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04-29-2010, 10:03 PM   #1

Join Date: Apr 2010
Location: Wangaratta, Victoria
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Extension tubes - image ratio tables?

Hi friends.
I've just acquired a set of (manual) extension tubes for use with my *ist DS.
Is there a table anywhere that shows image size ratios at varying lenghts of extension and varying focal lengths? Trial and error is good, but if there is a readily available table it would be helpful.

04-29-2010, 10:52 PM   #2
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Join Date: Aug 2008
Location: Michigan, USA
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How about This..

extension tube chart

Is that what you're looking for??

Google is your friend but it's good of you to ask because some of these questions are things I would have never thought of..

04-30-2010, 06:32 AM   #3
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It isn't hard to do in your head:

Magnification change = tube_length/focal_length.

So a 30mm extension tube on a 50mm lens adds 30/50=0.6x

A 30mm tube on a 90mm lens adds 30/90=0.33x

04-30-2010, 11:19 PM   #4

Join Date: Apr 2010
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Original Poster
Jeffjs & newarts;
Thanks on both counts - the source of the chart and the mental calculation. If there is one thing better that one solution it is two! This forum has a great breadth of photographic knowledge, (both Pentax specific and general) and makes a must read for newbies and old hands that have forgotten stuff, or never learnt fully. Thanks again both of you.


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