light passes through the opening in order to get to the image sensor (or film). The bigger the hole (the larger the aperture), the more light reaches the sensor. The smaller the hole (the smaller the aperture), the less light reaches the sensor.

F-stop

Every full stop, the aperture doubles.    "aperture doubles" means that the lens allows twice as much light in. So, if I change aperture from f/11 to f/8, f/8 allows twice the light in as f/11. When the aperture is closed by a full stop (via a higher f-stop),  ½ of the light is allowed in.  So if a switch from f/2.0  to f/2.8, f/2.8 lets ½ of the light in that f/2.0 lets in.

Important specifications are marked on the barrel of the objective, so students or researchers can easily identify the properties of an objective and determine the optical performance and working conditions for proper use. Figure 1 shows a diagram of an objective lens. A detailed discussion of the objection specifications is provided below.

Since the objective is closest to the specimen being examined, it will relay a real image to the ocular lens. While doing so, it contributes a base magnification of anywhere from 4x (for a scanning objective lens, typically used to provide an overview of a sample) to 100x (for oil immersion objectives).

Magnification is one important parameter. Magnification is usually denoted by an X next to a numeric value.  Objectives are available in a range of magnifications from 2X to 200X.

"Full Stops" are noted in bold, above. If you think these numbers are strange... be glad we traditionally round them off to only two significant digits. It could be worse. Strictly speaking, f2.8 should be f2.823427...

For keeping the objective at the proper position, there are mounting threads on almost all objectives. Commonly used mounting threads include RMS, M25 x 0.75, M26X 0.706, M32 x 0.75.