Pincushiondistortion

If taking a lot of photos from far away subjects is important to you, be sure that the camera you buy has an optical zoom.

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Distortion is an aberration commonly seen in stereoscopic microscopes, and is manifested by changes in the shape of an image rather than the sharpness or color spectrum. The two most prevalent types of distortion, positive and negative (often termed pincushion and barrel, respectively), can often be present in very sharp images that are otherwise corrected for spherical, chromatic, comatic, and astigmatic aberrations. In this case, the true geometry of an object is no longer maintained in the image.

Normal lenses have a focal length of around 50mm; it resembles the view of the human eye and creates a natural view — unlike wide-angle that distort and telephoto that compresses the view. These lenses usually have a very low f-number, which makes them perfect for photographing in low light conditions. Back in the days this was the standard lens everybody had, often a 50mm prime lens with an aperture of f/1–f/1. The fact that they were so widely used might be one of the reasons why they have now been left behind for most beginners and amateurs — they are just seen as too boring.

where M is the axial lateral magnification and M(l) is the off-axis magnification at the image plane. If the lateral magnification increases proportionally with the off-axis distance of the object, distortion is positive, producing a pincushion effect (Figure 1). In this instance, each image point is displaced radially outward from the center, with the peripheral image points being translocated the greatest distance. Alternatively, when magnification is decreased with the off-axis object distance, distortion is negative and a barrel aberration is observed. Barrel distortion corresponds to a situation where the transverse magnification decreases with axial distance and each image point moves radially towards the center of the image.

Macro photography is close-up photography. Macro is a word that has been severely abused lately, every photograph of an insect or flower is not macro, and many people seems to have missed the point of what macro is supposed to be. True macro photography is at the scale of 1: or greater — this means that the object you’re photographing should be the same size or larger on the sensor. Most macro lenses have a focal length between 50mm and 200mm, and they usually have a large maximum aperture (low f-number) that gives them both the ability to be fast as well as totally isolate the subject. The background and shallow depth-of-field is a very important part of macro photography and can take quite a lot of time to master. Many modern macro lenses can focus to infinity and are prime lenses which can make them ideal when it comes to portrait photography, so just because it’s a macro lens doesn’t mean it can only be used for that type of photography.

In general, thin lenses show little or no distortion, whereas thicker positive and negative simple lenses will suffer positive and negative distortion, respectively. The sign (or type) and magnitude of geometric distortion depend on the position of the aperture diaphragm with respect to the lens(es). When an aperture diaphragm is placed in front of a positive lens, the principal ray does not form an image at the predicted Gaussian point and barrel distortion occurs, whereas when the diaphragm is placed behind the lens (forming an exit pupil), a pincushion distortion effect is observed.

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Although modern research-grade microscopes are corrected in order to keep aberrations from spreading the image of a point source beyond the Airy disk, geometrical distortion of the image formed by microscope objectives tends not to be as well corrected when compared to photographic lenses at the same image angle. Distortion is produced mainly by the oculars (between 5 and 10 percent of the radial distance) in optical microscopy, although some distortion is also found in lower quality objectives. Microscopes can be monitored for distortion by imaging crossed grating lines, such as those found in haemocytometers, in the wide-field mode. When viewed through the eyepieces, the lines should appear straight and parallel over the entire image field.

Most objectives designed for use in biological microscopes can have pincushion distortions up to one percent, however objectives designed for imaging semiconductors are essentially distortion-free. This is necessary because the majority of integrated circuits have surface features with an abundance of grid-like structures composed of horizontal and vertical lines. Any distortion present in objectives when imaging these samples will be obvious in the eyepieces and in photomicrographs. Images produced by objectives, eyepieces and other optical components that have been corrected for geometrical distortion aberration are referred to as orthoscopic images.

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Fisheye lenses are extreme wide-angle lenses, having a 180° horizontal angle of view. There are both Circular and Full-frame fisheye lenses, the circular will create a round image in the center with unexposed (black) edges and the full-frame lens will fill the entire sensor but will only have 180° horizontal and not vertical.

Distortion is often found in systems utilizing compound lens systems containing meniscus, double gauss, telephoto, retrofocus, fisheye, and zoom lenses. In telephoto and retrofocus lens designs, the front group acts as an aperture stop for the rear group, producing a pincushion distortion for the negative rear group in telephoto lenses and a barrel distortion for the positive rear group in retrofocus lenses. Complex lens systems such as the zoom design can have rather pronounced distortion, which may vary with focal length, producing pincushion distortion at long focal lengths and barrel distortion at short focal lengths. For this reason, stereoscopic zoom microscopes classically have a significant amount of distortion present and microscope manufacturers have expended considerable effort in alleviating this aberration.

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subjects. Optical zoom gets up close and personal by using an actual lens adjustment and digital zoom adjusts the image in the camera itself (this figure shows the difference in the images you get):

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These lenses have a narrow view field and a long focal length. Telephoto lenses are great for wildlife and sport photography, and can be good to use for portrait when you want to isolate the model from the background. Telephoto lenses compresses the view which can be both positive and negative depending on the situation. Telephoto lenses with their longer focal length require better light conditions or the use of a tripod. There are fast telephoto lenses, like a 400mm f/2, but these are often very expensive and out of reach when it comes to most amateurs — and most of these lenses are too heavy to be handheld. The last decade most companies have started to produce these high end telephoto lenses with Image Stabilizer to make them more usable without tripods. Lately this feature has been implanted in more and more low-end lenses as well.

The origin of geometrical distortion lies in a difference between the transverse magnification of a lens and the off-axis image distance. When this distance deviates from that predicted by paraxial theory for constant transverse magnification, distortion can arise due to differences in focal lengths and magnifications through various parts of the lens. In the absence of other aberrations, geometric distortion is manifested by a mis-shaped image, even though each image point is in sharp focus, as discussed above. Quantitatively, distortion can be described by the following equation:

The tutorial initializes with a photomicrograph of a computer microprocessor integrated circuit loaded into the microscope viewport. To select a new image, use the Choose A Specimen pull-down menu. The Distortion slider is utilized to introduce either pincushion (slider to the left) or barrel (slider to the right) distortion into both the image and a horizontal/vertical grid pattern positioned above the slider. As the grid pattern changes to illustrate varying degrees of aberration, the approximate percentage of distortion is displayed beneath the pattern.

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Choosing a lens is often more difficult then choosing a camera when it comes to purchasing. One thing to remember is that a lens will last several times longer than digital cameras. A D-SLR has a limited lifespan of a couple of years, the prices on cameras just keeps on dropping and purchasing a new camera every 3-5 years is quite reasonable if you want good quality photographs & equipment. A lens on the other hand will (if handled correctly) last much longer then that, so a good lens will be an investment that you can use for a longer period of time. Another thing to think about is the fact that all the light that reaches the sensor needs to pass through the lens. A low quality lens on a high quality camera will result in bad image quality, but a good lens on a low quality camera (entry level D-SLR) can still produce good results. It’s important to realize that different lenses distort and compress the view. A wide angle lens will distort the view and distances can seem greater than they really are, while in contrast, a telephoto lens will compress the view and make far away object seem closer than they are.

Digital zoom: Some cameras offer a digital zoom, which is simply some in- camera image processing. When you use a digital zoom, the camera enlarges the image area at the center of the frame and trims away the outside edges of the picture. The result is the same as when you open an image in your photo- editing program, crop away the edges of the picture, and then enlarge the remaining portion of the photo. Enlarging the “zoomed” area also enlarges the pixels and reduces the image resolution and the image quality.

Introduction of distortion aberration is more obvious in specimens having a network of regular periodic features, such as diatoms or microprocessors. Other specimens lacking such periodicity do not appear to be dramatically distorted when viewed in the microscope, as evidenced by the biological specimens selectable with the pull-down menu. This does not mean that specimens with irregular features are immune to distortion aberration, just that the aberration must be far more severe to produce noticeable effects.

Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.

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In Photography, your lens is often your most important purchase. This photography tutorial outlines some important qualities of different lenses, and how each performs in identical situations.

There are two types of lenses, prime and zoom. A prime lens is a lens that has a fixed focal length, these lenses comes in all shapes and price classes. Zooms are more versatile. A zoom lens can be a wide-angle lens, a normal lens and a telephoto lens — all in one — where as a prime can only be what it is. High-end telephoto lenses as well as macro lenses are almost always primes. So why choose a prime instead of a zoom lens then? Most prime lenses are considerably sharper than the zooms in the same price class, even when you go to the very high-end lenses the primes are sharper but the difference is not as distinct. Not only are primes sharper but they often have a larger maximum aperture which makes them faster and ideal in low- light situations. However, the technology is moving forward at a great speed right now and the noise levels at high ISO isn’t as visible as it was before which makes zoom lenses able to be faster as well. All in all I would recommend that people have at least one prime in their camera bag, preferably a normal lens, which is the perfect lens for many situations — sharp, fast and light-weight. Most lenses have a “sweet spot” where the lens is performing better than on other settings. Zoom lenses are often best in the middle of their range and there can be some quality loss on both the maximal and minimal focal length, but it’s different from lens to lens so your best bet is to try and see where you find the sharpest results. The aperture will also affect the sharpness, and most lenses are softer when they are wide open (largest aperture). To prevent this you can always step down one or two f-stops, if the situation allows for it.

To show this effect see three photos taken with different lenses, the front object (a street sign) is kept at the same size but the background is drastically different.