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12-06-2007, 09:56 AM   #1
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Question about extension tubes

I know that extension tubes reduce the minimum focusing distance of a lens, but they also reduce the maximum focusing distance. However, I can't find any information about HOW MUCH the maximum focusing distance is reduced with each tube (12, 20 & 36). Does anyone know?

I have an FA77 and sometimes I'm frustrated by the minimum focusing distance. I shoot portraits with it, and I'm wondering what the minimum and maximum focusing distance would be with the 12mm extension tube.

I've checked Kenko's site, but there isn't much information. I figure there must be some formula for calculating how much the minimum and maximum focusing distance of a lens with focal length "A" is reduced by an extension tube with a given length "B" (i.e. 12mm, 25mm).

12-06-2007, 10:54 AM   #2
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Here is a link to a 2003 thread: Scroll down to the math guru posting and enjoy.

Extension Tube Math? - Canon Digital Photography Forums
12-06-2007, 11:48 AM   #3
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And some more Canon numbers... I suppose the same lens physics apply to Pentax?

The Canon EOS system - Macro and Closeup Work
12-06-2007, 12:17 PM   #4
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QuoteOriginally posted by Nesster Quote
And some more Canon numbers... I suppose the same lens physics apply to Pentax?

The Canon EOS system - Macro and Closeup Work
The brand does not matter at all. It's all optical physics. Move a lens further or closer and the focus distance changes.

12-06-2007, 12:24 PM   #5
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QuoteOriginally posted by Nesster Quote
And some more Canon numbers... I suppose the same lens physics apply to Pentax?

The Canon EOS system - Macro and Closeup Work
Thanks for the link. One thing I don't understand is the difference between focusing distance and working distance in the extension tubes tables.

For a 12mm extension tube attached to the 77, the magnification ratio would go from .14x to .3x; with a 25mm tube, it goes to .47x. The minimum focusing distance with the 12mm tube attached is approximately 13.8 inches; with the 25mm tube attached the minimum focusing distance is approximately 9.7 inches.

I found these formulas here: Nikon Forum: How to calculate new minimal focus distance with a extension tube? - photo.net

However, I can't find any information anywhere about the maximum focusing distance with extension tubes.
12-06-2007, 01:06 PM   #6
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QuoteOriginally posted by switters Quote
Thanks for the link. One thing I don't understand is the difference between focusing distance and working distance in the extension tubes tables.
The focusing distance is the distance from the lens node, a techie term meaning the center of a single element lens, a piece of glass shaped like this () with the same focal length as the actual lens. The working distance is the distance from the front of the lens barrel to the subject. They are identical with the single element lens, but not with a compound lens.

My 400 mm lens is only about 305 mm long, so the node is actually in front of the lens out in mid air 95 mm in front of the lens furthest tip (without hood, of course). That is the definition of a telephoto construction, by the way. If it wasn't telephoto construction, the lens focal center would be 400 mm from the film/sensor and the barrel would be something over 400 mm long. This means that the working distance of the lens is 95 mm more than the focal distance.

A telephoto design macro lens exaggerates this to no end. At 1:2, my 100mm macro has 273 mm of working distance. The focal distance is about 200 mm, so I get an extra 73 mm between the lens and the subject.

Fun ain't it?
12-06-2007, 01:11 PM   #7
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If I read the Canon tables right, an 85mm lens with 12mm extension tube will focus to about 50 - 55 cm at infinity.
12-06-2007, 01:21 PM   #8
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i'm really curious as to what kind of portraits you are shooting with a 77mm at 9 inches....

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