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06-20-2011, 06:42 PM   #1
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Extension Tube Question

So I dropped by the local thrift store yesterday and found an old Pentax Asahi case with a SMC Takumar 3.5/135mm as well as an off brand 70-230 lens. The bag also had a bunch of extras. some of those extras where 3 different extension tubes. I didn't know what you called them at first, but for some reason I knew from reading somewhere that they were for macro work.
Each of the 3 tubes has a number on them. 1,2, and 3. I tried them out with the 135 and they work well.
My question is this; When I attach lets say the extension with #3 on it, do I divide my focal length by 3 ( which would be 45) and enter that result when I turn my K10D on and it asks for the length? I'm thinking this may not be correct..??? Any help would be greatly appreciated.

Here are a couple of test shots. The bug in one of the photos is so tiny I didn't even see it until I went to take a macro of the flower it was on..He is literally 1/8" in body length roughly.
I took this handheld for time sake and didn't sharpen the photo at all. I was also still shaking from my favorite cup o joe, as well as it being slightly windy out. In the shade too..




Cropped 74%



06-20-2011, 07:45 PM   #2
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It asks for the length for SR purposes because there is no communication between the lens and the body so the camera doesn't know what focal length the lens is.
Some say to add the length of the tube to the focal length of the lens ( https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/32961-1-1-macr...ion-tubes.html ). I've also seen it said to enter the focal length of the lens.
SR might not be effective close up anyway so the usual advice of a tripod, if you can, and SR off is possibly the best.
Whatever you have done seems to have worked - great shots.
06-20-2011, 09:33 PM   #3
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Hi Barron and welcome!

You might want to check this post which explains a little about posting pictures here. Posting things full size tends to do weird things to people with different size monitors. On my monitor your post is about 4 monitor widths wide, making it very hard to read.

I believe if you want to set the SR correctly with an extension tube you would add the length of the tube in mm to the lens focal length. You would need to measure the tube as I do not think the numbers (1,2,3) always mean anything on all brands.

However, as mentioned above I am not sure SR is all that useful with macro and definitely not if you use a tripod. YMMV of course!
06-20-2011, 09:49 PM   #4
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SR is not effective at close focus distances, period. You can add the tube length in mm to the focal length in mm to tell the SR'bot if you wish, but it won't really matter. This applies to short, medium, and long lenses, when bringing focus down close, like under 20-50cm (depends a bit on the focal length).

But I may add just a 10mm tube to a 200mm lens to cut its far focus down to 50-100m and its near focus down to ~1m. In that range, SR is still effective. Adding the extension only slightly alters the focal length, so I still tell the SR'bot that it's 200mm.

In other words: Don't sweat it! Tell the SR-bot the actual lens focal length. Even though SR won't be effective when hand-holding close shots, it won't hurt either. And if you use a tripod or other stabilization, turn of SR entirely.

06-20-2011, 09:53 PM   #5
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I have been using an old screw mount tele-converter with the glass removed as an extension tube, and have been using my Tak's for the glass on it. I just enter the focal length of the actual lens, probably wrong but I don't think the SR has any effect on it anyways - been shooting handheld with rather impressive results:

https://www.pentaxforums.com/forums/pentax-dslr-discussion/88994-show-us-your...ml#post1546179

The bug in that pic is about the size of the bug in your pic...


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06-22-2011, 05:59 PM   #6
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Everyone - thanks a lot for all the info you provided. It helps a great deal.
I will also review how to post pictures appropriately.
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