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04-29-2009, 04:39 AM   #1
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Anyone using long ext. tubes ?

I have been experimenting with ext. tubes with my FA77, FA43 and the Sigma 100-300 F4...they work, but I feel like with long ext. tubes (100mm) the corners remain tack sharp, but the center gets fuzzy. Also, with the 100-300 I see a round lighter area in the middle of some images.

Anyone who has used ext. tubes longer than 50mm, can you please give me your experience with them and what lens you used.

This thread which clearly showcase some of the finest macro work ever, involves 100mm ext tubes, so it should work.

https://www.pentaxforums.com/forums/post-your-photos/51505-my-macro-photos-5.html

Thank you

04-29-2009, 05:08 AM   #2
mer
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I have used very long extension tubes, including different bellows, up to 20cm+ with many different lenses ( Pentax macro DFA 50mm , cine lenses ( D - C mount ) , special macro lenses like the tominons ) etc etc

I have never seen what you describe ( better corners , fuzzy center ) .

Maybe you could post an example to clearly show what you mean .

Longer tubes than 100mm will have a great impact on diffraction. Don't use very long lenses ( long lenses = longer than 100mm ) with ext.tubes to gain high magnifications. If you use very long tubes ( 20cm ) for high magnifications ( 10x magnification ) make sure you choose an aperture typically not more than F11, then diffraction kicks in and its really obvious , the image becomes very fuzzy and not sharp at all .

I can post you some examples of 20cm + reversed pentax dfa 50mm if you want , they are really sharp.
04-29-2009, 05:12 AM   #3
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Original Poster
Thank you...

I will post some images later....at times I take the time to upload to post these images only to find that the thread dies without a reply. DSo, wanted to make sure there are people in here who can help me.

I will take some shots later today.
04-29-2009, 06:13 AM   #4
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Without seeing samples and just guessing off the top of my head:

- the reflections from inside the tube may be messing with the center of the image
- field curvature at such a magnification may make it so that the edges are in focus while the center isn't, or vice versa

04-29-2009, 08:32 AM   #5
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Try reversing the lens on the extention tube. That MAY help depending on the particular lens combination. Sometimes. Maybe.

Actually, I've found ultra-macro to be a crap-shoot depending on the equipment combinations. But fun when you have the time to experiment.

H2
04-29-2009, 08:42 AM   #6
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I'm with Nesster on this one, it sounds like field curvature. I see it when I use my M50/1.7 on a macro bellows.
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