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10-17-2010, 06:00 PM   #16
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Steve,

I'm curious as to why you think the M* 800 uses a Flourite element or two. I know that early on Minolta experimented with Flourite, but went to anomalous dispersion glass in their big tele's. I thought Canon was the only manufacturer to use it regularly. Does Pentax have a history of using Flourite?

Scott

10-18-2010, 02:28 PM   #17
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Scott, there have been several lens producers that have used Fluorite elements. The 250mm Superachromatic lens from Zeiss used Fluorite as well.

There is no history of Pentax ever using Fluorite. Pentax has been fairly secretive about their lenses in general but the following is why I suspect the use of Fluorite in the 800 f/6.7. It is rare to see a negative element being low dispersion; nearly all are positive. The low dispersion element in the front group of the 800 is very likely positive. The fact that the positive element is surrounded by two negative elements, looks as though they are trying to protect it from the weather. Fluorite weathers easily. Using a Fluorite element as a front element is not done. It is possible that the positive ED element is some other type of Fluoride ( not Calcium Fluoride ). I'm betting on crystalline Calcium Fluoride though , AKA Fluorite.
10-20-2010, 06:34 PM   #18
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Pentax made a Takumar Ultra-Achromatic (Apochromat) using Calcium Flourite materials.

Ultra-Achromatic-Takumar 85mm F4.5 Lens Reviews - Pentax Lens Review Database

Ultra-Achromatic-Takumar 300mm F5.6 Lens Reviews - Pentax Lens Review Database

1973 prices were $1,400 for the 85mm and $1,600 for the 300mm.

The sought after SMC Takumar 85mm f/1.8 was only $259.50

A complete Pentax 6x7 with pentaprism and 105 f/2.4 was only $1,049.00

Where is that time machine?

Last edited by nightfly; 10-20-2010 at 06:46 PM. Reason: Correct information
10-21-2010, 03:04 PM   #19
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Thanks Nightfly, I had never seen that. The description says quartz fluorite. I think they meant that one element was Fluorite and another was quartz.

The fact that two earlier lenses used Fluorite makes the odds of the 800ED using it more likely.

10-21-2010, 04:06 PM   #20
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QuoteOriginally posted by nightfly Quote
...
A complete Pentax 6x7 with pentaprism and 105 f/2.4 was only $1,049.00

Where is that time machine?
You don't have to go back to the future that far. When I purchased my 6x7 (some 20 years ago), you could pick up a gray market system for about that price.
10-21-2010, 04:30 PM   #21
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QuoteOriginally posted by desertscape Quote
Thanks Nightfly, I had never seen that. The description says quartz fluorite. I think they meant that one element was Fluorite and another was quartz.

The fact that two earlier lenses used Fluorite makes the odds of the 800ED using it more likely.
Growing Calcium Flourite into large enough crystals for large elements is an expensive process. That might be one reason for using them in a slow lens design. They would seldom be used for front elements as they can deteriorate easily.

Modern ED glass is very good and lessens the need for flourite in the design, but flourite can correctly focus light beyond the violet and ultra-violet range. It is commonly used in expensive astronomical refractor telescopes. Again, expensive!
11-24-2010, 11:47 PM   #22
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Takumar 85mm 4.5

The Patent paperwork on the 85mm lens says that the two positive lens elements are fluorite and the negative elements are quartz (I think it says "fused silica"). I don't think that lenses newer than these used fluorite elements. The Pentax tech people that I talked to in the late 70's, 80's and 90's said that ED glass was preferred because of better durability. Anyway the the 85 is cool because it has no glass in it.
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