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Message: <blockquote data-quote="aroy" data-source="post: 296324" data-attributes="member: 16090">Here is what I have understood of the lense design.The simplest lense design is the Gaussian doublet. Two lenses separated by space will magnify a distant object and bring them to a focus at a plane. This is a design of two Groups. More Groups are introduced when you want to reduce the length or distortion.Lenses are cemented together to reduce aberration. Thus the simplest group is one convex and one concave lense cemented together. The reason is that a single element will have different focal length for each wavelength, which is called chromatic aberration. When two lenses, one convex and the other concave, of different refractive index are cemented, then the difference in focal length across the visible spectrum of one is counteracted by the other - they tend to cancel out and (at least theoretically) all the wavelengths focus at the same point. You have now got a chromatic aberration corrected lense - the achromatic lense. Planar design was one of the earliest successful design to improve upon the Gaussian doublet.Though the Gaussian doublet can be designed to be perfectly achromatic its length depends on the focal length and so does the rear focusing distance. For longer focal lengths you will get very long lenses, and for wide angles the lense will not focus on the sensor as the distance it too short, because the point where the image is formed is at the focal length distance behind the lens's optical centre.So now you bring in more lenses into the design.. To shorten the length of Super Telephotos. To create enough space for a wide angle to focus on the film/sensor. This is called the retro focus design. The extra elements ensure that the image plane is well behind the rear element, enough to clear the space taken up by the mirror.Extra lenses mean more groups thus larger and heavier construction.Here are some more cases for the designers to introduce more groups in the lense design.. Internal Focusing. If you do not want the lense barrel to change the length, you go for another group of lenses, which will move one particular group, while the rest of the groups remain static.. Faster Lense. For a larger aperture you use a larger front element and a larger opening for the aperture blades. This means that many elements will be bigger (not just the front element) and residual distortion which was insignificant at smaller apertures of the design, will now begin asserting it self. So you add another group to reduce this distortion.. Correction for sharpness fall off at edges. This is usually a problem with faster lenses. The older F4 or lower had pretty uniform sharpness. The reason is that, say for 50mm design, the internal elements were quite big compared to the aperture, so only the central portion of the glass was used. With faster aperture, without increasing the element diameters more of the lense diameter is used increasing both non linearity and decreasing sharpness. You have two methods of dealing with this. A) Introduce more groups or B) increase the element diameters. Both are used in the current high end fast lenses - Zeiss Otus and the Sigma Art series. That is why their lenses are larger in diameter, bigger over all and containing those extra elements/groups. With so much glass the lense is bound to be expensive.</blockquote>

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