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JohnBar
Joined: 21 Jun 2012 Posts: 581 Location: Liverpool
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Posted: Sun Oct 07, 2012 3:05 pm Post subject: It's not a close up filter but .... |
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JohnBar wrote:
a far, far away filter that I need
I'm hoping there are optics gurus that can help
I have an old projection lens which, because of mechnical restraints I can only get to focus up to 5 metre.
I think I need a negative achromat to focus further maybe infinity. The achromat will have to be mounted on the lens front.
If this is right, what diopter do I need? _________________ Rectilux 3FF Series single focus anamorphic attachments
http://www.transferconvert.co.uk/cinemania/rectilux-3ff.html
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Orio
Joined: 24 Feb 2007 Posts: 29545 Location: West Emilia
Expire: 2012-12-04
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Posted: Sun Oct 07, 2012 3:11 pm Post subject: |
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Orio wrote:
Have you tried to take a close-up lens and reverse it? _________________ Orio, Administrator
T*
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Minolfan
Joined: 30 Dec 2008 Posts: 3438 Location: Netherlands
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Posted: Sun Oct 07, 2012 3:18 pm Post subject: |
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Minolfan wrote:
Orio wrote: |
Have you tried to take a close-up lens and reverse it? |
When you reverse a close up lens it stays a positive lens, but mounted less good.
Cose up lenses are positive (convex) so you need a negative lens (concave).
Might be you find something usefull when you take an old stuck lens apart?
Easier in most cases is to shorten the barrel or the mount a bit. |
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JohnBar
Joined: 21 Jun 2012 Posts: 581 Location: Liverpool
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Posted: Sun Oct 07, 2012 3:31 pm Post subject: |
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JohnBar wrote:
Orio wrote: |
Have you tried to take a close-up lens and reverse it? |
Oops, that does not work _________________ Rectilux 3FF Series single focus anamorphic attachments
http://www.transferconvert.co.uk/cinemania/rectilux-3ff.html
Regular News on https://www.facebook.com/pages/Rectilux/704770636267200 |
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JohnBar
Joined: 21 Jun 2012 Posts: 581 Location: Liverpool
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Posted: Sun Oct 07, 2012 3:34 pm Post subject: |
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JohnBar wrote:
Minolfan wrote: |
Orio wrote: |
Have you tried to take a close-up lens and reverse it? |
When you reverse a close up lens it stays a positive lens, but mounted less good.
Cose up lenses are positive (convex) so you need a negative lens (concave).
Might be you find something usefull when you take an old stuck lens apart?
Easier in most cases is to shorten the barrel or the mount a bit. |
There is some cheap stuff @Surplus shed 80mm diameter, but I need to know what to order, for example there is a negative meniscus coated FL -91mm for $4
I think I need an achromat but a negative meniscus might be OK (but CA might be bad), its like my lens is short sighted and needs glasses _________________ Rectilux 3FF Series single focus anamorphic attachments
http://www.transferconvert.co.uk/cinemania/rectilux-3ff.html
Regular News on https://www.facebook.com/pages/Rectilux/704770636267200 |
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skida
Joined: 02 Mar 2012 Posts: 1826 Location: North East England
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Posted: Sun Oct 07, 2012 6:09 pm Post subject: |
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skida wrote:
Have you tried putting the projector lens away and mounting a good camera lens? |
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twinquartz
Joined: 11 Jun 2012 Posts: 316 Location: Sweden
Expire: 2013-10-29
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Posted: Sun Oct 07, 2012 6:13 pm Post subject: |
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twinquartz wrote:
Quote: |
I have an old projection lens which, because of mechnical restraints I can only get to focus up to 5 metre. |
What camera body?
What adapter? |
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s58y
Joined: 05 Sep 2010 Posts: 131 Location: Eastern NY
Expire: 2013-09-10
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Posted: Sun Oct 07, 2012 6:18 pm Post subject: |
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s58y wrote:
I guess you'd need something with a (negative) focal length shorter than 5 meters, but not too short. I see that Edmund Optics has negative achromats of -150mm focal length and shorter. These might be too short.
Maybe the negative lens from a Canon R/FD/FL to EOS adapter, which adds about 2 mm of back focus? There are also barlow lenses for telescopes, but I have no idea what their (negative) focal length is. Maybe old eyeglasses (-0.75 diopter, for example)? _________________
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JohnBar
Joined: 21 Jun 2012 Posts: 581 Location: Liverpool
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Posted: Sun Oct 07, 2012 9:23 pm Post subject: |
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JohnBar wrote:
Yes thats right.
After some study I learned that a -0.5 D will form a virtual image from infinity to just 2 metres, which I can focus upon. _________________ Rectilux 3FF Series single focus anamorphic attachments
http://www.transferconvert.co.uk/cinemania/rectilux-3ff.html
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