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#21
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#22
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"Hemi4268" wrote in message ... How about lateral color, which is a difference in image size with color of light. Yes this would be true and a true APO would correct for this regardless of f-stop. It makes for easy overlap of seperation negatives. I am not sure though that the APO correction would hold for much more the the design 1:1 scale that this lens is designed for. Larry The lateral color correction stays pretty good but also depends on the lens design. Lateral color is automatically corrected in a symmetrical lens, but like coma and geometrical distortion, the cancellation is complete only when the entire system is symmetrical, that is at equal image and object distances, but generally stays good at infinity. Not all Apo Artars are symmetrical. Most of the Red-Dot series were sold with some element spacing ajustment to optimise them for middle distances. The adjustment depended on the focal length and whether the lens was sold in a shutter or a barrel. Long barrel mounted lenses, which were used mostly for photomechanical use, were still symmetrical but shorter focal lengths in shutters were adjusted for magnification of up to about 1:10, which is effectively infinity. Kodak Commercial Ektar lenses are Tessar types, definitely NOT symmetrical but are corrected for lateral color. Kodak claimed that the color correction was good enough to allow their use in making color separation plates. Kodak also claims the corrections are good over a distance from ingfinity to around 5 times the lens focal length. I am sure you are aware that few of the lenses now being sold with "Apo" attached to their names are actually apochromatic. Apochromatic correction is defined as bringing three colors to a common focus and correcting for spherical aberration at two colors. Most of the "apo" lenses on the market currently are actually acromats (corrected for two colors for focus and one color for spherical) but having little deviation in color correction. The apending of apo to these lenses is really misleading. It turns out that somewhere in the German DIN standards is one written so sloppily that it allows for lens manufacturers to use this appelation and get away with it. The Apochromatic Artar is an actual apochromat, corrected for focus at three colors and being corrected for lateral color through symmetry or semi-symmetry. -- --- Richard Knoppow Los Angeles, CA, USA |
#23
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"Hemi4268" wrote in message ... How about lateral color, which is a difference in image size with color of light. Yes this would be true and a true APO would correct for this regardless of f-stop. It makes for easy overlap of seperation negatives. I am not sure though that the APO correction would hold for much more the the design 1:1 scale that this lens is designed for. Larry The lateral color correction stays pretty good but also depends on the lens design. Lateral color is automatically corrected in a symmetrical lens, but like coma and geometrical distortion, the cancellation is complete only when the entire system is symmetrical, that is at equal image and object distances, but generally stays good at infinity. Not all Apo Artars are symmetrical. Most of the Red-Dot series were sold with some element spacing ajustment to optimise them for middle distances. The adjustment depended on the focal length and whether the lens was sold in a shutter or a barrel. Long barrel mounted lenses, which were used mostly for photomechanical use, were still symmetrical but shorter focal lengths in shutters were adjusted for magnification of up to about 1:10, which is effectively infinity. Kodak Commercial Ektar lenses are Tessar types, definitely NOT symmetrical but are corrected for lateral color. Kodak claimed that the color correction was good enough to allow their use in making color separation plates. Kodak also claims the corrections are good over a distance from ingfinity to around 5 times the lens focal length. I am sure you are aware that few of the lenses now being sold with "Apo" attached to their names are actually apochromatic. Apochromatic correction is defined as bringing three colors to a common focus and correcting for spherical aberration at two colors. Most of the "apo" lenses on the market currently are actually acromats (corrected for two colors for focus and one color for spherical) but having little deviation in color correction. The apending of apo to these lenses is really misleading. It turns out that somewhere in the German DIN standards is one written so sloppily that it allows for lens manufacturers to use this appelation and get away with it. The Apochromatic Artar is an actual apochromat, corrected for focus at three colors and being corrected for lateral color through symmetry or semi-symmetry. -- --- Richard Knoppow Los Angeles, CA, USA |
#24
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"Bob Salomon" wrote in message ... In article , (Hemi4268) wrote: Since f-stop controls the amount of depth of focus Depth of focus or depth of field? -- To reply no_ HPMarketing Corp. Well, these are really the same thing. Depth of field is the depth in the object space, depth of focus is the depth in the image space. The way Larry used the term it is correct, he is talking abuot the image space. Since longitudinal chromatic aberration is a measure of the variation in focus with wavelength a reduction of the effect of defocusing by stopping down would tend to alleviate the fringing produced by chromatic although that is not the same as correcting it. Stopping down changes the rate at which the size of the blur spot changes with change in focus. The smaller the stop, that is, the smaller the hole in the iris, the narrower the angle of the fan of light rays from lens to film and, hense, the smaller the blur spot will be for a given amount of defocusing. Since chromatic correction also affects spherical correction, and since spherical is proportional to the stop size, stopping down will also tend to reduce the effect of spherochromatism. Lateral color is a measure of the size of the image at different wavelengths. It is possible (and actually pretty common) to have a lens which is correced for focus at different colors but where the iamges vary in size despite being in common focus. This is sometimes called chromatic variation of magnification. Symmetry will correct this but it can be corrected in asymmetrical lenses by very careful design. stopping down has no effect whatever on lateral color. -- --- Richard Knoppow Los Angeles, CA, USA |
#25
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"Bob Salomon" wrote in message ... In article , (Hemi4268) wrote: Since f-stop controls the amount of depth of focus Depth of focus or depth of field? -- To reply no_ HPMarketing Corp. Well, these are really the same thing. Depth of field is the depth in the object space, depth of focus is the depth in the image space. The way Larry used the term it is correct, he is talking abuot the image space. Since longitudinal chromatic aberration is a measure of the variation in focus with wavelength a reduction of the effect of defocusing by stopping down would tend to alleviate the fringing produced by chromatic although that is not the same as correcting it. Stopping down changes the rate at which the size of the blur spot changes with change in focus. The smaller the stop, that is, the smaller the hole in the iris, the narrower the angle of the fan of light rays from lens to film and, hense, the smaller the blur spot will be for a given amount of defocusing. Since chromatic correction also affects spherical correction, and since spherical is proportional to the stop size, stopping down will also tend to reduce the effect of spherochromatism. Lateral color is a measure of the size of the image at different wavelengths. It is possible (and actually pretty common) to have a lens which is correced for focus at different colors but where the iamges vary in size despite being in common focus. This is sometimes called chromatic variation of magnification. Symmetry will correct this but it can be corrected in asymmetrical lenses by very careful design. stopping down has no effect whatever on lateral color. -- --- Richard Knoppow Los Angeles, CA, USA |
#26
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Depth of focus or depth of field?
DEPTH OF FOCUS!!! |
#27
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Hemi4268 wrote:
As far as telescopes, we all know that mirror telescopes by design have no color errors "what-so-ever" and are APO by defult. That is why we see very little glass lens telescopes except ones sold by K-mart for $99. Apo or not Apo isn't the real question when deciding between a mirror or a lens telescope. The real problem is making large lenses to _real_ tight tolerances... The largest refractors commonly used (for an amateur) are 6" diameter, the cost rising exponentially with diameter. A mirror telescope with 8" is a beginners tool, costing about 20% of the cost for a 6" refractor. Martin |
#28
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Hemi4268 wrote:
As far as telescopes, we all know that mirror telescopes by design have no color errors "what-so-ever" and are APO by defult. That is why we see very little glass lens telescopes except ones sold by K-mart for $99. Apo or not Apo isn't the real question when deciding between a mirror or a lens telescope. The real problem is making large lenses to _real_ tight tolerances... The largest refractors commonly used (for an amateur) are 6" diameter, the cost rising exponentially with diameter. A mirror telescope with 8" is a beginners tool, costing about 20% of the cost for a 6" refractor. Martin |
#29
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On 23 Sep 2004 06:38:15 -0700, (brian) wrote:
True apochromatic correction is a big deal in the realm of refracting telescope objectives, where f/9 is considered *fast*. Then my old 6" f/8 must be a real mover or a good spotting scope ;)) Regards, John S. Douglas, Photographer - http://www.puresilver.org Please remove the "_" when replying via email |
#30
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On 23 Sep 2004 06:38:15 -0700, (brian) wrote:
True apochromatic correction is a big deal in the realm of refracting telescope objectives, where f/9 is considered *fast*. Then my old 6" f/8 must be a real mover or a good spotting scope ;)) Regards, John S. Douglas, Photographer - http://www.puresilver.org Please remove the "_" when replying via email |
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