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Forum: Pentax SLR Lens Discussion
12-15-2017, 11:16 AM
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FYI- Spherical is corrected by 3 means; 1. Using an element with the opposite sign as the element that is causing the spherical. If a positive element (plano convex as an example) is causing the marginal rays to focus in front of the focal plane, adding a negative element will mostly compensate for it. 2. Bending the elements toward the diaphragm reduces both spherical and coma. 3. Using aspherical surfaces reduces spherical but in most cases does not completely eliminate it.
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Forum: Pentax SLR Lens Discussion
12-10-2017, 12:08 PM
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You are basically saying the same thing I just said above. BTW, most LF lenses are Plasmat designs.
---------- Post added 12-10-2017 at 12:20 PM ----------
The reason why telescopes are so limited by magnification has to do with the fact that they use two optical systems (the telescope's optical design combined with an eyepiece) When an eyepiece is used that magnifies too much and causes unsharpness, it is due to the aberrations from the telescope being magnified. In the photographic world, it would be like using a 10X teleconverter on the back of a long lens.
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Forum: Pentax SLR Lens Discussion
12-09-2017, 02:01 PM
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Optical physics tells us that a larger diameter optic will outperform a smaller one. This is the main reason why larger telescopes will out resolve smaller ones. This is proven; however in photographic optical design, a more important factor is the degree of aberration correction. Aberrations determine performance more than diameter.
Wide lenses (Retrofocus, reverse telephoto, etc) do not fit in the typical "focal length divided by diameter" computation for focal ratio because of the use of a negative lens group in front of the stop. This negative group reduces the size of the entrance pupil. That entrance pupil must be used to figure the focal ratio, not the diameter of the front element. Since the negative group reduces the entrance pupil size, it also reduces the light, therefore the front element must be made larger to compensate.
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