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it's still stock here too. there was a time when it was out of stock but thats not the dame as discontinued. I've been looking at samples and I agree the canon isn't near the quality of the minolta
Joined: Sun Jun 17, 2007 6:58 am Posts: 1524 Location: San Jose, CA, USA
David, Vacuum filled? Hmm!! How does one go about doing it? Couldn't help notice the contradiction in terms. Purely in jest. Nothing else worthwhile to add, I am afraid.
With best regards, Sury
David Kilpatrick wrote:
alphaPDX wrote:
70-300 f/4, SSM, with helium-filled chambers to keep it below 450g. With all the patents flying, Sony could do this, right? MAke it watertight too, so if I drop it in a lake it'll float!
Helium filled? Not necessary. A vacuum is lighter than helium, and unlike balloons, lens construction does not call for pressure to retain shape. A vacuum also has a different refractive index to air, and by definition could not admit sucky-sucky dust particles from zooming, or host any airborn particles, or admit any moisture, or allow fungus to develop.
The vacuum filled lens is the future!
David
_________________ Minimize avoidable sufferings - Sir Karl Popper
Joined: Sat Nov 22, 2008 6:36 am Posts: 598 Location: Southern California
David Kilpatrick wrote:
Helium filled? Not necessary. A vacuum is lighter than helium, and unlike balloons, lens construction does not call for pressure to retain shape. A vacuum also has a different refractive index to air, and by definition could not admit sucky-sucky dust particles from zooming, or host any airborn particles, or admit any moisture, or allow fungus to develop.
The vacuum filled lens is the future!
I assume that this was said in jest, but just in case it was not, I'll point out some issues with a vacuum-filled lens.
First, as was discovered by the rocket scientists 50-some years ago, special precautions have to be taken to prevent surfaces in contact in a vacuum from bonding to each other. Second, given a design service life of many years, it would be a challenge to prevent air from leaking into the interior of a mechanism that includes a rotating seal that is subjected to a differential pressure of 15 pounds per square inch.
A helium-filled lens would have its own set of problems with meeting the design service life, as helium is the most slippery molecule in nature - it will leak past seals that will stop any other molecule. Manufacturing costs for helium-filled or vacuum-filled lenses would be significantly higher than for "normal" lenses, and this would probably doom the possibiity of such designs in an era where the bean counters seem to have the final say on product design.
The most practical approach to providing most of the benefits that you ascribe to a vacuum-filled lens would be to fill a sealed lens with nitrogen at atmospheric pressure. Of course, this would also be somewhat more expensive to manufacture than a normal lens, and it would not offer the weight savings of the vacuum-filled or helium-filled designs.
BTW, stepping back from the tongue-in-cheek vewpoint, I cannot resist pointing out that the weight saving of even a super-telephoto size vacuum-filled lens would be on the order of only about 10 grams - and that assumes that there is no weight penalty associated with the additional sealing required.
Joined: Sat May 19, 2007 1:14 pm Posts: 6049 Location: Kelso, Scotland
Not vacuum-filled in the barrel, of course! Like water-filled lenses - just a vacuum in place of air spaces found in the completely sealed 'group' units which make up some lenses. In theory of course these are hermetically sealed anyway, but the presence of a gas within the inter-element voids allows particles (if they detach from blackening) to be temporarily suspended before landing on the lens.
Your comment about materials sticking together is interesting. Balsam was used for many years to glue compound elements - you can see it as a yellow discolouration in many old lenses, or 'oyster mark' delamination starting at the edges. Balsam also gets invaded by fungi. Today bonding polymers are used not natural balsam.
But what if two glass elements were simply placed in contact with sufficient external pressure and a deep vacuum created between them? Glass is liquid, and after a few years maybe two elements would be physically bonded so well that nothing could ever get in between.
These are not serious suggestions, just idle thoughts. A pressurized lens, not a vacuum, might be more useful anyway - it could have a tiny pump constantly maintaining positive pressure within the lens, so dust was never sucked into the camera. Revise that, pressurise the camera too...
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