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Yamaji , the to-be president of Canon published a paper on zoom lenses that I think still stands the test of time today. Ironically, Yamaji later became a proponent of keeping technical information in-house in favour of disclosing them as patents, and good papers such like his own became scarce to this day. Optically speaking, zoom lenses can be traced back to the converter.
We see the wide converter and teleconverter lenses that convert the focal length of a system as an attachment. The Fuji X series comes to mind, which is a fixed lens digital camera that has both a wide angle conversion lens and a tele conversion lens to change the focal length of the lens. This is actually a simple way to think about the zoom lens, because the zoom lens has a number of fixed focal length groups in the lens system that change position to give a different magnification.
Just like if you were to move a magnifying lens from your eye front to back, you'd see the image get bigger or smaller. The optical compensation zoom system has one linearly moving part and essentially does not use a cam system for movement. The focus is within the depth of field by using clever positioning of the lenses. The mechanical compensation zoom system has at least two moving parts with a cam system, usually a variator lens and a compensator lens.
That means that a lot of photography lenses are not zoom lenses, they are varifocal lenses, because we usually need to refocus the lens. For a video system, staying in focus while zooming is critical as the picture would go out of focus as we zoom in or out. This makes zoom lenses sorry, varifocal lenses for photography have a higher degree of freedom, and more compact and large zoom ratios can be made.
Some zoom systems have a compensator that doubles as a focusing lens, and some zoom systems have two variators with one variator doubling as a compensator. But even if a lens group doubles functions, the components are still there. It may be a little tricky to decode these lenses, but if we know the properties of each component it is easy to find them in the system. A wide-angle zoom lens shares a lot of properties with the retrofocus lens. And not coincidentally, wide-range focal length zoom shares the properties of the retrofocus lens on the wide angle side, and also shares the properties of the telephoto lens on the tele side.
In most cases, a challenging retrofocus or telephoto lens is more difficult than the single focal length of a wide angle zoom or a tele zoom. The difficulty of the zoom lens is the balance between all of the focal lengths, and often the individual focal lengths are not too difficult on their own. Making the front element the focusing lens, or making the lens an inner focusing system.
Figure out which system you need for the focus system. The front focus is most straightforward and robust. It is also the optically logistic way to focus a lens. However, if the front element is large, this can mean moving a large chunk of glass to focus, which is harder to do mechanically, requires more precision, and can cause focus backlash overshooting the focus point. Also, some focusing systems do not work well when the distance of the object to the front of the lens cannot change. Some systems require that the total length of the lens from the first element to the last element not change.
Which brings us to inner focus, which seems like the perfect solution, since the focus is in the inner location of the lens.
However, inner focus is harder to achieve optically, since the focus must work for all zoom positions, and the focus is likely on the same pathway as the variator or the compensator. Gaussian brackets themselves are a mathematical tool, and when used for lens systems they can be extremely powerful. Gauss came up with the mathematical algorithm, and it can be used to evaluate the focal length, magnification, and back focus of a lens system with some given parameters like the thickness or lens power.
The Gaussian brackets were introduced in optics by M. Herzerberger in , and then expanded upon by K. Tanaka in the s. Gaussian brackets are too involved to get into here, but defining a zoom system with Gaussian brackets and is useful. I might have a lot of fun trying to explain this someday. Maybe an epic post on zoom lenses? If we can answer these questions, then we can get a better idea of the requirements of the system. The 4 group zoom system is the most basic, and has the four components — focus, variator, compensator, master lenses clearly defined in the system.
Close observation of the lenses shows that the variator covers the most distance, and is close to the focusing lens for the wide end and moves ever so close to the master lens in the tele end. The variator is usually a negative lens group, mainly because the zoom system can be made more compact. The master lens group usually has an afocal component , and the first-second-third group is the zooming part. The 2 group zoom system has two main forms, a negative-positive group and a positive-negative group form.
The picture below is a negative-positive zoom since the first group has negative power and the second group has positive power. The concept is much like the retrofocus lens. For a wide-angle zoom, a 2 group zoom lens form is typical, with a negative focusing lens, and a positive variator.
The other 2 group zoom , a positive first group and a negative second group is mostly used for compact zoom lenses. The 3 group zoom system is a variation of the 2 group zoom, where the first group is split in two for a higher degree of freedom.
By using a 3 group zoom, it is possible to achieve a larger zoom range in a compact form. Multi-group zooms are zooms that many groups that move for zooming. These multi-group zooms typically are used for a large zoom range. As far as modern zoom lenses are concerned, just go to any famous lens maker Canon , Nikon , Olympus , Sigma , FujiFilm , Panasonic , Cooke , etc and you will find plenty of zoom lenses.
Nowadays, there are so many variations that it is hard to keep up. But the essence of lens design is still there, and the four components in focusing lens, variator, compensator, and the master lens are used. I think I have an epic post on zoom lens design in me, waiting to come out. I have too many ideas bouncing around in my head, so stay tuned while I try to figure things out.
How can I use those? Afocal systems alone may not dazzle us too much, but they are used in many situations, and making a system afocal or making a portion of the system afocal has many benefits like making the system simpler to understand, or to simpler to systematically build the optical system. A lens that has parallel incident rays and has parallel exit rays is called an afocal system. Simple enough, right?
Well, there are a few variations on how we achieve this. In an afocal system, the object is at infinity, and the focal length is also infinity. What to do? Well, the most typical way to express an afocal system is by magnitude, not focal length. The most general afocal system is made up of two lenses, and the focal point of the image of the first lens is placed at the focal point of the object of the second lens.
We can actually have a positive-negative lens combination and a negative-positive lens combination. The former, the positive-negative afocal system shrinks the beam width if it were a beam expander , while the negative-positive afocal system expands the beam. Since the focal length of afocal systems is infinity, we use the angular magnification as a unit of measure.
When the system is a positive lens and negative lens combination, the magnification is positive, while for a positive lens and positive lens system the angular magnification is negative. For both examples, we can see the parallel rays incoming to the front lens, and it is parallel going out, for all zoom positions. Below is a laser beam expander, and it takes a narrow beam and widens it significantly. If we look closely, it is the same as the above examples and the parallel rays incoming to the front lens, and it is parallel going out also.
And FYI, this afocal imaging system where the principal ray is parallel to the optical axis is a telecentric system. Afocal systems are used for a specific purpose, so instead of figuring out how to spec out an afocal system, it is more important to think about where an afocal system is useful within the lens design. If the rays on the optical axis to the marginal rays change angles, that means that the system induces spherical aberration.
In an actual optical lens design, unless we are using it with our eye, the simplest way to express an afocal system is with an imaging lens. Take the zoom lens above for example. We can see the parallel rays incoming to the front lens, and it is parallel going out, regardless of zoom position. In actual use, there is another group of lenses after the exiting parallel beams forming an image to a sensor.
A conversion lens that is mounted on the back of the lens system is a rear conversion lens or alternatively called a rear converter. Rear converters that lengthen the back focal length are common. Rear converters that shorten the back focal length did not exist for a long time, because it was not practical to shorten the back focal length to widen the focal length, we would much rather use a front converter for that.
For mirrorless camera systems, there are rear converters that take legacy lenses, widen the focal length, and by proxy shorten the back focal length. Thanks for the tip, Hans! When a rear conversion lens is mounted, the resulting F-number is the product of the magnification and the F-number of the master lens. Note that the F-number when using a front conversion lens does not change. Rear converter. Usually, the Petzval sum of the master lens is small, and this rear converter has a negative Petzval sum, so the resulting Petzval sum is negative as well.
Therefore, the positive lenses should have as low of an index of refraction as possible, and the negative lenses should have as high as an index of refraction as possible. The aberration correction of rear converter lenses is difficult since the rays usually pass above the optical axis, or most of the ray bundles pass through one side of the optical axis.
A lens system that has its focal point at either the entrance pupil or the exit pupil is called a telecentric system. For the former, a telecentric system with the focal point at the entrance pupil, the principal ray is parallel on the image side. This is an image side telecentric system. For the latter, a telecentric system with the focal point at the exit pupil, the principal ray is parallel on the object side. This is an object side telecentric system.
When both the object side and image side are telecentric, this is an afocal lens system. Therefore, the magnification is always constant, and the displacement or tilt of the object or the image does not affect the system.
Since the LSI fabrication process requires extreme precision the image side telecentricity is important. Systems that require optical fiber bundles, such as endoscope imaging require image side telecentricity. The fiber bundle requires that the rays are not oblique, since this will reduce the transmission. Therefore an image side telecentric optical system with parallel rays to the fiber bundle works nicely. Some projection systems have object side telecentricity.
This is useful since the object can move a bit and the resulting image will still be good. A microscope objective is designed with a very small object, so designing it with object side telecentricity is useful as well. Super LSI optics require extremely balanced optics that can withstand some minute displacements or tilt of the object plane and the image plane, and bi-telecentric lenses are very useful here.
Image side telecentric systems have zero chief ray angles for all fields. The easiest way to achieve object side telecentricity is to flip the image side telecentric system around. For optical design software settings, it is sometimes easier to design an image side telecentric system and flip it around.
This means that the object is placed at the focal point of the first positive lens group, and the image is placed at the imaging surface of the second positive lens group.
Usually, the same positive lenses are used, just opposing each other. Also, most tandem lens systems are the same scale in most cases. But if a tandem lens is telecentric, it is certainly bi-telecentric. Since tandem systems have parallel rays in between the two lens groups, there are a few characteristics:. To design a tandem system, we first design with the object at infinity, with the aperture stop in front of the lens, and make an image.
We then take this lens to duplicate it and flip it around the aperture stop. Since ancient times, mirrors were used as reflective optics. There is a hypothesis that Archmides used collective mirrors as a heat ray to burn ships with the sunrise. Since sir Isaac Newton , reflective telescopes were used to observe the stars. In Japan, for example, optics made its development via photographic lenses, so it was mainly refractive optics. In Physics, reflection can be explained in a line or two, but from a lens design standpoint, there are so many angles to look at lens design.
There are plane mirrors, spherical mirrors, aspherical mirrors, ellipsoidal mirrors, parabolic mirrors, hyperbolic mirrors, and toric mirrors. These mirrors can be further classified into convex shaped mirrors and concave shaped mirrors. The reflective surfaces are lapped and polished for high surface precision, and coated with reflective materials.
Aluminum is common, but some applications use silver, gold and other materials. As lens designers, we need to keep in mind the reflective properties of the material and make sure that it is usable in the wavelength range and the manufacturing. Not only Newtonian telescopes that use mirrors exclusively, but catadioptric systems use reflection and refraction in the optical system. Prisms and reflectors are used in laser applications, binocular system, and foldable optics as well.
In a reflective system, there can be one surface that reflects light twice, or reflects light to a refractive lens for the second time. In the lens design, we number the surfaces in the order that the ray hits the surface, so some surfaces will have two numbers associated with it, depending on the number of times the ray hits the surface.
Keeping all of the sign conventions consistent , a reflected ray will move through the system negatively, and subsequent surfaces that it hits will reverse in sign. Some reflective systems have a hole in the center of the mirror. We will still trace the rays near the optical axis for paraxial calculations. As far as the F-number, it is the ratio of the reflective area with the entire system. Also, reflective mirrors deprecate with time, as the reflection percentage can change with time, and dirt, fingerprints, dust, scratches all contribute to the reflection loss of the mirror.
If possible, the best way to solve this problem is to place the reflective surface behind a sheet of glass, as a means to protect the reflective surface.
This is called a Mangin mirror. Interestingly, the spherical aberration of a Mangin mirror is much smaller than a simple reflective mirror. Total internal reflection occurs when a ray passes through a higher index of refraction material to a lower index of refraction material. There is a critical angle at which this happens, and any angle larger than the critical angle continues to have internal reflection.
Pretty straight forward, when we are concerned with the chromatic aberration it is good to consider a reflective system. We have to be aware of the tolerances of the system and see if it makes sense as an overall system. Along with names like the Tessar and Sonnar, some of the most creative lens names belong to the symmetric wide angle lenses. Lens forms are:. On the other hand, Leica named their lenses after lens speed. The modern Leica lenses use Summarit for F2. Our eyes work a little differently than photographic lenses with sensors, so we have to take a different approach in thinking about the lens design form.
In particular, optical lenses for the eye are afocal since the eye itself has focus properties, whereas photographic lenses focus on a plane, either film in the old days or a digital sensor today. Optical systems that are made for direct viewing with our eyes, such as Loupes a simple magnifier , viewfinders, telescopes , microscopes , and many others, all share issues with the characteristics of the eye.
The angle between the center of the entrance pupil of the eye and the object, in terms of the principal ray. The size of what we see is proportional to the tangent of the angle u. This angle increases as the object is moved closer to the eye, and there is a limit to how close we can get this to our eyes, but it depends on person to person, and their nearpoint. Our eyes focus by moving the muscles around the lens to change the shape of the lens and increase the optical power.
There is contraction that makes the curvature of the lens larger and nearpoint is achieved. The eye continuously focuses on the retina so we see things in focus.
Although there are some differences from person to person, in general younger people can accomodate for a larger range between their nearpoint and farpoint, and this range gets shorter as we get older. Usually, we use the reciprocal of metre as the unit of measure of how far we can see. This is called the diopter. The distance of distinct vision is the closest nearpoint we can get while being able to see the details of the object.
Usually we use the distance -4 diopters for lens design, but for a camera viewfinder we may use -1 diopters instead. The magnification of a loupe or simple magnifier is the ratio between the tangent of the viewing angle of the object through the magnifier and the tangent of the viewing angle of the object without the magnifier. In exact optical ray path terms, the focal point of an eyepiece and the entrance pupil of our eye does not match up perfectly, but this is the equation used to get the numbers for magnification in general.
I think I have talked about a lot of different types of magnification like transverse magnification and longitudianl magnification, and even angular magnification, but this magnification is a little different. The other three magnifications are all conjugate systems, while the optical system for the eye is not a conjugate system.
Most eyepiece optics are afocal. The aberrations for an eyepiece and optics using an eyepiece allow about 3 minutes of arc in general. This means that a perfect eyepiece designed into the eyes have parallel rays, but we can afford less than 3 minutes of arc from parallel rays and still be fine, in most cases.
This is because it is said that the human eye has a resolution of one minute of arc , and for observational optical systems 3 minutes of arc is an acceptable compromise. Telescopes use a telescope objective and an eyepiece. Binoculars are optically similar to telescopes, but use two lens systems together and a Porro prism. Optical view finders and prisms for SLR camera objectives are optically similar to the microscope. They have the image of the film surface on a ground glass, and the eyepiece takes the image of the groundglass into our eye.
Lens assembly, 2. Mirror, 3. Focal-plane shutter, 4. Focusing screen, 6. Condensing lens, 7. Pentaprism, 8. For that matter, electronic view finders are also similar to microscope systems and OVF systems, since there is an LCD in the digital camera that shows the image on the sensor. This image on the LCD is then imaged to the eye, which is exactly the same thing. Riflescopes are a long-range finite to finite conjugates, and there are zooms as well.
And of course, normal eyeglasses whether they be near-sighted or far-sighted, use the same optical principles for lens design. When we design an eyepiece, it is often useful to flip the lens design around and trace the rays from infinity where our eye will eventually be to the lens, and the focal point is where the object will be. Set the first surface as the entrance pupil of the eye, and build the lenses from there. For a riflescope, where the kickback from the rifle is large after shooting, a longer eye relief of about 90mm might be more suitable.
I have some examples of typical eyepiece lens design forms below. As a preface, the focal lengths are all mm. Singlet eyepiece. Huygens eyepiece. The Huygens eyepiece is originally two plano-convex lenses with the convex side facing the objective lens. The first Huygens eyepieces were made of more simpler glass, like BK7. The objective side lens is called the field lens, and the eye side lens is called the eye lens.
Interestingly, the Huygens eyepiece has a virtual image in between the two lenses, so in a sense, the field lens can optically be part of the objective lens. It is possible to correct the transverse chromatic aberration very well. Later, the Huygens eyepiece used meniscus lenses, which slightly improves the performance. The most important point of the Huygens eyepiece is its ease in manufacturing, especially with two plano-convex lenses. Some disadvantages of the Huygens eyepiece is the longitudinal chromatic aberration and large field curvature.
The Ramsden eyepiece is two plano-convex lenses with the convex surface facing each other. There is a significant amount of chromatic aberration which does not make it ideal for telescopes. Compared to the Huygens eyepiece, the focal point of the Ramsden eyepiece is outside of the two lenses, so placing patterns like a scale of a scope is easier.
Kellner eyepiece. The Kellner eyepiece tried to solve the chromatic aberration problem by adding a doublet, and can be used for a wider field of view. Although there is significant astigmatism, field curvature, and distortion, it can still be used for a fairly wide field of view.
Plossl eyepiece. Plossl eyepieces are a low-cost solution because of the symmetrical shape, and we only have to design one doublet, flip one, and then put them together.
The cemented lenses are used for colour correction, and can be many variations of glass materials. Of the more classic eyepiece lens design forms, the Plossl is still used in many eyepieces today.
Abbe orthoscopic eyepiece. The Orthoscopic eyepiece, also called the Abbe eyepiece, since Abbe presented this lens design for a microscope eyepiece.
Although expensive to make due to the three-cemented lenses, it is very well corrected chromatically and has low distortion.
Erfle eyepiece. The Erfle eyepiece is a wide angle eyepiece that was first designed for binoculars but is also used for astronomical eyepieces. Bertele eyepiece. Bertele eyepiece wide. Astronomical eyepiece. Telescopes are one of the oldest lens design forms, a lot earlier than photographic lenses.
This is probably because film emulsion technology developed much later, and the camera obscura only had limited applications compared to a telescope. The telescope had scientific applications for studying distant objects in astronomy, and military applications as well. Johannes Kepler developed the Keplerian telescope, and it is a positive objective lens and positive eyepiece lens combination.
Since it is two positive lenses, the image is flipped around top-to-bottom and also left-to-right. This is okay as far as astronomical observation, but not practical for landscape viewing, especially in a military situation. Galileo Galilei developed a different configuration that solved the image flipping issue, he used a positive objective lens and a negative eyepiece lens. Although the image is upright in this configuration, there is no place we can place a physical aperture stop, since the exit pupil is a virtual image that is inside the lens.
The field of view of this configuration diminishes considerably. You can see that the pupil position play a great role in telescopes. Interestingly, the Galilean telescope is not invented by Galilei, but was filed for a patent a year earlier by Hans Lippershey and Jacob Metius independently from each other. Most of the telescope analysis in the rest of this chapter focuses on the Keplerian telescope, which is more complicated with a wide use case, and frankly more interesting than the Galiean telescope configuration Sorry Gallileo.
Along with the eyepiece, the telescope is also an important lens form that uses stops and pupils in order to work the way that they do. The typial telescope is shown below. If you remember, I talked about the achromatic doublet as a telescope objective. The telescope has two components to it, the objective and the eyepiece.
The objective creates an enlarged image of the object at infinity, and the eyepiece takes that intermediate image and creates a virtual image at infinity that we can see. To illustrate the system, the image above shows the object and virtual image at a finite distance. I think this is a better conceptual description. In order for the eyepiece to function properly, the focal plane of the eyepiece is matched to the focal plane of the objective lens.
The telescope is an objective lens combined with an eyepiece. These are two lenses with distinct focal lengths each. A similar configuration with different focal lengths is a microscope system, which enlarges a close-focus object with an eyepiece. The only optical difference with the telescope is the focal length of the objective lens. Some large range zoom systems have an intermediate image to help with the magnification.
In this case, the second lens is not an eyepiece, but a focus lens, and a similar type of logic with an intermediate image applies. Some projector systems create an intermediate image of the LCD, for example with one lens, and then project the intermediate image onto a screen with a projection lens, another similar optical system.
Most telescopes and binoculars have the entrance pupil on the objective lens, and the exit pupil is after the eyepiece.
This distance of the eyepiece to the entrance pupil is called the eye relief. The aperture stop is usually located at the intermediate image. The blue rays are the on-axis, and the green rays are at an angle. The blue rays focus at the intermediate focal plane which is the rear focal plane of the objective lens. This focal plane is also the front focal plane of the eyepiece, so the blue rays then become parallel rays after exiting the eyepiece lens. The green rays also focus at the intermediate focal plane which is the rear focal plane of the objective lens.
Again, this focal plane is also the front focal plane of the eyepiece, but this time the green rays exit the eyepiece at an angle, but also parallel. If you have a chance to look at a pair of binoculars or a telescope, try to look at the exit pupil of the lens.
You can see the exit pupil floating in the air if you get the angle right. I do a lot of practice lens designs during my work time. I have a lens design mentor who is an advisor to our company, and he regularly gives me problems to solve.
Doing these exercises has really upped my lens design game. The telescope system I was tasked to design was not a immensly difficult design, and I proceeded to design the system in two parts: The objective lens, and the eyepiece. I improved the objective the best I could, which is not difficult because it is a long focal length system and all I had to worry about was the spherical aberration and the chromatic aberration.
And a little bit of coma. I then proceeded to design the eyepiece, as I tested a few eyepiece design forms that was the best. I made sure the chromatic aberration, spherical aberration, and field curvature were as small as possible. This is typical for commercial eyepices and telescope objectives, because we have to have multiple objectives that work with multiple eyepieces, mix and match for different magnifications.
All I had to do now was to dock the two systems together, right? I flipped the eyepiece design around in the software this is a useful tool when we design lenses in the opposite configuration of actual use , and added it to the objective lens. Since both systems had the minimum error possible, I was convinced that I had a winner of a design. There was just no way in my mind, because both were maximally optimized! We can leave a little bit of chromatic aberration in the objective lens and then correct it with the eyepiece.
Sometimes that becomes an unnecessary constraint on our lens design. In commercial systems with interchangeable lenses, it may make sense to make two perfect lens designs and just stick them together. But for a custom system, it may be better to balance the performance as a system. Even for a commercial system, if two or three objectives share the same aberration properties, it would be sensible to make the eyepieces match those aberrations to counteract the aberrations and make a nice image overall.
Another lesson in lens design all part of the journey. Binoculars fix the image flipping issue with a porro prism to make the virtual image upright to the eye. By Antilived — Own work based on: Binocular-optics. In modern binoculars there are a lot of innovations to make the prism as small as possible, for better handling in use.
Photographic objectives are generally reduction optical systems, but in contrast, microscopes are enlargement optical systems. In that sense, it may seem like they are opposites of each other, but the method to approach lens design is fundamentally the same.
Historically, people would try to enlarge an object with a single lens, usually a loupe. The shorter the focal length, the larger the magnification, but at a certain point, the distance between the object and the lens working distance will be extremely close, and therefore the distance between the lens and your eye eye relief will also be small. In the early s, someone had the bright idea the inventor is disputed to place an objective lens very close to the object, but instead of sticking the eye close to the image, another magnifier lens was used to look at the image the first magnifier produced.
This other magnifier is called the eyepiece. This entire system is called a compound microscope. The invention of the compound microscope is brilliant. The objective lens magnifies the object to an image, but the eyepiece gives an additional magnification so that the image can be enlarged with high magnification while the distance from the object to the eye is far away. For a microscope, if we think about the back focal point of the objective lens being the stop, we can place the image where the light source is going to be.
The chief ray goes through the stop, so the chief ray at the object is parallel to the optical axis. The entrance pupil is at infinity, and we call this situation telecentric.
If the chief rays are not perpendicular to the image parallel to the optical axis , the defocus blur during focusing will be different across the screen, an is a problem for usability. The NA is the numerical aperture and represents how much light enters the system, and is usually represented with the equation. NA and F-number are interchangeable, but from my experience, I usually like to define NA as an object side parameter and F-number as an image side parameter.
Not exclusively, though. The microscope is an objective lens combined with an eyepiece. A similar configuration with different focal lengths is a telescope system, which reduces a far object like a star in the sky with an eyepiece. The only optical difference with the microscope is the focal length of the objective lens.
The field of view of a microscope is small, so the aberrations that we concentrate on are the spherical aberration , the coma , and the chromatic aberration. Although the focal length is short, since the microscope is an enlargement optical system the longitudinal aberrations become pronounced.
It is good practice to design the lens as if it is a reduction optical system, in the opposite direction of actual use. Like for the image above, the object to be magnified clearly should be at the right.
But if I were using optical design software, the rays are going left to right, so in fact I have the setup that has starts opposite of what it should. Regardless of the configuration, it is important to eliminate the aberration. Also, the final design should be based on wavefront aberration rather than ray aberration, although ray aberration is faster and therefore better to use in the early stages of the lens design.
As the magnification increases, the lens power increases, and the magnification affects the transverse chromatic aberration. In general, the transverse chromatic aberration is difficult to correct with the objective lens alone, so Abbe took the method of leaving the opposite transverse chromatic aberration in the eyepiece to cancel out the aberrations as an entire system.
This is called the compensation method. The compensation method does have a disadvantage in that the objective lens needs to be paired with the eyepiece if they correct each other.
The obvious thing to do was to make a chromatic aberration free method. Also, if there are any oils or other liquids used for immersion, then we have to take into account the index of refraction of those liquids as well. The line thickness of the circles is kept thin to avoid interference in reading.
If you make the bubble outlines very thick the software might read all bubble edges as response because ultimately software is made for reading dark areas. DO NOT change location of index points as exported by the software. In any method of printing: Laser, Offset or Photocopies, ensure that 4 Black Index Points on 4 corners of sheet are properly printed. If the index points are not properly printed, they will not be scanned properly and it will give trouble while reading.
Print to centre of page so that there is sufficient white space outside 4 index points in all 4 corners. In case if sheet includes a cutting or tearing margin, it should be sufficiently far from the index points. The font should be as small and thin as possible. The sheets using tick marks or thin impression responses should be preferably printed in 2 colours.
In case of Offset printing, the corner black index points are printed in both colours, such that the exactly overlap and there is no relative displacement i. Stamped numbers cannot be read. OCR text for optical character recognition should be printed in font size in black colour only. Keep the sheet holding plastic brackets in the OMR sheet scanner pocket close to prevent from unnecessary tilting, straying or tangling in the OMR sheet scanner.
Keep the scanning area wide enough so that the corner 4 index points are properly scanned and there is sufficient white space outside them. DO NOT increase brightness or contrast unless guided. Scan at default settings. It will also work only file size will be larger. These instructions are for sheets with 2 colour printing or sheets with photograph pasted on them.
Send Query. Set the printer page to A4.
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