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#51
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Fewer elements - lesser Bokeh?
Gordon Moat wrote in message ...
brian wrote: Gordon Moat wrote in message ... There is a mention in the above article that a switch from a Sonnar type design to a Xenotar type design improved aberration correction. While this makes sense to me, is that what effected the reduction in Coma? Would that reduction in Coma directly relate to improvements in defocus rendition? Thanks! Ciao! Gordon: Coma correction for a single object distance is a trivial task with either the Sonnar or double-Gauss (Xenotar) design forms. However, since the double-Gauss has a greater degree of symmmetry the coma correction remains stable over a wider magnification range. Thanks for the awesome reply, I think I am finally understanding these lens types more. Since the Planar type seems to be quite common in medium format choices, how does that fit in? The double-Gauss design type is a natural choice when you want high image quality and reasonable production economy in a lens with medium to large aperture and moderate field angle. It is markedly superior to Tessars in virtually all respects. Sonnars tend to be preferred for longer focal lengths because they are more compact, and they lend themselves more conveniently to apochromatic correction. The old Nikon 135mm f/2 AI/AIS is an example of a Sonnar type design which is degraded by coma at close focusing distances, but which is very well corrected for all aberrations at small magnifications. Brian www.caldwellphotographic.com Since you bring up the close focusing issue, it makes me wonder why Nikon ? did not apply their close range correction (CRC) idea to the 135 mm. If I remember correctly, one group between the aperture and film plane moved. While the construction of the 135 mm you mentioned might not allow that to work, I notice that the 85 mm f1.4 did employ CRC. Perhaps cost was an issue. I suspect that the Nikon designers felt that the 135/2 AI design was good enough at the time. It is a very nice lens, and edge performance degradation due to coma even at full aperture and closest focus is still not what I would call dreadful. The later 135/2.0 DC Nikkor is a double-Gauss derivative which does use CRC to control coma in a manner very similar to the 85/1.4. Note that alot of the wide angle Nikkors have CRC, but here the motivation is to keep astigmatism under control while focusing rather than coma. Ciao! Gordon Moat Alliance Graphique Studio http://www.allgstudio.com Brian www.caldwellphotographic.com |
#52
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Stranger and stranger (was Fewer elements - lesser Bokeh?)
jjs wrote:
In article , wrote: Beautiful lens! The engraved information says that it's an f/5 lens, yet the aperture ring has an f/4 setting, or have I misread something? Good eye, James. It is more mysterious than that. See this - it is the right side of the dial: http://wind.winona.edu/~stafford/2.jpg (Sorry four the lousy underexposed picture, but I just snapped it by the window as the sun had about set.) So the lens is marked "F5" in one place and the widest aperture "1,5". (The barrel and lens are not mismated. They are as much a single part as one can imagine.) I'm going to take a guess that the "1,5" is European decimal notation. No. We Europeans have always used the f-value as it is. I'm not sure when it was adopted, but possibly already in the 17th century when the first telescopes were built. Could the lens be "1,5" under a the manufacturer's convention, but branded "F5" to follow the American standards by the American distributor? Stranger and stranger! Yes... -- Lassi |
#53
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Fewer elements - lesser Bokeh?
Stacey wrote:
Gordon Moat wrote: Since you bring up the close focusing issue, it makes me wonder why Nikon did not apply their close range correction (CRC) idea to the 135 mm. If I remember correctly, one group between the aperture and film plane moved. While the construction of the 135 mm you mentioned might not allow that to work, I notice that the 85 mm f1.4 did employ CRC. Perhaps cost was an issue. Several high end lenses do have designs (floating elements) where multiple elements are moved when focusing to help closeup performance. My zuiko 28mm f2, 50mm f2 and their 90mm f2 macro come to mind. The were all cost over $700 20 years ago. I'm sure there are others as well made by other manufacturers. Cost is exactly why they don't do this more as it makes the mechanical design of the lens much more complex. Olympus seems to have solved the cost issue. Their IS series cameras have an integrated zoom lens that has several lens groups moving around in all directions, and still the whole camera costs far less than $700. -- Lassi |
#54
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Stranger and stranger (was Fewer elements - lesser Bokeh?)
In article , Lassi
=?iso-8859-1?Q?Hippel=E4inen?= wrote: [...] We Europeans have always used the f-value as it is. I'm not sure when it was adopted, but possibly already in the 17th century when the first telescopes were built. Okay, if you say so, but there were other aperture metrics in the USA and possibly elsewhere. I do not mean the EV marks on some lenses, but something else entirely. For example, I find it hard to believe the lens in question really has an F 1:1.5 aperture. |
#55
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Fewer elements - lesser Bokeh?
brian wrote:
Gordon Moat wrote in message ... brian wrote: Gordon Moat wrote in message ... There is a mention in the above article that a switch from a Sonnar type design to a Xenotar type design improved aberration correction. While this makes sense to me, is that what effected the reduction in Coma? Would that reduction in Coma directly relate to improvements in defocus rendition? Thanks! Ciao! Gordon: Coma correction for a single object distance is a trivial task with either the Sonnar or double-Gauss (Xenotar) design forms. However, since the double-Gauss has a greater degree of symmmetry the coma correction remains stable over a wider magnification range. Thanks for the awesome reply, I think I am finally understanding these lens types more. Since the Planar type seems to be quite common in medium format choices, how does that fit in? The double-Gauss design type is a natural choice when you want high image quality and reasonable production economy in a lens with medium to large aperture and moderate field angle. It is markedly superior to Tessars in virtually all respects. Sonnars tend to be preferred for longer focal lengths because they are more compact, and they lend themselves more conveniently to apochromatic correction. Definitely makes sense then why I see so many of each type in their respective ranges. The old Nikon 135mm f/2 AI/AIS is an example of a Sonnar type design which is degraded by coma at close focusing distances, but which is very well corrected for all aberrations at small magnifications. Brian www.caldwellphotographic.com Since you bring up the close focusing issue, it makes me wonder why Nikon ? did not apply their close range correction (CRC) idea to the 135 mm. If I remember correctly, one group between the aperture and film plane moved. While the construction of the 135 mm you mentioned might not allow that to work, I notice that the 85 mm f1.4 did employ CRC. Perhaps cost was an issue. I suspect that the Nikon designers felt that the 135/2 AI design was good enough at the time. It is a very nice lens, and edge performance degradation due to coma even at full aperture and closest focus is still not what I would call dreadful. The later 135/2.0 DC Nikkor is a double-Gauss derivative which does use CRC to control coma in a manner very similar to the 85/1.4. Considering that even used, the DC version is quite a bit more than the more normal AI/AIS version, it makes one wonder whether the added cost and complexity really hold many benefits. I have used the 105 mm DC lens once, and it was such a pain (slowing down the pace of shots too much) that I stuck with the more normal 105 mm F2.5 AIS. Note that alot of the wide angle Nikkors have CRC, but here the motivation is to keep astigmatism under control while focusing rather than coma. Very interesting differences. Considering the uses to which the different focal lengths would often be used, I can see why there would be a particular bias in correction. Of course, the more amazing (to me) aspect is that much of that correction work was done several decades ago. Thanks for the information. Ciao! Gordon Moat Alliance Graphique Studio http://www.allgstudio.com |
#56
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Fewer elements - lesser Bokeh?
Lassi Hippeläinen wrote in message ...
Olympus seems to have solved the cost issue. Their IS series cameras have an integrated zoom lens that has several lens groups moving around in all directions, and still the whole camera costs far less than $700. -- Lassi Moving groups of elements around is really not very costly in high volume production. I was once involved in a 3:1 zoom lens project in which the complete opto-mechanical package cost less than $15.00. The truth is that mechanical parts are quite cheap. The main difficulty in my experience is getting a first-class mechanical design. Brian www.caldwellphotographic.com |
#57
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Stranger and stranger (was Fewer elements - lesser Bokeh?)
jjs wrote:
In article , Lassi =?iso-8859-1?Q?Hippel=E4inen?= wrote: [...] We Europeans have always used the f-value as it is. I'm not sure when it was adopted, but possibly already in the 17th century when the first telescopes were built. Okay, if you say so, but there were other aperture metrics in the USA and possibly elsewhere. I do not mean the EV marks on some lenses, but something else entirely. For example, I find it hard to believe the lens in question really has an F 1:1.5 aperture. Wasn't the lens made in Kansas :-) Do you know for certain that the numbers are f-stops? Maybe the maker had developed a system that indicates how long the exposure time is when compared to some standard lens. If the reference lens is an f/4, this lens (f/5) would then need 1.5 times that time when fully open, etc. Should be testable with an exposure meter. -- Lassi |
#58
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Fewer elements - lesser Bokeh?
nicholas wrote in message ...
brian wrote: So, my conclusion is that all Xenotar-type lens designs have this particular rendering... I'm going to assume that Xenotar means double-Gauss. Not quite... A Xenotar-type design is 5/4 and not symmetrical as you have me believe... _SNIP_ Nicholas: Double-Gauss designs come in a very wide variety of subtypes, and it wasn't clear to me whether you were referring to the general design form or to the particular 5/4 subtype. So, if you want to restrict the conversation to the 5/4 type, then its OK with me. Xenotars have a degree of symmetry very similar to other double-Gauss designs. The fact remains: a 5/4 Xenotar subtype need not have harsh bokeh. The reason for this is that the design flexibility is nearly as great as with the more complex double-Gauss forms. A competent lens designer can vary the character of the defocused background highlights at will. For example U.S. Patent 2,844,072 by Lowenthal discloses an f/2.8 Xenotar type lens with moderate undercorrected spherical aberration. This design will produce beautifully soft-edged defocused background highlights. Although Schneider adopted the Xenotar type fairly early, the design was pioneered by Charles Wynne of Wray Optical back in the 1940's. At the time it was hailed as a significant achievement since it was apparently simpler than the similar 6/4 Gauss type. Oddly enough, the reason that the 5/4 Xenotar design form isn't used more widely is that it is more difficult to make and hence more expensive than the more common 6/4 form. The thickness tolerance on the 4th element almost always turns out to be unusually tight, and this element is also quite difficult to center. The formula for the Vivitar Series I 90mm macro lens is given in U.S. Patent 3,942,875 (example 6 of 6). There are a couple of minor typos in the prescription, and I can give you the correct version if you're interested. As you guessed, this design is not a Xenotar derivative since the rear shell is a cemented doublet. However, changing that cemented doublet to a singlet is easier than you might imagine. Brian www.caldwellphtographic.com |
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