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It's not a close up filter but ....
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PostPosted: Sun Oct 07, 2012 3:05 pm    Post subject: It's not a close up filter but .... Reply with quote
a far, far away filter that I need

I'm hoping there are optics gurus that can help Smile

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?


PostPosted: Sun Oct 07, 2012 3:11 pm    Post subject: Reply with quote
Have you tried to take a close-up lens and reverse it?


PostPosted: Sun Oct 07, 2012 3:18 pm    Post subject: Reply with quote
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.


PostPosted: Sun Oct 07, 2012 3:31 pm    Post subject: Reply with quote
Orio wrote:
Have you tried to take a close-up lens and reverse it?



Oops, that does not work Sad


PostPosted: Sun Oct 07, 2012 3:34 pm    Post subject: Reply with quote
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 Smile


PostPosted: Sun Oct 07, 2012 6:09 pm    Post subject: Reply with quote
Have you tried putting the projector lens away and mounting a good camera lens? Laughing


PostPosted: Sun Oct 07, 2012 6:13 pm    Post subject: Reply with quote
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?


PostPosted: Sun Oct 07, 2012 6:18 pm    Post subject: Reply with quote
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)?


PostPosted: Sun Oct 07, 2012 9:23 pm    Post subject: Reply with quote
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.
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