Question about achromat - MFlenses forum

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Author Message raay04





Joined: 08 Dec
Posts: 340
Location: India

Joined: 08 Dec Posts: 340Location: India

Posted: Mon Nov 30, 8:48 pm

 

   Post subject: Question about achromat

raay04 wrote:

hello folks , i have a question about achromat closeup lenses , i have a elnikkor 50 /2.8 enlarger which i bought in "as is" condition and found out that the 1st optical doublet block is missing , it has the the first elelmnt ,2nd cemented doublet and the last element , now my question is can i use the cemented doublet as high quality achromatic closeup filter ??My main intention is to get a achromatic close up lens for free

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hemeterfilms





Joined: 04 Jul
Posts: 80
Location: Mexico City

Joined: 04 Jul Posts: 80Location: Mexico City

Posted: Tue Dec 01, 2:29 am

 

   Post subject:

hemeterfilms wrote:

The remaining doublet looks as if it has quite small radius curve and so may be quite powerful.

You can roughly estimate its focal length by holding it up to a bright window some distance away and then measuring the distance to a piece of paper with the image of the window focussed on it as sharp as you can get it.

The diameter seems quite small, so using it as a close up lens might be a problem

dan_





Joined: 05 Dec
Posts:
Location: Romania
Supporting Member

Joined: 05 Dec Posts: Location: Romania

Expire: -12-19

Posted: Tue Dec 01, 3:42 am

 

   Post subject:

dan_ wrote:

Not always a good idea. Not every doublet is an achromat. IMO the front and rear lens groups are supposed to corect each other (allmost symetric lens) and therefore the dublet alone may not be an achromat. Anyway, you should actually test it to see if it's O.K. for you.

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Joined: 12 Mar
Posts:
Location: Weinheim, Germany
Supporting Member

Joined: 12 Mar Posts: Location: Weinheim, Germany

Expire: -03-09

Posted: Tue Dec 01, 5:44 am

 

   Post subject:

kds315* wrote:

That's a cheap and very enlarger lens, just get another one...

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Gerald





Joined: 25 Mar
Posts:
Location: Brazil

Joined: 25 Mar Posts: Location: Brazil

Posted: Tue Dec 01, 10:53 am

 

   Post subject:

Gerald wrote:

In a double-Gass lens, the menisci have negative optical power. They have no use as close-up lens.

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Amith said:

Homework Statement

1)If we want to form an achromatic combination of lenses in which both are made of same material, one should be convex and the other should be concave, isn&#;t it? Is there any other condition to be satisfied along with this?


For the first question,I think the reason behind having one convex and other concave in order to form an achromatic doublet is that, they disperse the beam in opposite directions as one is converging and other is diverging. According to the condition for achromatism of lenses,
w1/w2 = -f1/f2
as w1 & w2 are different, f1 & f2 should be different. Is it right?

For the second question,
a) If both are made of same material, then their dispersive powers will be same. From the condition for achromatism of lenses,
f1=-f2
If both are convex, then this is not possible, isn't it?
b) Similarly both cannot be concave.
c) Similarly this cannot be true.
So for achromatism of 2 lenses made of same material placed in contact:
# one should be convex and other should be concave.
# Both should be of same focal length.

3) I think the third question is wrong because 2 lenses made of same material cannot have different dispersive powers. Please correct me if I am wrong.

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