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#11
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Newbie question: lenses
I don't have any lenses of this kind at all, but I believe I've
read that they're relatively difficult to make well - something about precise centering of the cemented elements. And at any rate, for whatever reason, one lens might be quite a bit better or worse than the next. Whole different world from today's Schneider large format lenses like the Symmar XL. So, you never know, your odds are good but it's something to check. Donn |
#12
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Newbie question: lenses
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#13
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Newbie question: lenses
"Donn Cave" wrote in message news:1075615453.736833@yasure...
I don't have any lenses of this kind at all, but I believe I've read that they're relatively difficult to make well - something about precise centering of the cemented elements. And at any rate, for whatever reason, one lens might be quite a bit better or worse than the next. Whole different world from today's Schneider large format lenses like the Symmar XL. So, you never know, your odds are good but it's something to check. Donn Cemented lenses must be centered so that all of the elements are exactly coaxial and the axis is perfectly concentric. This is done by precision grinding of the edges. The Angulon is particularly difficult because the front and rear elements are larger than the others. So, not only must it be accurately ground but the lens must be clamped in a precision fixture after cementing. All lens elements are centered but those to be cemented must have much greater precision of gringing of the edges since the edge is the reference surface for cementing. The old procedure is to place the element on a tube of something less than the lens diameter. The lens is held on by a thick resin. A point source light is shown on the lens and its reflection observed through a telescope. If the lens is not centered the reflection rotates. The lens is moved on the tube until the reflections from both surfaces stand still during rotation. The lens is then clamped by another tube from the top and the edge ground so that it is exactly concentric. In the usual case where the diameters of all the cemented elements are of the same diameter the edge is used to clamp the lenses together. The more elements to be cemented the more critical the centering operation becomes. For air spaced lenses the precision of the edge is not so important because it is not used to center the lens. The usual method of mounting lenses is to clamp the element between two rings, one on each side. The pressure of the rings on a spherical surface will force the lens to be centered in them. The edge has just enough clearance to allow this. This trick works only for spherical surfaces, aspherical surfaces must be individually centered in the mount by optical inspection methods. Cemented lenses must also be individually polished to final figure so that the cemented surfaces fit accurately. This, plus the additional centering operation, and the additional operations needed for the actual cementing, make cemented surfaces expensive. Cemented surfaces were popular with lens designers before good lens coatings became available. A cemented surface reflects very little light so their use is a way of reducing lens flare. Lens coatings reduce glass-air surface reflection to nearly the same level so air spaced designs are much more popular now than before about 1945. |
#14
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Newbie question: lenses
question about the lens, a Schneider Angulon 90mm f/6.8. I know it's old; the serial number is is mid 5 million, making it a two years my senior. The trouble is, I can't seem to find any information about it on the web. What I did find conflicted with everything else. I'm looking for just the angle of coverage and coverage at infinity focus. It was mere curiosity before; now that I have it in my hands, I'm concerned. http://www.schneideroptics.com/info/...format_lenses/ http://www.schneideroptics.com/info/...enses/angulon/ http://www.schneideroptics.com/info/.../6,8-90mm.html |
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