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06-17-2015, 02:44 PM   #31
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The classic combination with fluorite is to use a high index Barium glass. The other element could possibly be Lanthanum. Some of these combinations yield near zero tertiary spectrum (residual color). A major problem with long photographic lenses is that color passing through the marginal area of the lens does not focus at the same plane as that same color passing through the paraxial area of the lens. This leads to longitudinal chromatic wide open but much reduced when stopped down. If you have a diaphragm, that can be used to truncate the poorly corrected marginal rays and reduce fringing. If however you have no diaphragm (telescope) then this higher order aberration (spherochromatism) must be solved for wide open shooting.

06-17-2015, 03:47 PM   #32
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QuoteOriginally posted by desertscape Quote
This leads to longitudinal chromatic wide open but much reduced when stopped down. If you have a diaphragm, that can be used to truncate the poorly corrected marginal rays and reduce fringing.
I told the builders that this optic was not for astro work but only for terrestrial photography of very colorful birds at prime focus where accuracy was essential.
They assured me no problem. Simply start with a 90mm optic, optimize that and then mask it down to a maximum effective clear aperture of 80mm.
They said if I do see obvious CA it's much more likely to be the sensor than the optics. As I recall the the front objective assembly which could be simply screwed directly into any appropriate tube cost about $1100.
It was made by a small custom shop in Mainz Germany.

So far I'm a happy camper.

Last edited by wildman; 06-17-2015 at 04:04 PM.
06-17-2015, 04:26 PM   #33
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I never knew custom lens making was a thing. If you don't mind me asking.. How much was it?
06-17-2015, 08:28 PM   #34
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QuoteOriginally posted by trevorg Quote
I never knew custom lens making was a thing.
It was my understanding that this was not custom but simply one of the optional standard methods they use in preparing optics.
They did not do their own coating if I recall right. I've been told Taiwan is full of small operations like this.
About $1100 for the optics only not the entire scope. Relatively cheap compared to a 600mm Canon.

06-17-2015, 08:43 PM   #35
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QuoteOriginally posted by trevorg Quote
Hey guys,


Why do lenses that have wide apertures, zooms or primes, generally have better sharpness than small aperture lenses (like the kit lenses)? I'm talking about wide open, ie. the 50mm prime is sharper wide open (f1.8 for example) than the kit lens at 50mm wide open (f5)?


Is it just because lenses with small base apertures are generally cheaper, so the optics are "cheaper" too?
Telescopes with larger primary optics have more resolving power. For lenses you start from the premise that a large front optical element has a greater resolving power than a smaller one and that it's all downhill after that. Depending on coatings, RI, how well corrected each element is (to within fractions of a wave etc) how many elements/air surfaces there are, the more (or less) degradation there is depending on the quality of the work. Obviously there are tradeoffs and where one lens is let down with a smaller aperture it can also compensate to a degree by the quality of the work.

Cheap kit lenses won't be hand figured for the ultimate quality BTW.
06-17-2015, 09:07 PM - 2 Likes   #36
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QuoteOriginally posted by bossa Quote
Cheap kit lenses won't be hand figured for the ultimate quality BTW.
And don't forget the final polishing done on the thighs of virgins at a remote Shinto temple high in the mountains of northern Japan.
06-17-2015, 11:20 PM   #37
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QuoteOriginally posted by wildman Quote
And don't forget the final polishing done on the thighs of virgins at a remote Shinto temple high in the mountains of northern Japan.
Damn! How could I forget that?
06-18-2015, 12:43 AM   #38
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QuoteOriginally posted by wildman Quote
And don't forget the final polishing done on the thighs of virgins at a remote Shinto temple high in the mountains of northern Japan.
Yup, that final step provides the pixie dust!

06-18-2015, 01:10 AM   #39
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I have been very impressed with simple triplet lenses. Think of the Tessar design (actually four bits of glass in three groups). The only reason to have a lot of glass is to correct for issues like wide apertures (Tessar seem limited to f2.8) and flat field or of course zoom lens compensation. The less glass, the less reflections and distortions, and for general photography, a flat field is of little consequence. I wonder if my 6 element Xenon is really better than the 4 element Zenar from Schneider. Can't say I have noticed so far.
06-18-2015, 07:59 AM   #40
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QuoteOriginally posted by pathdoc Quote
Making a prime lens with f/4 is almost like tying a brick around Usain Bolt's ankle. In the field, such a low speed would be a deal-breaker. Under the controlled conditions prevailing for its intended use, I guess it's a non-issue.
The Pentax100/4 macro is much better in the field, as you have a greater film/lens to subject distance because of the longer focal length.

Using the 50mm macro with extension tubes will put you cm/inches from your subject.

Phil.
06-18-2015, 11:36 AM   #41
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QuoteOriginally posted by arnold Quote
The only reason to have a lot of glass is to correct for issues like wide apertures
Many elements are required for wide lenses and long lenses although there are exceptions. Wide lenses have problems with the oblique aberrations and long lenses with color correction. It is difficult to accomplish this with only 4 elements unless the designer has a huge budget for expensive glass and uses aspheric surfaces.
One of the biggest misconceptions in photography is curvature of field. It is an aberration at the focal plane, where the image is not flat but curved. All lenses must be corrected for it so the image lays flat on the film/sensor. Having a flat subject plane is another issue. Some macro lenses do this and also projection lenses.
06-18-2015, 01:41 PM   #42
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QuoteOriginally posted by arnold Quote
I have been very impressed with simple triplet lenses.
Sometimes less is more...

...a classic Fraunhofer doublet, f/8, 1000mm, No special technique. The moon was there so I just spot metered on it and pulled the trigger for a single RAW file. Got rid of CA in PS. Just a snap shot really.

Last edited by wildman; 06-23-2015 at 07:42 PM.
06-18-2015, 04:32 PM   #43
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Pretty sure I can see an American flag flapping in the breeze there...
06-18-2015, 06:44 PM   #44
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QuoteOriginally posted by trevorg Quote
Pretty sure I can see an American flag flapping in the breeze there...
Not possible - I cloned it out along with the footprints.

So what's the final verdict - is fast glass necessarily better all else being equal?
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