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04-22-2008, 07:20 AM   #1
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I hate math, so help me....

How many extension tubes do I have to add to the DA*200mm to get it to 1:1?

Or should I just not even ask?

04-22-2008, 07:31 AM   #2
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get a reverse ring!!
04-22-2008, 07:39 AM   #3
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QuoteOriginally posted by jshurak Quote
get a reverse ring!!
Does that work without an aperture control? I'm actually just curious as I don't have a DA here to see what it does when its off anymore. I thought they closed down.
04-22-2008, 07:45 AM   #4
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200mm extension tube

04-22-2008, 07:55 AM   #5
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Just to make the answer better understood.

Any lens produces 1:1 magnification when the distance from the front element, to the subject is exactly 2 times the focal length. This requires an extension tube equal to the focal length of the lens to allow an in focus image.
04-22-2008, 07:58 AM   #6
Ed in GA
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QuoteOriginally posted by Lowell Goudge Quote
Just to make the answer better understood.

Any lens produces 1:1 magnification when the distance from the front element, to the subject is exactly 2 times the focal length. This requires an extension tube equal to the focal length of the lens to allow an in focus image.

Thanks,

That is good information to know.

But, this begs another question from me.

If I add a 70mm extension tube to a 70mm lens, do I now have the equivalent of a 140mm lens?


Ed
04-22-2008, 08:06 AM   #7
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Exactly what Lowell said. Personally, I think you should try the extension tube on the 105 macro instead. On a macro lens, things get more complicated. There is a formula floating around:

(Lens FL + length of the extension)/Lens FL.

However, in reality, the magnification is higher than this formula. It's probably due to the real focal length of the lens when it's at 1:1.
04-22-2008, 08:07 AM   #8
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QuoteOriginally posted by Ed in GA Quote
Thanks,

That is good information to know.

But, this begs another question from me.

If I add a 70mm extension tube to a 70mm lens, do I now have the equivalent of a 140mm lens?


Ed
Without the corrected glass element, you can't focus to infinity. So your setup is not exactly equivalent to a 140mm lens.

04-22-2008, 08:11 AM   #9
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QuoteOriginally posted by Ed in GA Quote
Thanks,

That is good information to know.

But, this begs another question from me.

If I add a 70mm extension tube to a 70mm lens, do I now have the equivalent of a 140mm lens?


Ed
Ed

No what you have is a 70mm lens that can focus from a range of about 125-140 mm depending upon the close focus of the lens without extension tubes.
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