Starman1
Vendor (EyepiecesEtc.com)
Reged: 06/24/03
Loc: Los Angeles
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Re: Comparison of 24mm-26mm Eyepieces
[Re: bluedandelion]
#5081932 - 02/20/12 12:48 PM
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Andreas,
Eek! You're right. But the issue is further confused by the discussion of distortion in lenses here:
http://en.wikipedia.org/wiki/Distortion_(optics)
in which the radial distortion coefficient K(sub n) is negative for pincushion distortion and positive for barrel distortion.
Though I do understand the idea in that book that pincushion results from a lateral (positive) stretching of the image, and barrel the opposite, I cannot reconcile that with the math that shows if the distortion coefficient is negative you get pincushion.
Perhaps this should become a different thread so not to hijack the discussion of the review of eyepieces.
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Daniel Mounsey
Vendor - Vixen Optics
Reged: 06/12/02
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Re: Comparison of 24mm-26mm Eyepieces
[Re: bluedandelion]
#5081940 - 02/20/12 12:51 PM
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One of the things I admire about Bill is his dedication. Regardless of any topics others question in a review, it takes a tremendous amount of work and time to complete this kind of essay. I personally see eye to eye on many of his impressions which remind me a lot of the glory days of Charlton Flats.
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andydj5xp
Pooh-Bah
Reged: 05/27/04
Loc: 52.269 N/10.571 E
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Re: Comparison of 24mm-26mm Eyepieces
[Re: Starman1]
#5082281 - 02/20/12 04:13 PM
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Quote:
Andreas,
Eek! You're right. But the issue is further confused by the discussion of distortion in lenses here:
http://en.wikipedia.org/wiki/Distortion_(optics)
in which the radial distortion coefficient K(sub n) is negative for pincushion distortion and positive for barrel distortion.
Though I do understand the idea in that book that pincushion results from a lateral (positive) stretching of the image, and barrel the opposite, I cannot reconcile that with the math that shows if the distortion coefficient is negative you get pincushion.
Perhaps this should become a different thread so not to hijack the discussion of the review of eyepieces.
Don,
you are right about not wanting to hijack the discussion about Bill's magnificent review.
Thus only as a short remark: the Wikipedia formula derives the position of the undistorted point x(sub u) from the position of the distorted point x(sub d). In case of barrel distortion (negative RD) the distorted point is too near to the center (too low). Therefore, it needs a positive K(sub n) to yield the position of undistorted point. And vice versa.
Andreas
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DanaJ
super member
Reged: 12/11/11
Loc: Idaho, USA
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Re: Comparison of 24mm-26mm Eyepieces
[Re: andydj5xp]
#5083103 - 02/21/12 02:12 AM
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Canon's excellent book "Lens Work" has photos showing barrel and pincushion distortion, as well as photos and discussion of many more optics issues. Daniel's photos show basically what they do for barrel and pincushion distortions. The text related to these:
"[...] Distortion which stretches (+) the shape in the diagonal direction is called pincushion distortion, and that which compresses (-) the shape in the diagonal direction is called barrel distortion. In rare cases with super-wide-angle lenses, these two distortion types may coexist to produce a shape which is both stretched and compressed. [... text relating to lens design, zoom lenses, aspherical elements, and diagram of subject/lens/image formation ...] Since this type of aberration is caused by refraction abnormalities of the principle light rays passing through the center of the lens, its effect cannot be reduced by stopping down the lens."
Not that much different than Wikipedia I suppose (though I had my copy of Lens Work II before Wikipedia existed).
On direct topic: Great review, Bill, and thanks for spending all the time on it, and sharing with us. Looks like it was much harder than the earlier ones, with so many different targets and qualities to examine.
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rguasto
professor emeritus
Reged: 11/18/10
Loc: Long Island, NY
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Re: Comparison of 24mm-26mm Eyepieces
[Re: Starman1]
#5083238 - 02/21/12 07:31 AM
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Yes! and that is exactly what I think every time I look through an eyepiece in my telescope. 
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BillP
Postmaster
Reged: 11/26/06
Loc: Vienna, VA
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Re: Comparison of 24mm-26mm Eyepieces
[Re: DanaJ]
#5084197 - 02/21/12 05:00 PM
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Quote:
Looks like it was much harder than the earlier ones, with so many different targets and qualities to examine.
In a sense, it was actually a bit easier. This time I got a voice recorder and just talked out loud in the back yard in the middle of the night as I observed (wondered what neighbors were thinking  ). So I didn't have to divert my attention between observing and writing all the time. Made it go much more smoothly. Then next day just sat in front of computer and transcribed my audio to notes. Have an audio editing program so was easy to just find the spots where I was talking so didn't have to listen 3 hours for a 3 hour observing session. Typically took about 30 minutes of transcribe time to catch all the previous evening's audio notes. made the whole process much more enjoyable also as I could have fun while testing instead of writing.
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BillP
Postmaster
Reged: 11/26/06
Loc: Vienna, VA
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Re: Comparison of 24mm-26mm Eyepieces
[Re: Daniel Mounsey]
#5084203 - 02/21/12 05:02 PM
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Quote:
Barrell (left) Pincushion (right)
In pictures of people, the left one usually happens with age to most of us
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Daniel Mounsey
Vendor - Vixen Optics
Reged: 06/12/02
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Re: Comparison of 24mm-26mm Eyepieces
[Re: BillP]
#5086543 - 02/22/12 11:25 PM
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that's very true!
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CounterWeight
Postmaster
Reged: 10/05/08
Loc: Cloudyopolis, OR.
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Re: Comparison of 24mm-26mm Eyepieces
[Re: Daniel Mounsey]
#5108195 - 03/06/12 10:57 AM
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Bill your review is an outstanding effort. Very enjoyable, really liked the format, layout, and content. Given the nuances discussed it is remarkably uncomplicated and easy to read. Very well done and many thanks!
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BillP
Postmaster
Reged: 11/26/06
Loc: Vienna, VA
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Re: Comparison of 24mm-26mm Eyepieces
[Re: Jason Martin]
#5125699 - 03/16/12 03:29 PM
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I'm please with them all and find them consistent bewtween each other.
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Pod
member
Reged: 04/17/12
Loc: An American abroad
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Re: Comparison of 24mm-26mm Eyepieces
[Re: BillP]
#5177181 - 04/17/12 03:25 PM
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Hi, first post. I had an account a few years ago but forgot my user name. Just to say what a fantastic review, very concise, comprehensive, well laid out and very easy to read.
Thanks
Edited by Pod (04/17/12 03:27 PM)
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astroneil
Pooh-Bah
Reged: 07/28/09
Loc: res publica caledoniae
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Re: Comparison of 24mm-26mm Eyepieces
[Re: Pod]
#5260390 - 06/07/12 03:40 PM
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Hello Bill or should I say, Mr. Eyepiece. 
My apologies for chiming in so late. I've been away on the frontier for most of that time.
What can I say? A most excellent and unique project, superbly illustrated and easy enough for a eyepiece duffer like myself to understand. 
I now realise there's a brave new world of oculars out there. And it's nice to know that there's still some bargains to be had.
With very best wishes,
Neil. 
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